Variable aperture assembly

Abstract

A variable aperture assembly comprising multiple blades 40 wherein a portion of each blade nearer the central axis O is further along that axis than a portion further from the axis over at least part

Description

Field The present application relates to a variable aperture assembly. Background A variable aperture assembly can be used to provide an aperture of a controlled size. For example, in the context of a camera, a variable aperture assembly may be used to control the size of the aperture. For example, different aperture sizes may be used for different focal lengths. When a variable aperture assembly is part of an optical system, the variable aperture assembly may increase the overall size of the optical system along the optical axis. For example, when a variable aperture assembly is applied to a camera, the variable aperture assembly may increase the size of the camera. When the camera is part of a handset (e.g. for a mobile device), then the presence of the variable aperture assembly may increase the thickness of the handset. It is desirable to decrease the extent by which a variable aperture assembly increases the size of an optical system. Summary According to an aspect of the present invention, there is provided a variable aperture assembly comprising: a plurality of blades arranged to define a variable aperture with a central axis and a size that can be changed within a size range; wherein the variable aperture assembly is arranged such that, for at least part of the size range, a radially inner part of the blades is further along the central axis in a first direction parallel to the central axis than a radially outer part of the blades by a distance greater than a thickness of the blades. By providing that a radially inner part of the blade is further along the central axis than a radially outer part of the blades, features of the mechanism of the variable aperture positioned further along the central axis than the radially outer part of the blades can be moved back along the central axis (e.g. moved down). This can reduce the extent by which the variable aperture assembly takes up space along the central axis. Optionally, the blades are angled relative to a plane perpendicular to the central axis such that the radially inner part of the blades is further along the central axis in the first direction than the radially outer part of the blades. By arranging the blades in an inclined plane / cone, variable aperture mechanism features positioned above the blades can be moved down. Optionally, the blades form a truncated cone shape around the central axis. By providing a truncated cone shape, space is provided for parts of the mechanism above the radially outer part of the blades. By providing that the cone is truncated, the aperture may be maintained. Optionally the radially inner part of the blades is further along the central axis in the first direction when the size of the variable aperture is smaller than when the size of the variable aperture is greater. This may allow the angle of inclination of the blades to be maintained across the range of sizes of the variable aperture. This may help the variable aperture assembly to be more robust and / or reliable. Optionally, the variable aperture assembly comprises a support surface arranged to prevent the blades from moving beyond the support surface in a direction opposite to the first direction. By providing the support surface, the blades may be prevented from touching other components, for example, a lens of an optical system to which the variable aperture assembly is applied. This may allow the clearance between the blades and the component to be minimal, without risking undesirable contact between the blades and the component. Optionally, the variable aperture assembly comprises an actuator assembly configured to drive the blades so as to change the size of the variable aperture, wherein at least part of the actuator assembly is located in the first direction relative to the blades. By providing at least part of the actuator assembly in the first direction relative to the blades, less space is required below the blades to accommodate the actuator assembly. This may allow the radially outer part of the blades to be positioned further back along the central axis so as to reduce the height of the variable aperture assembly. Optionally, the actuator assembly comprises at least one shape memory alloy, SMA, element arranged on contraction to drive the blades, wherein the at least one SMA element is located in the first direction relative to the blades. For example, the SMA element which may be a wire may be located in a space above the blades. This may allow the blades to be located lower so as to reduce the height of the variable aperture assembly. Optionally, the blades are flexible. By providing that the blades are flexible, the radially inner part of the blades may be bent on contact with another component for small aperture sizes. This may allow the blades to be located further back along the central axis so as to reduce the overall height of the variable aperture assembly. Optionally, the blades have a thickness of at most 0.1 mm, and optionally at most 0.05 mm. By providing particularly thin blades, the blades may bend more readily so as to form an appropriate shape which allows the size of the aperture to be varied while allowing the blades to be retained further back along the central axis. Optionally, the variable aperture assembly comprises a base, and a rotatable part arranged to rotate relative to the base about the central axis, wherein the blades are connected to the base and to the rotatable part such that rotation of the rotatable part relative to the base drives the blades so as to change the size of the variable aperture. By providing the base