JPH0361166B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention has a telephoto ratio smaller than 1 and a half angle of view.
It concerns a wide-angle lens with a five-group configuration that extends to 35 degrees. (Prior art) In order to widen the photographic lens and reduce the telephoto ratio for use in so-called compact cameras, attempts have been made to develop a type of lens that is the opposite of the retrofocus type used as a wide-angle lens. . Among these types of lenses, there are many lenses with a five-group configuration, which have a strongly curved negative meniscus lens with a convex surface facing the image side as the rear group.
No. 56-133711, No. 56-95207, Japanese Patent Application Publication No. 1983-
No. 1117, No. 58-62608, and No. 44016 of the same patent are known, but all of these have an angle of view of 64°.
These are as follows. Furthermore, although the angle of view of JP-A No. 58-219509 reaches 39 degrees, the telephoto ratio is as large as 1.1, which cannot be said to be sufficient for compactness. Furthermore, the strongly curved negative meniscus lens in the rear group is made of resin, and although it has a four-group configuration, Japanese Patent Application Laid-open No. 56-75612, No. 56-94317, and No.
No. 58-57106 can be seen. However, a disadvantage of resin lenses that are easy to process is that the refractive index changes significantly due to temperature changes, which causes back focus fluctuations.Increasing the power of the negative lens in order to shorten the overall length of the lens eliminates this disadvantage. It becomes noticeable. In order to avoid this drawback, there is an example of using a resin lens having no power by making it aspherical, as in Japanese Patent Application Laid-Open No. 59-44016, but this inevitably imposes restrictions on lens design. (Problems to be Solved by the Invention) This invention achieves a half angle of view of 35° by introducing aspherical surfaces into the front and rear groups in a so-called reverse retrofocus type lens, and achieves a telephoto ratio of 35°. but
The aim is to obtain a compact wide-angle lens of 0.96 or less. A secondary objective is to create a lens with an aspherical surface that is made of resin that is easy to process, and which is also extremely susceptible to temperature changes. Structure of the Invention (Means for Solving Problems) In this invention, the structure of the lens system is as follows: in order from the object side, a positive lens L ₁ , a negative lens L ₂ , a positive lens L ₃ , and a positive lens L ₄ and a rear group consisting of _L5 , a negative meniscus lens with a strongly convex surface facing the image side, and at least one of the refractive surfaces of lenses _L3 and _L4 , and a fifth lens.
At least one surface of L ₅ is made aspherical, and 0.45<f/|f ₅ |<2.5, f ₅ <0...(1) 1.7<N ₂ ...(2) However, f ₅ :Fifth lens L ₅ The focal length N ₂ of the second lens L ₂ is characterized by satisfying the condition of the refractive index of the glass material of the second lens L 2 . The number of aspherical surfaces is not limited to one for each of the front group and the rear group, but may be provided for three or more surfaces in total. In this case, the total amount of displacement from the spherical surface of each of the front group and the rear group becomes a problem. This displacement amount is -1.0×10 ^-2 <△X ₅ (0.1) + △X ₇ (0.1) - with respect to the front group.
△X ₆ (0.1)−△X ₈ (0.1)／＜−0.5×10 ^-4 ……(
3) For the rear group, 0.2×10 ^-3 < △X ₉ (0.13) − △X ₁₀ (0.14) / < 0.3 ... (4) However, for the j-th surface satisfies the conditions of Furthermore, when one of the positive lenses L ₃ and L ₄ and the negative meniscus lens L ₅ are resin lenses, the following conditions are satisfied. 0.5＜ _Lp ／｜ ₅ ｜＜5.0……(5) However, _Lp is the focal length (effect) of _L3 or _L4 which is a resin lens.The front group has a positive/negative/positive/positive configuration, and the rear group In a wide-angle lens that is a negative meniscus lens whose lens is strongly curved toward the image side, in order to further reduce the telephoto ratio, it is necessary to increase the positive refractive power of the front group and the negative refractive power of the rear group. However, if the refractive power of the front group is strengthened, spherical aberration will be under-corrected, extroverted coma flare will occur, field curvature will be under-corrected, and distortion will become strong and pincushion-like. Furthermore, if the refractive power of the rear group is strengthened, extroverted coma flare will occur and pincushion distortion will increase. Condition (1) relates to the negative refractive power of the fifth lens _L5 in the rear group, and when the lower limit is exceeded, the movement of the image-side principal point toward the front of the lens becomes small, and the telephoto ratio becomes large.
On the other hand, if the upper limit is exceeded, the curvature of field will be overcorrected.
The occurrence of extroverted coma and pincushion distortion becomes significant. Condition (2) is for keeping the Petzval sum appropriate; in general, compact lenses tend to have too small a Petzval sum. Therefore, by setting the refractive index of the second lens _L2 , which is a negative lens, within the range of this condition, the curvature of field can be kept within an appropriate range. Condition (3) attempts to correct the various aberrations mentioned above by making the positive lenses L ₃ and L ₄ belonging to the front group aspheric. By forming an aspherical surface that is displaced toward the object side with respect to the paraxial radius of curvature surface, there is a tendency to correct the various aberrations occurring in the front group. Furthermore, the image-side surface has an aspherical shape that is displaced toward the image side from the paraxial radius of curvature surface to correct each aberration. Condition (4) relates to the aspherical shape of the negative meniscus lens _L5 in the rear group. This reduces pincushion distortion. Similarly, the above-mentioned aberrations are corrected by displacing the image plane side toward the object side rather than the paraxial radius of curvature surface. As is clear from conditions (3) and (4), the above effects are not required independently for each aspect. When there is only one aspherical surface, the above is the case, but when multiple surfaces are made aspherical, the surface that mainly contributes to aberration correction is displaced as described above, but the surface that contributes to aberration correction in an auxiliary manner is displaced as described above. In many cases, the displacement is the opposite. However, it is sufficient that it falls within the range of conditions (3) and (4) as a whole. The upper limit of condition (3) and the lower limit of condition (4) are the limits at which aberration correction by aspherical deformation is effectively performed, and condition
The lower limit of (3) and the upper limit of condition (4) are conditions to avoid overcorrection. Furthermore, in the lens system of this invention, the positive lens in the front group
One of L ₃ and L ₄ and the negative lens L ₅ of the rear group are resin lenses. As mentioned above, the back focus of resin lenses changes due to changes in refractive index caused by changes in temperature, which has been a problem when used in cameras. On the other hand, by making both the positive and negative lenses made of resin, the effects of temperature changes can be canceled out from each other. In this invention, the positive lens L ₃ ,
Alternatively, L ₄ and negative lens L ₅ may be made of resin, but positive lens L ₃ or L ₄ has smaller power than negative lens L ₅ , but because it is located in front of negative lens L ₅ , negative lens L ₅ , the arrangement is such that it is possible to suppress fluctuations in the back focus due to temperature changes. Condition (5) is a range in which such fluctuations in back focus can be offset. (Example)

