JPH0412444B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has an angle of view of 90° to 100° and an F number of 2.8.
The present invention relates to a wide-angle lens with a long back focus, especially a wide-angle lens used underwater. Conventionally, underwater wide-angle lenses that reach an angle of view of 90° to 100° and an F number of 2.8 are known to use a symmetrical lens with concave and convex convexities as the master lens, but the back focus is around 0.55f (f: (focal length of the entire system), which has caused problems when a long backfocus is required to install a photometry mechanism or a quick return mirror in a single-lens reflex camera between the lens and the image plane. Furthermore, as a general aerial photography lens with an angle of view exceeding 90°, a lens using a retrofocus type is known, and this is suitable for the purpose of elongating the back focus. However, when used underwater, the refractive index of the physical world changes, and the refraction of light rays entering the lens from water changes, resulting in fluctuations in aberrations, making it impossible to obtain sufficient performance. Another known method is to attach a concentric spherical window centered at the entrance pupil position on the front surface of a lens for aerial photography, but this method has the advantage of not changing the angle of view and causing no chromatic aberration of magnification. , as shown in FIG. 1, since the surface 1a of the spherical window 1 in contact with the air has a diverging effect, the object plane becomes a spherical virtual image P with the concave surface facing the lens 2, and the image Q formed by the lens 2
As shown in the figure, the lens has a positive field curvature aberration as a whole. Furthermore, it is desirable to make the window smaller in order to increase water pressure resistance and make the lens more compact, but the smaller the radius of curvature of the spherical window, the stronger the curvature of field aberration occurs, and the angle of view is wide-angle exceeding 90°. The lens deteriorates the image in the periphery, making it unusable. An object of the present invention is to provide an underwater wide-angle lens with a back focus of 1.4 f or more, an angle of view of 90° to 100°, and a well-balanced aberration that reaches an F number of 2.8. In consideration of the above-mentioned difficulties, the present invention is capable of sufficiently correcting each aberration including the first component lens that comes into contact with water under various conditions. Specifically, from the object side, the first component L ₁ has a convex surface facing the object side, the second component L 2 has a positive meniscus lens with a convex surface facing the object side, and the second component L ₂ has a convex surface facing the object side. The third component L ₃ of the negative meniscus lens, the fourth component L ₄ of the negative meniscus lens with the convex surface facing the object side, the fifth component L ₅ of the positive lens, the sixth component L ₆ of the biconvex positive lens, the object side a seventh component L ₇ of a negative lens having a concave surface on the object side, an eighth component L ₈ of a positive meniscus lens with a concave surface facing the object side, and a ninth component L ₉ of a positive lens having a convex surface facing the image side; The radius of curvature of the image-side lens surface of the first component, third component, and fourth component is r ₂ , r ₆ , r ₈ , respectively.
The radii of curvature of the object-side and image-side lens surfaces of the sixth component are r ₁₁ and r ₁₂ , respectively, the focal lengths of the third and fourth components are f ₃ and f ₄ , and the first
When the axial distance from the image-side lens surface of the component to the object-side lens surface of the fifth component is D, the wide-angle lens is characterized by satisfying the following conditions. (1) 0.6＜f ₃ ／f ₄ ＜1.7 (2) r ₆ ＞r ₈ (3) r ₁₁ ＞1.5｜r ₁₂ ｜ (4) 0.6＜r ₂ ／D＜1.2 Each condition will be explained in more detail below. state Condition (1) defines the ratio of the focal lengths of the lenses of the third component _L3 and the fourth component _L4 . Each lens of the third component L ₃ and the fourth component L ₄ has the function of gradually diverging the light rays incident parallel to the optical axis and lengthening the back focus. Therefore, negative distortion and overcorrected spherical aberration tend to remain in the entire optical system. In order to achieve good aberration correction, it is best to suppress this tendency as much as possible, and the third component L ₃ and the fourth component L ₄ can be adjusted without placing too much load on either the third component L 3 or the fourth component L ₄ . Fourth
It is desirable to keep the ratio of the focal lengths of component _L4 close to 1. If condition (1) exceeds the upper limit and the refractive power (reciprocal of the focal length) of the third component _L3 of the lens becomes relatively small, the effect of lengthening the back focus of the lens becomes weaker and becomes necessary. I can't get back focus. In addition, since the light beam incident parallel to the optical axis is diverged by the third component _L3 , the fourth component _L4 is incident at a height further away from the optical axis.
Since the refractive power of the fourth component _L4 is relatively large, it is more strongly influenced by the divergence caused by the fourth component _L4 , and spherical aberration occurs in an excessive manner and cannot be corrected. on the other hand,
If the lower limit of condition (1) is exceeded, it is advantageous to obtain a back focus, but the third component L ₃ of the oblique ray
Since the incident position is farther from the optical axis than that of the fourth component _L4 , the negative distortion due to the relatively stronger refractive power of the third component _L3 increases, It becomes impossible to correct it. Condition (2) is a conditional expression that satisfactorily corrects the field curvature aberration of the optical system by distributing the divergence effect of the oblique ray to the third component L ₃ and the fourth component L ₄ .
Since both components have a strong divergent effect, they generally tend to cause positive field curvature aberration in optical systems, but the condition (2) of r ₆ > r ₈ suppresses the divergent effect. It has become possible to correct the field curvature aberration of the optical system in a well-balanced manner by having the third component _L3 and the fourth component _L4 share the same function. If condition (2) does not hold and the radius of curvature r ₆ of the image-side lens surface of the third component L ₃ becomes relatively small, the divergence effect on this surface becomes excessive and the curvature of field aberration is overcorrected. Therefore, it cannot be corrected. Condition (3) is a conditional expression for correcting coma aberration. The oblique rays are subjected to a diverging effect by the third component L ₃ and the fourth component L ₄ , but according to condition (2), the lens component L ₃