and the rotatable part, a convenient mechanism may be provided for controlling the size of the variable aperture. The rotatable part may be provided to have a low weight, such that less end stop material may be required. This may help to reduce the overall size of the variable aperture assembly. According to another aspect of the present invention, there is provided a camera assembly comprising the variable aperture assembly described above and a lens assembly configured to receive light in a light receiving direction opposite to the first direction. The variable aperture assembly may be mounted on the lens assembly such that, for the at least part of the size range, the radially inner part of the blades is further along the central axis, in the first direction, than the radially outer part of the blades. By providing a camera assembly comprising the variable aperture assembly, the size of the camera assembly may be reduced while providing for a variable aperture function. This may help to reduce the size of a device in which the camera assembly is provided. For example, the thickness of a handset comprising the camera assembly may be reduced. Optionally, the variable aperture assembly is mounted on the lens assembly such that, for the at least part of the size range, the blades bend due to contact with the lens assembly such that the radially inner part of the blades is further along the central axis, in the first direction, than the radially outer part of the blades. By allowing the blades to be driven into the front lens element, the variable aperture mechanism features positioned above the blades can be moved down. This helps to reduce the overall size of the camera assembly. Optionally, at least 50%, optionally at least 80% and optionally at least 90% of the variable aperture assembly overlaps with the lens assembly along the central axis. This helps to reduce the extent by which the presence of the variable aperture assembly increases the size of the overall camera assembly. The variable aperture assembly may largely be located in sections of the central axis which would in any case be taken up by the lens assembly of the camera assembly. Optionally, the camera assembly is arranged such that contact between the blades and the lens assembly generates frictional forces that constrain size change of the variable aperture at any size within the at least part of the size range when the variable aperture assembly is not actuated. The contact between the blades and the lens assembly may provide added friction that can help with holding the configuration of the camera assembly with zero hold power, i.e. when power is not provided to maintain the configuration of the camera assembly. Optionally, the lens assembly has an optical axis that coincides with the central axis. The variable aperture assembly may be applied to an optical system of the camera assembly. According to another aspect of the present invention, there is provided a variable aperture assembly comprising: a plurality of blades arranged to define a variable aperture with a central axis and a size that can be changed within a size range, wherein the plurality of blades are arranged in at least two layers that are spaced apart along the central axis, and wherein the blades in a layer that is further along the central axis in a first direction are movable further towards the central axis than the blades in an adjacent layer. By providing the blades in multiple layers, direct contact between the blades may be avoided. The blades thus do not need to bend and twist on changing of the variable aperture size. Friction between the blades may be reduced, which improves adjustment of the variable aperture and reduces particle generation by the variable aperture. Allowing blades in a layer that is further along the central axis in a first direction to move further towards the central axis allows the aperture assembly to be mounted on a lens assembly with reduced height. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of a variable aperture assembly mounted on a lens assembly; Figure 2 is a schematic view of a comparative variable aperture assembly; Figure 3 is a schematic view of a variable aperture assembly mounted on a lens assembly; Figure 4 is a schematic view of a variable aperture assembly in a first position; Figure 5 is a schematic view of the variable aperture assembly of Figure 4 in a second position; Figure 6 is a schematic view of a variable aperture assembly mounted on a lens assembly; and Figure 7 is a schematic view of another variable aperture assembly mounted on a lens assembly. Detailed description Height reduction Figure 1 schematically depicts a variable aperture assembly 1. The variable aperture assembly 1 shown in Figure 1 is mounted on a lens assembly 50. Figure 1 shows a camera assembly 2 comprising the variable aperture assembly 1 mounted on the lens assembly 50. It is not essential for the variable aperture assembly 1 to be mounted on a lens assembly or to be part of a camera assembly. The variable aperture assembly 1 may be used in other contexts in which it is desirable to provide an aperture having a variable size. As shown in Figure 1, the variable aperture assembly 1 comprises a plurality of blades 40. The blades 40 may alternatively be referred to as leaves. The blades 40 are arranged to define a variable aperture with a central axis O. The variable aperture has a size that can be changed within a size range. The variable aperture assembly 1 is controllable to increase or decrease the size of the aperture defined by the blades 40. Edges of the blades 40 define the perimeter of the aperture. The aperture may have a generally circular shape when viewed along the central axis O. However, the aperture may not be precisely circular. The shape of the aperture may depend on the shape