JPH047485B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a wide-angle photographic lens, and particularly to a wide-angle lens using an aspheric surface for small screen sizes. (Prior art) Angle of view57 used as an optical system for relatively small screen sizes such as disk cameras and facsimiles.
Wide-angle lenses with a brightness of about F2.8 and a focal length of about F2.8 have a small focal length, so no consideration was given to making the lens compact, and lenses with large telephoto ratios were often used. Recently, the telephoto ratio has been reduced by making the glass member aspherical for disc camera lenses.
A model that corrected various aberrations appeared. Also, there are some that use plastic lenses that have refractive power. However, making the glass aspherical requires a complicated manufacturing process, and if the plastic lens has refractive power, the optical system will be easily affected by temperature and humidity. A problem arises when an imaging lens is not used. (Purpose of the Invention) This invention consists of a so-called wide-angle telephoto lens system with a small telephoto power, and a plastic lens with a very small refractive power is inserted between the glass lenses, and its surface is made into an aspherical surface. The objective is to obtain a bright wide-angle telephoto lens that is less affected by temperature and humidity, and in which various aberrations are well corrected by the aspheric surface. (Structure of the Invention) The lens system of this invention is shown in FIGS. 1, 2, and 3.
As shown in the figure, the basic configuration is a first lens group that is a convex lens with a strongly convex surface facing the object side, a second lens group that is a concave lens with a strongly concave surface facing the image side, Consists of the third lens group, which is a biconvex lens, and the fourth lens group, which is a negative meniscus lens with a strongly concave surface facing the object side. f: Focal length of the entire system f ₁₂₃ : From the first lens group to the third lens group Synthetic focal length f ₄ : Focal length of the 4th lens group D ₆ : Distance between the 3rd lens group and the 4th lens group 1.3/f<1/f ₁₂₃ <1.6/f ……(1) 1.0 /f<1/|f ₄ |<1.4/f ...(2) 0.20f<D ₆ <0.32f ...(3) A lens system that satisfies the following: between the first lens group and the second lens group, or between the second lens group and the third lens group.
A plastic lens with at least one aspherical surface is inserted between the lens groups or between the third and fourth lens groups, and the focal length f _A of the plastic lens is 1/|f. _A |<0.05/f...(4) This lens system consists of the first, second, and third lenses that make up the front group.
By using a so-called telephoto type lens configuration with the fourth group that makes up the rear group and the fourth group that makes up the rear group, the telephoto ratio can be reduced to approximately
1.2, and various aberrations are kept small with just this basic configuration. The conditions for this are conditions (1) to (3). When the lower limit of condition (1) is exceeded, the telephoto ratio becomes large, the lens system becomes large, and the curvature of field becomes overcorrected. On the other hand, if the upper limit is exceeded, the lens system will become more compact, but spherical aberration will be insufficiently corrected. In addition, strong pincushion distortion occurs. Condition (2) relates to the fourth lens, and when the lower limit is exceeded, the telephoto ratio increases and the curvature of field becomes overcorrected. On the other hand, if the upper limit is exceeded, the curvature of field will be insufficiently corrected, and coma flare and pincushion distortion will occur. If the lower limit of condition (3) is exceeded, the curvature of field will be insufficiently corrected, the telephoto ratio will become large, and the lens system will become large. Conversely, if the upper limit is exceeded, spherical aberration will be insufficiently corrected. Furthermore, it is necessary to increase the outer diameter of the fourth lens, which is disadvantageous in manufacturing. Condition (4) relates to aspherical plastic, which is a feature of this invention. As described above, in the lens system of the present invention, a plastic lens with a weak refractive power is inserted into one of the four lens groups. Plastic lenses are relatively easy to mold into aspherical shapes and have been put into practical use, but they have the disadvantage that they are easily affected by temperature and humidity, causing changes in the refractive index and expansion and contraction of the radius of curvature, making them susceptible to environmental changes. be. In order to avoid this, condition (4) is a condition for weakening the refractive power of the plastic lens to almost eliminate the effects of environmental changes. Making this plastic lens aspherical can be used to correct various aberrations, but in the wide-angle lens of the type of this invention, it is most effective to use it to reduce coma flare at off-axis image points. Embodiment 1 shows an example in which a plastic lens is inserted between the first lens group and the second lens group.