Even if L 4 and L ₄ share the diverging effect, coma aberration occurs because the lower side of the oblique light beam (the ray passing below the center of the aperture) receives a particularly strong diverging effect.
Condition (3) provides a strong convergence effect to the sixth component L ₆ by limiting the radius of curvature of the sixth component L ₆ of the positive lens, thereby correcting coma aberration.
If condition (3) is not satisfied and the radius of curvature |r ₁₂ | of the image-side lens surface becomes relatively large, the convergence effect on the lower side of the oblique light beam on this surface becomes weaker, and the third
The divergence effect by the fourth components L ₃ and L ₄ becomes relatively strong, and comatic aberration cannot be corrected. Condition (4) defines the radius of curvature of the image-side lens surface of the first component L ₁ with respect to the sum of the lens surface spacings d ₂ to d ₃ regarding the lens components L ₁ to L ₄ that form a divergent group as a whole. It is. When the value of the radius of curvature r ₂ of the image side lens surface of the first component L ₁ is a relatively large value, the divergence effect on this surface becomes small for oblique rays, and this occurs in the entire optical system. This is advantageous for correcting the positive field curvature aberration that tends to occur, but the back focus becomes shorter, and the diverging effect of the third and fourth components L ₃ and L ₄ becomes stronger in order to extend it to the required value. need arises, and higher-order aberrations are more likely to occur. If condition (4) exceeds the upper limit, the third and fourth
Strong coma aberration occurs over the entire screen due to the strong diverging effect of components L ₃ and L ₄ , but coma aberration around the screen can be corrected by strengthening the effect of the converging surface within the lens system. However, coma aberration at intermediate angles of view cannot be corrected in a well-balanced manner due to the effects of higher-order aberrations. If the radius of curvature r ₂ of the image-side lens surface of the first component L ₁ is relatively small, it is advantageous for increasing the back focus, but the divergence effect of oblique rays becomes stronger on this surface. Therefore, positive field curvature aberration increases. If condition (4) exceeds the lower limit, positive field curvature aberration will occur, but the field curvature will be particularly noticeable at the periphery of the screen, and the field curvature aberration at intermediate field angles will need to be corrected using other methods. Even if it were possible to do so, positive field curvature aberration remains at the periphery of the screen and cannot be corrected satisfactorily. The grounds for conditions (1) to (4) have been described above, but in the wide-angle lens according to the present invention, the conditions for maintaining good coma aberration, especially coma aberration in the upper part of the oblique ray bundle (rays passing above the center of the aperture), are as follows: When the focal length of the seventh component L ₇ is f ₇ and the radius of curvature of its image side lens surface is r ₁₄ , it is desirable to satisfy −5.0<r ₁₄ /f ₇ <−1.5. Especially the upper side of the oblique light beam is the eighth,
The ninth component L ₈ and L ₉ are converging, but the image-side surface r ₁₄ of the seventh component L ₇ has a diverging effect, especially on the upper side of the oblique light beam, and coma aberration. is corrected in a well-balanced manner. If the upper limit of this condition is exceeded, the divergence effect becomes strong, and if the lower limit is exceeded, the divergence effect becomes too weak, and in either case, coma aberration cannot be corrected well. Next, embodiments according to the present invention will be described. In general, underwater lenses do not use a system that extends the entire lens because it is advantageous in terms of waterproof and water pressure resistant structure, but the distance is adjusted by fixing the front group lens. In the first embodiment shown in FIG. 2, the first component _L1 is fixed, and the second to ninth components _L2 to _L9 can be fed out as one unit. In this case, the lens component
By extending L ₂ to _{L 9} , the entrance pupil position approaches the first component L ₁ , and the incident height of the oblique ray approaches the optical axis, so that the curvature of field aberration is directed in a negative direction. In general, retrofocus type lenses have a strong tendency for positive field curvature to occur at short distances, and this is the first step in correcting this positive field curvature aberration.
It is advantageous to keep the component L ₁ fixed and to feed out the lens components L ₂ to L ₉ as one unit. The lens configurations of the second and third embodiments of the present invention are shown in FIGS. 3 and 4, respectively. In both embodiments, the first component _L1 to the third component _L3 are fixed front groups, After the fourth component, L ₄ to _{L 9} are integrated into a rear group, and by extending this, it is possible to perform close-range photography. The entrance pupil position is the third component L ₃
By approaching , it also has the function of correcting fluctuations in field curvature aberration, as described in the first embodiment. In the first embodiment, as shown in FIG. 2, the fifth component L5 of the positive lens is composed of a negative meniscus lens with its convex surface facing the object side and a biconvex lens. In the second embodiment, as shown in FIG.
The fifth component _L5 of the positive lens is composed of a combination of three lenses: a biconvex lens, a biconcave lens, and a biconvex lens, and the seventh component _L7 of the negative lens is composed of a positive meniscus lens with its convex surface facing the image side, a biconcave lens, and a biconcave lens. It is made up of a combination of . In the third embodiment, as shown in FIG. 4, the second component _L2 of the positive meniscus lens is a combination of a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens with a convex surface facing the object side. The fifth component _L5 of the positive lens is composed of a negative meniscus lens and a biconvex lens laminated together as in the first embodiment, and the seventh component _L7 of the negative lens is the same as in the second embodiment. Similarly, it is constructed by laminating a positive meniscus lens and a biconcave lens. The specifications of each example are shown below. However, r ₁ , r ₂ ,
r ₃ , ... are the radius of curvature of each lens surface sequentially from the object side, d ₁ , d ₂ , d ₃ , ... are the center thickness and spacing of each lens, n ₁ , n ₂ , n ₃ ... and ν ₁ , ν ₂ , ν ₃ , . . . each represents the refractive index and Abbe number of each lens with respect to the d-line λ=587.6 nm).