of the curves at the edge of the blades 40. The aperture may be polygonal, for example hexagonal (e.g. when there are six blades 40 provided). The number of blades may be varied. For example, optionally the variable aperture assembly 1 comprises at least three, optionally at least four, optionally at least six and optionally at least eight blades 40. In general, a greater number of blades 40 may allow the shape of the aperture defined by the blades 40 to be more circular. Optionally, the variable aperture assembly 1 is arranged such that, for at least part of the size range, a radially inner part 41 of the blades 40 is further along the central axis O in a first direction parallel to the central axis O than a radially outer part 42 of the blades 40 but by a distance greater than a thickness of the blades 40. The radially inner and radially outer parts of the blades 40 are defined with respect to the central axis O. The radially inner part 41 of the blades 40 is closer to the central axis O. The radially outer part 42 of the blades 40 is further from the central axis O. In the arrangement shown in Figure 1, the first direction is upwards (from bottom to top). In the orientation shown in Figure 1, the radially inner part 41 of the blades 40 is higher than the radially outer part 42 of the blades 40. As shown in Figure 1, the blades 40 have a thickness. The thickness refers to the dimension of the blades 40 in the direction perpendicular to the plane of each blade 40. As shown in Figure 1, the distance along the central axis 0 between the radially inner part 41 of the blades 40 and the radially outer part 42 of the blades 40 is greater than the thickness of the blades 40. This means that the difference in position along the central axis O between the radially inner part 41 and the radially outer part 42 of the blades 40 is greater than would be due to the blades 40 simply overlapping each other. Optionally, the radially inner part 41 is further along the central axis O in the first direction than the radially outer part 42 of the blades 40 by a distance at least twice, and optionally at least five times, the thickness of the blades 40. As shown in Figure 1, optionally the variable aperture assembly 1 comprises a variable aperture mechanism 5. The variable aperture mechanism 5 may comprise a plurality of components. The variable aperture mechanism is configured to facilitate movement of the blades 40 so as to change the size of the aperture defined by the blades 40. An implementation of the variable aperture mechanism is described in more detail with reference to Figures 4 and 5. As shown in Figure 1, at least part of the variable aperture mechanism 5 may be provided above the blades 40. In particular, at least part of the variable aperture mechanism 5 may be located above the radially outer part 42 of the blades 40. The term "above" is used to refer to the position further along the central axis O in the first direction. The variable aperture mechanism 5 comprises at least one component that is further along the central axis O in the first direction relative to the radially outer part 42 of the blades 40. As shown in Figure 1, by providing that the radially inner part 41 is further along the central axis O compared to the radially outer part 42 of the blades 40, there is more space above the radially outer part 42 that overlaps with the blades 40. There is space above the radially outer part 42 of the blades 40 to accommodate at least part of the variable aperture mechanism 5 while allowing the variable aperture mechanism 5 to overlap with the axial position of the blades 40. The variable aperture mechanism 5 above the radially outer part 42 of the blades 40 may to a greater extent overlap with the rest of the blades 40. This can help to reduce the extent by which the variable aperture assembly 1 adds to the height of the camera assembly 2 along the central axis 0. Figure 2 illustrates a comparative example of a camera assembly 2 comprising a variable aperture assembly 1 mounted on a lens assembly 50. The variable aperture assembly 1 comprises blades 40 and a variable aperture mechanism 5. Part of the variable aperture mechanism 5 is located above the blades 40. In the example shown in Figure 2, the blades 40 are arranged such that substantially the whole of each blade 40 is located at the same position along the central axis O. As a result, the variable aperture mechanism 5 which is above the blades 40 adds to the height of the camera assembly 2. A comparison 7 between Figure 1 and Figure 2 shows that the arrangement in Figure 1 reduces the overall height of the camera assembly 2. By reducing the height of the camera assembly 2, the thickness of a device (e.g. a handset of a mobile device) comprising the camera assembly 2 may be reduced. The extent by which the camera assembly 2 bumps out from the thickness of the handset may be reduced. As shown from a comparison between Figure 1 and Figure 2, the amount of overlap in the axial direction between the variable aperture assembly 1 and the lens assembly 50 may be increased. Optionally, at least 50%, optionally at least 80% and optionally at least 90% of the variable aperture assembly 1 overlaps with the lens assembly 50 along the central axis O. So, at least 50%, optionally at least 80% and optionally at least 90% of the variable aperture assembly 1 overlaps with the lens assembly 50 when viewed orthogonally to the central axis O. As shown in Figure 1, substantially all of the variable aperture assembly 1 overlaps with the lens assembly 50 along the central axis O. The variable aperture assembly 1 may add substantially no height to the overall camera assembly 2. Optionally, the rotatable part 20 and / or the blades 40 are formed of a relatively light material. For example, the blades 40 may be made from a flexible