Imaging lens

Abstract

The present invention is an imaging lens in which various aberrations are favorably corrected, the optical length is short, and a sufficient back focus is secured. The imaging lens is constituted by arranging a first lens L1, an aperture diaphragm S1, a second lens L2, and a third lens L3 in succession from the object side to the image side. The first lens L1 is a lens having a positive refractive power and a meniscus shape in which the convex surface faces the object side, the second lens L2 is a lens having a negative refractive power and a meniscus shape in which the convex surface faces the image side, and the third lens L3 is a lens in which the convex surface faces the object side. The imaging lens satisfies the following conditions:

• • 0.35<r₁/r₂<0.45 (1)
  • 0.07<D₂/f<0.1 (2)
  • 0.01<D₄/f<0.04 (3) 1.00<d/f<1.30 (4)
  • 0.3<b_f/f<0.5 (5) where f is the combined focal length of the imaging lens,
  • r₁ is the radius of curvature (axial radius of curvature) of the object-side surface of the first lens L1 in the vicinity of the optical axis,
  • r₂ is the radius of curvature (axial radius of curvature) of the image-side surface of the first lens L1 in the vicinity of the optical axis,
  • D₂ is the distance between the first lens L1 and second lens L2,
  • D₄ is the distance between the second lens L2 and third lens L3,
  • d is the distance (atmospheric) from the object-side surface of the first lens L1 to the imaging surface, and
  • b_f is the distance (atmospheric) from the image-side surface of the third lens to the imaging surface.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to an imaging lens, and more particularly to an imaging lens which is suitable for installation into an image input device of a portable telephone or personal computer, a digital camera, a CCD (charge-coupled device) camera used for monitoring purposes, a surveying device, or similar which uses a CCD or CMOS (complementary metal-oxide semiconductor) as an imaging device. [0003] 2. Description of Related Art [0004] In such an imaging lens, the optical length, which is defined as the distance from the entrance surface on the object side of the imaging lens to the imaging surface (the image-forming surface of a CCD or the like), must be short. In other words, during design of the lens, a method of reducing the ratio of the optical length to the combined focal length of the imaging lens is required. An imaging lens having a short optical length and a small optical length to focal length rat...

AI Technical Summary

Benefits of technology

[0050] Polycarbonate has a higher refractivity and a smaller Abbe number than cycloolefin plastics. Hence, by forming the first lens L1 and third lens L3 from cycloolefin plastics and forming the second lens L2 from polycarbonate, the optical length can be shortened, and the resolution can be further increased.

[0051] Moreover, by constituting the first lens L1 and third lens L3 by lenses formed using cycloolefin plastics, constituting the second lens L2 by a lens formed using polycarbonate, and constituting the third lens L3 by a lens having a negative refractive power and a meniscus shape in which the convex surface faces the object side, the optical length can be shortened more reliably, and an imaging lens having a high resolution can be realized. This will be described in further detail below.

Problems solved by technology

However, the refractive power of these three lenses, which are constituted by first, second, and third lenses arrayed in succession from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens, and hence the distance (optical length) from the surface of the first lens on the object side to the imaging surface is too long.

Further, a diaphragm is disposed on the object-side surface of the first lens, and hence the effective diameter of the third lens cannot be reduced.

As a result, a compact lens cannot be produced.

Hence, although these imaging lenses are set with a short combined imaging lens focal length, the back focus is long, and thus the optical length is too long.

In addition, these lenses use glass materials, and are therefore expensive.

In addition, the lenses use glass materials, and are therefore expensive.

However, the refractive power of the three lenses in this imaging lens, which are arranged as first, second, and third lenses in succession from the object side, is weak in the first lens, weak in the second lens, and positive in the third lens, and hence the refractive power of the first lens and second lens cannot be fully compensated for by the third lens alone.

As a result, the back focus lengthens, causing an increase in the optical length.

Furthermore, the third lens uses a glass material, and hence cost reduction is incomplete.

This is unsuitable for installation in image input devices of portable telephones or personal computers, digital cameras, CCD cameras used for monitoring purposes, surveying devices, and so on.

Since the angle of incidence of the light entering an intermediate position from the peripheral portion of the lens, which forms an important part of an image, deviates greatly from a right angle, the light enters the imaging device in a diagonal direction to the imaging device, thereby increasing the amount of reflection on the entrance surface such that the light reaching a photoelectric conversion surface of the imaging device is low in energy.

As a result, this part of the image becomes dark.

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