[Table] ⁱ⁼²

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[Table] ⁱ⁼²

【table】

[Table] ⁱ⁼²
Various aberration diagrams of the above embodiments are shown in FIG. 5 (first embodiment), FIG. 6 (second embodiment), and FIG. 7 (third embodiment), respectively. From the aberration diagrams, it is clear that all examples have an angle of view of 96°, an F number of 2.8, a sufficiently long back focus, and excellent imaging performance. In addition, the above-mentioned 1st - 3rd Examples were entered into the data table as a lens only for underwater. Although aberrations are corrected for the values of n ₀ and ν ₀ , it can be used not only in fresh water but also in seawater, alcohol, and ether. As long as the refractive index n ₀ of the object space is in the range of 1.2<n ₀ <1.4, each embodiment can be put to practical use as is. As described above, according to the present invention, the back focus is 1.4 f or more, the angle of view is 90° to 100°, and the F number is
It is possible to obtain an underwater wide-angle lens with a good aberration balance of 2.8.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram for explaining the present invention, Figure 2 is a conceptual diagram for explaining the present invention.
, 3 and 4 are the first to third figures of the present invention, respectively.
5, 6, and 7 are aberration diagrams showing various aberrations of the first to third embodiments, respectively. [Explanation of symbols of main parts], L ₁ to _{L 9} ... lens component, f ₃ , f ₄ ... focal length of lens component, r ₂ , r _5
, r ₈ , r ₁₁ , r ₁₂ ... radius of curvature of lens surface, D...
On-axis distance.

Claims (1)

[Claims] 1. In order from the object side, a first component L ₁ whose image side surface is convex toward the object side, a second component L ₂ of a positive meniscus lens whose convex surface is directed toward the object side, and a convex surface toward the object side. The third component L ₃ of a negative meniscus lens with its convex surface facing the object side, and the fourth component of a negative meniscus lens with its convex surface facing the object side.
L ₄ , the fifth component L ₅ of a positive lens, the sixth component L ₆ of a biconvex positive lens, the seventh component L 7 of a negative lens with a concave surface facing the object side, and the seventh component L ₇ of a positive meniscus lens with a concave surface facing the object side. It has 8 components L ₈ and a 9th component L ₉ of a positive lens having a convex surface on the image side, and the radius of curvature of the image side lens surface of the first component, third component, and fourth component is r ₂ and r, respectively. ₆ , _r8 ,
The radii of curvature of the object-side and image-side lens surfaces of the sixth component are r ₁₁ and r ₁₂ , respectively, the focal lengths of the third and fourth components are f ₃ and f ₄ , and the first
A wide-angle lens that satisfies the following conditions, where D is the axial distance from the image-side lens surface of the component to the object-side lens surface of the fifth component. (1) 0.6＜ _f3 ／ _f4 ＜1.7 (2) _r6 ＞ _r8 (3) _r11 ＞1.5｜ _r12 ｜(4) 0.6＜r2／ _D＜ 1.2