plastic. By reducing the mass of the moving section of the variable aperture assembly 1, less in stock material may be required. This can help to reduce the overall height of the variable aperture assembly 1. Optionally, the variable aperture assembly 1 is arranged such that as the size of the aperture is changed (e.g. increased or reduced), the blades 40 rub against each other and / or the blades 40 rub against the rotatable part 20 and / or the rotatable part 20 rubs against the base 30. This causes friction. Such friction may be undesirable. However, by allowing such friction to be produced, the axial spaces between the moving parts of the variable aperture assembly 1 may be reduced. This can help to reduce the overall height of the variable aperture assembly 1. Inclined plane / cone As shown in Figure 1, optionally the blades 40 are angled relative to a plane perpendicular to the central axis O such that the radially inner part 41 of the blades 40 is further along the central axis O in the first direction than the radially outer part 42 of the blades 40. In the orientation shown in Figure 1, a plane perpendicular to the central axis 0 is horizontal (with the central axis O shown vertically). As shown in Figure 1, the blades 40 are angled relative to the plane perpendicular to the central axis 0. The blades 40 are arranged such that an angle between the blades 40 and a plane perpendicular to the central axis O is greater than zero and less than 90°. The angle between the blades 40 and a plane perpendicular to the central axis O may be in the range between 0° and 15°, for example between 1° and 10°, preferably between 2° and 8°, optionally about 5°. Figure 1 shows a cross-sectional view of the camera assembly 2. By angling the blades 40, the radially inner part 41 is at a different axial location compared to the radially outer part 42 of the blades 40. Optionally, the blades 40 are inclined relative to a plane perpendicular to the central axis O, or around the circumference of the variable aperture. The blades 40, or at least part of the blades 40, may form a truncated cone shape around the central axis O. The cone shape is truncated such that the aperture is provided, for example, to allow light to reach the lens assembly 50. As shown in Figure 1, optionally the blades 40 are arranged such that the blades 40 are distanced from the lens assembly 50. The variable aperture assembly 1 may be arranged such that when mounted on the lens assembly 50, the blades 40 avoid contact with any lens element (in particular the final lens element) of the lens assembly 50. This can help to reduce any possibility of the blades 40 undesirably damaging the lens assembly 50. This can also help to reduce any undesirable friction between the blades 40 and the lens assembly 50 which could otherwise undesirably affect the variable aperture mechanism 5. As shown in Figure 1, optionally the inclination angle of the blades 40 remains substantially constant from the radially outer part 42 to the radially inner part 41 of the blades 40. This can help to simplify the construction of the variable aperture mechanism 5 driving the blades 40 so as to change the size of the aperture. Alternatively, the inclination angle may vary. For example, the inclination angle may be lower at the radially outer end of the blades 40. This may help to increase the amount of space above the radially outer part 42 of the blades 40 to accommodate the variable aperture mechanism 5. The blades 40 may be arranged in an inclined plane / cone around the front lens element of the lens assembly 50. This allows the variable aperture mechanism features positioned above the blades 40 to be moved downwards in Figure 1, in particular along the central axis O opposite to the first direction. This helps to reduce the height of the camera assembly 2. The blades 40 are positioned at an angle to create a shallow cone around the lens and allow the variable aperture mechanism 5 to be positioned so as not to add much (or optionally any) height to be lens assembly 50. Optionally, the radially inner part 41 of the blades 40 is further along the central axis O in the first direction when the size of the variable aperture is smaller than when the size of the variable aperture is greater. The radially inner part 41 of the blades 40 may move along the central axis O in the first direction as the variable aperture is reduced in size. The angle of inclination of the blades 40 may remain substantially constant throughout the size range of the variable aperture. As the size of the variable aperture is reduced, the radially inner part 41 moves radially inwards. As a result of the angle of inclination, this also causes the radially inner part 41 to move in the first direction along the central axis O. By providing that the angle of inclination remains substantially constant throughout the size range, the variable aperture mechanism 5 may be simplified. However, it is not essential for the radially inner part 41 to move along the central axis 0 as the size of the variable aperture varies. In an alternative embodiment, the radially inner part 41 may remain at the same location along the central axis O throughout the size range of the variable aperture. The angle of inclination of the blades 40 may vary. For example, the angle of inclination may be greater for greater sizes of the variable aperture. As shown in Figure 1, optionally the variable aperture assembly 1 comprises a support surface 6. This support surface 6 is arranged to prevent the blades 40 from moving beyond the support surface 6 in a direction opposite to the first direction. The support surface 6 may be fixed relative to the base 30 or the rotatable part 20. For example, as shown in Figure 1, optionally the support surface 6 is located between the blades 40 and the lens