【table】

[Table] In this example, the second lens group is a meniscus lens with weak negative refractive power, so even if the radius of curvature of the positive lenses in the first and third lens groups is somewhat large, f ₁₂₃ can be achieved. It can be made small. However, this alone results in insufficient correction of various aberrations, so the aspheric surface is designed to function as a negative lens. Aspherical shape When However, C ^* =C+2A (pi=2) represents a reference spherical surface, and k is a quadratic surface coefficient. When the plastic lens is located between the first lens group and the second lens group as in this example, it is desirable that △X〓=X〓−X〓 be in the following range: 0.2×10 ^-4 <|ΔX(0.1f)|/f<1.0×10 ^-4 ...(5) If the lower limit of this condition is exceeded, spherical aberration will be undercorrected and overcoma flare will occur. Conversely, when the upper limit is exceeded, undercoma flare occurs and spherical aberration becomes overcorrected. An example in which an aspherical plastic lens is inserted between the second lens group and the third lens group is shown as a second embodiment.

[Table] In this case, △X is preferably within the following range: 0.1×10 ^-3 <|△X (0.15f) |/f<0.3×10 ^-3 ...(6) If the upper limit is exceeded, undercoma flare occurs. increases and exceeds the lower limit, overcoma flare becomes noticeable. A third embodiment shows an example in which an aspherical plastic lens is inserted between the third lens group and the fourth lens group.

[Table] The aspheric surface in this case has a shape in which the radius of curvature becomes smaller off-axis, and △X is preferably in the following range: 0.1×10 ^-3 <|△X (0.15f) |/f<0.4 ×10 ^-3 ...(7) When the upper limit is exceeded, overcoma flare becomes noticeable.
Conversely, if the lower limit is exceeded, undercoma flare occurs. The reason for the absolute values in the conditional expressions (5), (6), and (7) above is that the signs will be reversed when the aspheric surface of the plastic lens is placed on the opposite side of each example. (Effects of the Invention) Although the present invention is a bright wide-angle lens as described above, the telephoto ratio is as small as about 1.2, and various aberrations are corrected in a well-balanced manner as shown in the aberration diagram. Furthermore, since the aspherical surface is a plastic lens surface with almost no refractive power, it is not only easy to mold, but also has the effect of being extremely little influenced by the environment such as temperature and humidity.

[Brief explanation of drawings]

1, 2, and 3 are cross-sectional views of lenses of the first, second, and third embodiments of the present invention, respectively.
Figures 5 and 6 are the first, second, and third figures, respectively.
It is an aberration diagram of an example.

Claims (1)

[Claims] 1. A first lens group that is a convex lens with a strongly convex surface facing the object side from the object side, a second lens group that is a concave lens with a strongly concave surface facing the image side, and a third lens group that is a biconvex lens. and a fourth lens group which is a negative meniscus lens with a strongly concave surface facing the object side, f: focal length of the entire system f ₁₂₃ : composite focal length from the first lens group to the third lens group f ₄ : th Focal length of 4 lens groups D ₆ : Distance between 3rd lens group and 4th lens group 1.3/f <1/f ₁₂₃ <1.6/f 1.0/f <1/|f ₄ | <1.4/f A lens system that satisfies 0.20f < D ₆ < 0.32f, and one of the lens groups of the above lens system, that is, between the first lens group and the second lens group, or between the second lens group and Third
A plastic lens with at least one aspherical surface is inserted between the lens groups or between the third and fourth lens groups, and the focal length f _A of the plastic lens is 1/|f. _A | A wide-angle lens characterized by <0.05/f.