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[Table] The values of f/|f ₅ | for each example are as follows. Example 1 1.065 Example 2 1.051 Example 3 1.070 Example 4 1.164 Example 5 1.061 Example 6 0.974 Example 7 0.967 Example 8 1.236 Example 9 1.756 Example 10 1.171 Example 11 1.171 Among the above, Examples In Example 10 and Example 11, both surfaces of the fifth lens _L5 have a gentler curvature off-axis than the paraxial radius of curvature surface. This is the angle of view
This is to flatten coma aberration at angles of 28° or more.
If the side surface of the object is largely displaced, the occurrence of coma aberration can be suppressed, but the curvature of field will be insufficiently corrected. By displacing this so that the curvature of the image side surface becomes gentler off-axis as well, coma aberration also becomes flat.
Field curvature has also been corrected. Effects of the Invention The lens of this invention has a telephoto ratio as small as 0.96 to 0.92 while reaching an angle of view of 35°, and various aberrations are well corrected as shown in each aberration diagram. Although the lens with an aspherical surface is made of resin and is easy to mold, it is remarkable that the influence of temperature can be eliminated by appropriately selecting the refractive power and placement of the resin lens. This has the following effects.

[Brief explanation of drawings]

In FIGS. 1 to 11, a indicates a cross-sectional view of the structure of Examples 1 to 11 of the lens of the invention, and b indicates various aberrations thereof.

Claims (1)

[Claims] 1. In order from the object side, a positive lens L ₁ , a negative lens L ₂ ,
a front group consisting of a positive lens L ₃ and a positive lens L ₄ ;
Negative meniscus lens L ₅ with a strongly convex surface facing the image side
The rear group consists of lenses L ₃ and
At least one of the refractive surfaces of L ₄ , and the fifth
At least one surface of the lens L ₅ is aspherical, and 0.45<f/|f ₅ |<2.5, f ₅ <01.7<N ₂ , and for the front group -1.0×10 ^-2 <△X ₅ (0.1f )＋△X ₇ (0.1f)−△X
₆ (0.1f)−△X ₈ (0.1f)/f<−0.5×10 ^-4 For the rear group, 0.2×10 ^-3 < △X ₃ (0.13f)−△X ₁₀ (0.14f)/f< A wide-angle lens characterized by satisfying the following conditions: f: Focal length of the entire system f ₅ : Fifth lens L ₅ N ₂ : Refractive index of the glass material of the second lens L ₂ △X _j (φ) : The amount of displacement of the refractive surface due to asphericalization at the height φ from the optical axis of the j-th surface, and is expressed by the following formula. In an orthogonal coordinate system (XYZ) with the vertex of the refractive surface as the origin and the optical axis direction as the X axis, the apex curvature is C, the conic constant is k, the aspheric coefficient is Ap _i , and the power of the aspheric surface is p _i time