assembly 50. The support surface 6 may be provided so as to reduce the possibility of (or prevent) the blades 40 touching the lens (e.g. the final lens element of the lens assembly 50). This may help to reduce the possibility of the blades 40 undesirably damaging the lens assembly 50. Optionally, friction between the support surface 6 and the blades 40 may constrain movement of the blades 40 when the actuator assembly 10 for driving movement of the blades is not actuated, e.g. is unpowered. Figure 1 shows a cross-sectional view of the camera assembly 2. The support surface 6 may be provided as a truncated cone shape around the central axis O. The support surface 6 may be arranged to guide the shape of the blades 40, particularly as the blades 40 move so as to vary the size of the variable aperture. However, it is not essential for the support surface 6 to be provided. Optionally, the blades 40 are sufficiently stiff and securely fixed relative to the lens assembly 50 such that the possibility of contact between the blades 40 and the lens assembly 50 is sufficiently low. Optionally, the blades 40 are made of a material that is sufficiently unlikely to damage the lens assembly 50 so that some contact between the lens assembly 50 and the blades 40 is acceptable. Flexible blades Figure 3 schematically depicts a camera assembly 2 comprising a variable aperture assembly 1 mounted on a lens assembly 50. As shown in Figure 3, optionally the blades 40 are flexible. The blades 40 may comprise a flexible material. For example, the blades 40 may be made out of, or formed from, a flexible material such as a flexible plastic. During use of the variable aperture assembly 1, the blades 40 are driven (in particular moved) so as to control the size of the variable aperture. By providing that the blades 40 are flexible, the blades 40 may be driven into the front lens element of the lens assembly 50, as shown in Figure 3. This can allow the variable aperture mechanism features positioned above the blades 40 to be moved down (i.e. in the direction opposite to the first direction along the central axis O). This helps to reduce the height of the camera assembly 2. Figure 3 shows the variable aperture assembly 1 with a small aperture size. For a small aperture size, the blades 40 may come into contact with the lens assembly 50. As the blades 40 are moved so as to reduce the size of the aperture, the contact between the blades 40 and the lens assembly 50 causes the blades to bend. As a result, the radially inner part 41 of the blades 40 is moved in the first direction (i.e. upwards in the orientation shown in Figure 3) along the central axis O. This allows the radially outer part 42 of the blades 40 to be located further down, leaving space for at least some of the variable aperture mechanism 5 to be above the blades 40 and overlap with the lens assembly 50. This helps to reduce the overall height of the variable aperture assembly 1. Optionally, friction between the blades 40 and the lens assembly 50 may constrain the blades from moving when the actuator assembly 10 that is used for driving the blades 50 is not actuated, e.g. is unpowered. When the size of the variable aperture is increased from its configuration shown in Figure 3, the blades 40 are retracted away from the central axis O. As the blades 40 are retracted, the radially inner part 41 of the blades 40 may move in a direction opposite to the first direction (i.e. downwards in the orientation shown in Figure 3) along the central axis O. Optionally, for the larger end of the size range of the variable aperture, the blades 40 do not contact the lens assembly 50. For example, for the largest aperture sizes, the blades 40 may be oriented to be substantially planar, i.e. in a plane perpendicular to the central axis O. Hence, it is not essential for the blades 40 to be permanently inclined relative to a plane perpendicular to the central axis O. Instead the flexibility in the material of the blades 40 may be used to accommodate the height difference by simply bending, as illustrated in Figure 3. Optionally, the blades 40 have a thickness (in particular along the central axis 0) of at most 0.5mm, optionally at most 0.2 mm, further optionally at most 0.1 mm, and further optionally at most 0.05 mm. For example, optionally the blades 40 may have a thickness of about 0.04 mm. By providing that the 11 blades 40 are thin, they may be made more flexible which may facilitate bending of the blades 40 as shown in Figure 3. However, thicker blades may be provided, particularly if the material from which the blades 40 are formed is inherently more flexible. The blades 40 may have a varying thickness. For example, the thickness of the radially inner part 41 of the blades 40 may be less than the thickness of the radially outer part 42 of the blades 40. The blades 40 may taper towards the central axis O. This may be particularly useful in the embodiment of Figure 3, because the relatively thinner portion that is proximal to the central axis O may bend on contact with the lens assembly, whereas the relatively thicker portion that is distal to the central axis O may provide structural rigidity to the blades. As shown in Figure 3, optionally the interaction between the variable aperture assembly 1 and the lens assembly 50 allows the overall height of the camera assembly 2 to be reduced without impeding the function of the variable aperture. In particular, the lens assembly 50 may be configured to receive light in a light receiving direction opposite to the first direction. The variable aperture assembly 1 may be mounted on the lens assembly 50 such that, for at least part of the size range, the radially inner part 41 of the blades 40 is further along the central axis O, in the first direction, than the radially outer part 42 of the blades 40. Optionally, the variable aperture assembly 1 is mounted on the lens assembly 50, such that, for the at least part of the size range, the blades 40 bend due to contact with the lens assembly 50 such that the radially inner part 41 of the blades 40 is further along the central axis O, in the first direction, than the radially outer part 42 of the blades 40. As mentioned above, for larger sizes of the aperture, the variable aperture assembly 1 may be arranged such that the blades 40 do not contact the lens assembly 50. The radially inner part 41 of the blades 40 may be further along the central axis O, in the first direction, than the radially outer part 42 of the blades 40 for only part of the size range, e.g. for only the smaller end of the size range. For the larger end of the size range, the radially inner part 41 may be at substantially the same axial location as the radially outer part 42 of the blades. In alternative arrangements, such as shown in Figure 1, the radially inner part 41 may be further along the central axis O in the first direction compared to the radially outer part 42 across substantially the whole of the size range of the variable aperture. Variable aperture mechanism Figure 4 schematically depicts an axial view of a variable aperture assembly 1, i.e. a view along the central axis O. The variable aperture assembly 1 may be the variable aperture assembly 1 shown in Figure 1 or Figure 3, for example. Figure 5 schematically depicts the variable aperture assembly 1 shown in Figure 4 with a larger aperture size. As shown in Figure 4, optionally the variable aperture assembly 1 comprises a base 30 and a rotatable part 20 that is arranged to rotate relative to the base 30 about the central axis 0. Optionally, the blades 40 are coupled to the base 30 and to the rotatable part 20 such that rotation of the rotatable part 20 relative to the base 30 drives the blades 40 so as to change the size of variable aperture. In the specific embodiment of Figures 4 and 5, the blades 40 are coupled to the rotatable part 20 and to the base 30 by a pair of pins, herein referred to as moving pins 21 and pivot pins 31. The pins 21, 31 are arranged in corresponding holes in the blades. Relative movement of the pins 21, 31, for example due to rotation of the rotatable part 20 relative to the base 30, results in movement of the blades 40 and thus adjusting of the size of the variable aperture. In general, any other mechanism for coupling the blades 40 between the base 30 and rotatable part 20 that allows movement on the blades 40 on rotation of the rotatable part 20 may be used. For example, the blades 40 may be coupled to one of the base 30 and rotatable part 20 by a pin-in-slot mechanism (not shown). In particular, as shown in Figure 4, the plurality of blades 40 may be coupled to the base 30 via a plurality of pivot pins 31 and connected to the rotatable part 20 via the plurality of moving pins 21. The blades 40 may be distributed around the central axis O. In alternative embodiments, the plurality of blades 40 may be coupled to the base 30 via the plurality of moving pins 21 (i.e. pins connected to connecting arms 22), and connected to the rotatable part 20 (i.e. pins about which the plurality of blades rotate about) via the plurality of pivot pins 31. As shown in Figure 4, optionally the base 30 is nested within the rotatable part 20. Alternatively, the rotatable part 20 may be nested within the base 30. The moving pins 21 are configured to rotate (e.g. move in a circular arc) around the pivot pins 31 (when the rotatable part 20 is rotated relative to the base 30). As shown in Figure 4, optionally the moving pins 21 are connected to the rotatable part 20 via connecting arms 22, wherein the connecting arms 22 are configured to allow (e.g. deform to allow) rotation of the moving pins 21 around the pivot pins 31. However, in alternative embodiments, the 13 moving pins 21 may be connected to the base 30 via connecting arms 22, wherein the connecting arms 22 are configured to allow (e.g. deform to allow) rotation of the moving pins 21 around the pivot pins 31. Although Figure 4 only show the plurality of blades 40 being connected to the base 30 via the plurality of pivot pins 31 and connected to the rotatable part 20 via the plurality of moving pins 21, in alternative embodiments the plurality of blades 40 could instead be connected to the base 30 via a plurality of moving pins 21 and connected to the rotatable part 20 via the plurality of pivot pins 31. The moving pins 21 are connected to the blades 40 in a manner that prevents relative translational movement between each connected moving pin 21 and blade 40, but allows each connected moving pin 21 and blade 40 to rotate relative to each other, i.e. the moving pins 21 are rotatably held by the blades 40. Additionally or alternatively, the moving pins 21 may be rotatably held by the connecting arms 22. The moving pins 21 and the pivot pins 31 are configured (e.g. are close enough to each other) to provide, per degree of rotation of the rotatable part 20 about the central axis O, at least 5, 10, or 20 degrees of rotation of the blades 40 about the pivot pins 31. A bearing (e.g. a sliding / plain bearing, ball bearing, or a roller bearing) may be provided between the base 30 and the rotatable part 20. The variable aperture assembly 1 may comprise a holding arrangement configured to releasably (i.e. temporarily) hold the rotatable part 20 at one or more positions within the range of positions that the rotatable part 20 is capable of being driven to relative to the base 30 by the actuator assembly 10. The actuator assembly 10 may comprise one or more SMA elements configured to, upon contraction, (directly or indirectly) drive the rotation of the rotatable part 20 relative to the base 30. For example the actuator assembly 10 may comprise (a total of) four SMA elements. The SMA elements may be SMA wires in the arrangement disclosed in WO 2013 / 175197 Al, for example. This particular arrangement of SMA elements may be used to drive rotation of the rotatable part 20 relative to the base 30. Optionally, the SMA elements are configured to, directly or indirectly, drive the rotation of the rotatable part 20 relative to the base 30. The SMA elements may be arranged in a loop at different angular positions around the central axis O. Successive SMA elements around the central axis O may be configured, on contraction, to, indirectly apply a force to the rotatable part 20 in alternate senses around the central axis O. However, in general, the actuator assembly 10 may comprise other arrangements of SMA elements to drive rotation of the rotatable part 20. The actuator assembly 10 may comprise actuator components other than SMA elements, such as piezoelectric actuators or voice coil motors (VCMs). Actuator assembly above blades Figure 6 is a schematic view of a camera assembly 2 comprising a variable aperture assembly 1 mounted on a lens assembly 50. Optionally, the variable aperture assembly 1 comprises an actuator assembly 10. The actuator assembly 10 is configured to drive the blades 40 so as to change the size of the variable aperture. As shown in Figure 6, optionally the actuator assembly 10 comprises at least one SMA element. The SMA element is arranged on contraction to drive the blades 40. For example, in an arrangement the SMA element is an SMA wire. One or more SMA wires may be provided as part of the actuator assembly 10. One or more of the SMA wires may be fixed, directly or indirectly, at one end to the base 30 and at the other end to the rotatable part 20. Drive signals may be provided to the SMA wires to as to control contraction of the SMA wires. As the SMA wires contract, rotation of the rotatable part 20 relative to the base 30 may be controlled. This allows driving of the blades 40 so as to control the size of the aperture to be controlled accurately. As shown in Figure 6, optionally at least part of the actuator assembly 10 is located in the first direction relative to the blades 40. The actuator assembly 10 may be located above the blades 40, in particular above the radially outer part 42 of the blades 40. For example, when the actuator assembly 10 comprises at least one SMA element, then the at least one SMA element 11 may be located in the first direction relative to the blades 40. As shown in Figure 6, the blades 40 may be located in a low position to as to avoid increasing the overall height of the camera assembly 2. This may allow space above the blades 40 for the at least one SMA element 11 of the actuator assembly 10. However, it is not essential for the SMA element 11 to be above the blades 40. In an alternative arrangement, the SMA elements may be located below the blades 40, for example between the blades 40 and the lens assembly 50. As mentioned above, optionally the variable aperture assembly 1 comprises an actuator assembly 10 which may comprise one or more SMA elements. Such SMA elements may be suitable for providing a relative high force for controlling the movements of the variable aperture assembly 1. This can allow the amount of friction between moving parts of the variable aperture assembly 1 to be increased while still allowing for accurate control of the variable aperture size. Blades in multiple layers In some variable aperture assemblies 1, the blades 40 may be arranged in two or more layers that are separated by a gap. One embodiment of this is shown in Figure 7, in which the blades 40 are arranged in two layers that are spaced apart along the central axis. The blades 40 overlap when viewed along the central axis O. For example, when viewed along the central axis, adjacent blades 40 may be arranged in different layers. As such, the blades 40 of different layers need not contact each other, or contact between the blades 40 is reduced compared to a situation in which all blades 40 are provided in a single layer. Friction between the blades 40 is thus reduced or avoided, reducing the force required to move the blades as well as reducing particle generation on driving of the blades. According to the present invention, the blades 50 of at least one of the layers, preferably of all of the layers, overlap the lens assembly 50 when viewed orthogonally to the central axis O. This is schematically shown in Figure 7. The height of the combination of variable aperture assembly 2 and lens assembly 50 may thus be reduced. Furthermore, the blades 40 in a layer that is arranged further along the first direction may be arranged to move radially inward by a larger amount than the blades 40 in a layer that is arranged less further along the first direction. As such, the inner edge of the blades 40 in different layers may effectively be arranged along the surface of the lens assembly 50. In Figure 7, for example, the blades 40 in an upper layer are arranged to move further towards the central axis O that the blades 40 in the lower layer. There is thus provided a variable aperture assembly 2 comprising a plurality of blades 40 arranged to define a variable aperture with a central axis O and a size that can be changed within a size range, wherein the plurality of blades are arranged in at least two layers that are spaced apart along the central axis, and wherein the blades in a layer that is further along the central axis in a first direction are arranged so as to move further towards the central axis 0 than the blades 40 in an adjacent layer. SMA element The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, the actuator assembly 10 may, for example, be a voice coil motor (VCM) actuator assembly or a piezoelectric actuator assembly, instead of a SMA actuator assembly.

Claims

1. A variable aperture assembly comprising:a plurality of blades arranged to define a variable aperture with a central axis and a size that can be changed within a size range;wherein the variable aperture assembly is arranged such that, for at least part of the size range, a radially inner part of the blades is further along the central axis in a first direction parallel to the central axis than a radially outer part of the blades by a distance greater than a thickness of the blades.

2. A variable aperture assembly according to claim 1 wherein the blades are angled relative to a plane perpendicular to the central axis such that the radially inner part of the blades is further along the central axis in the first direction than the radially outer part of the blades.

3. A variable aperture assembly according to claim 1 or 2 wherein the blades form a truncated cone shape around the central axis.

4. A variable aperture assembly according to any preceding claim wherein the radially inner part of the blades is further along the central axis in the first direction when the size of the variable aperture is smaller than when the size of the variable aperture is greater.

5. A variable aperture assembly according to any preceding claim comprising a support surface arranged to prevent the blades from moving beyond the support surface in a direction opposite to the first direction.

6. A variable aperture assembly according to any preceding claim comprising:an actuator assembly configured to drive the blades so as to change the size of the variable aperture, wherein at least part of the actuator assembly is located in the first direction relative to the blades.

7. A variable aperture assembly according to claim 6 wherein the actuator assembly comprises at least one shape memory alloy, SMA, element arranged on contraction to drive the blades, wherein the at least one SMA element is located in the first direction relative to the blades.8.A variable aperture assembly according to any preceding claim wherein the blades are flexible.

9. A variable aperture assembly according to any preceding claim wherein the blades have a thickness of at most 0.3mm, and optionally at most 0.05mm.

10. A variable aperture assembly according to any preceding claim comprising: a base; anda rotatable part arranged to rotate relative to the base about the central axis;wherein the blades are coupled to the base and to the rotatable part such that rotation of the rotatable part relative to the base drives the blades so as to change the size of the variable aperture.

11. A camera assembly comprising:the variable aperture assembly of any preceding claim; anda lens assembly configured to receive light in a light receiving direction opposite to the first direction;wherein the variable aperture assembly is mounted on the lens assembly such that, for the at least part of the size range, the radially inner part of the blades is further along the central axis, in the first direction, than the radially outer part of the blades.

12. A camera assembly according to claim 11, wherein the variable aperture assembly is mounted on the lens assembly such that, for the at least part of the size range, the blades bend due to contact with the lens assembly such that the radially inner part of the blades is further along the central axis, in the first direction, than the radially outer part of the blades.

13. A camera assembly according to any of claims 10-12 wherein at least 50%, optionally at least 80% and optionally at least 90% of the variable aperture assembly overlaps with the lens assembly along the central axis.

14. A camera assembly according to any of claims 10-13, arranged such that contact between the blades and the lens assembly generates frictional forces that constrain size change of the variable aperture at any size within the at least part of the size range when the variable aperture assembly is not actuated.

15. A camera assembly of any of claims 10-14 wherein the lens assembly has an optical axis that coincides with the central axis.

16. A variable aperture assembly comprising:a plurality of blades arranged to define a variable aperture with a central axis and a size that can be changed within a size range,wherein the plurality of blades are arranged in at least two layers that are spaced apart along the central axis, and5 wherein the blades in a layer that is further along the central axis in a first direction are movablefurther towards the central axis than the blades in an adjacent layer.

17. A variable aperture assembly according to claim 16, comprising an actuator assembly configured to drive the blades so as to change the size of the variable aperture, wherein the actuator assembly10 comprises at least one shape memory alloy, SMA, element arranged on actuation to drive the blades.

18. A variable aperture assembly according to claim 16 or 17, comprising:a base; anda rotatable part arranged to rotate relative to the base about the central axis;15 wherein the blades are coupled to the base and to the rotatable part such that rotation of therotatable part relative to the base drives the blades so as to change the size of the variable aperture.

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