JPS5832361B2 - Retrofocus lens - Google Patents

Description

[Detailed Description of the Invention] An ultra-wide-angle lens used in a single-lens reflex camera with a flip-up mirror requires a back focal length that is considerably long compared to the focal length, so a diverging lens group,
A so-called retrofocus type lens with a converging lens group placed at the rear is required.

In order to lengthen the back focus of a retrofocus lens and obtain good off-axis coma, astigmatism, distortion, etc., the negative refractive power of the front diverging lens group should be weakened, and the distance from the rear lens group should be increased. Although it is advantageous to increase the diameter of the front lens, it also has the drawbacks of increasing the diameter of the front ball and increasing the overall length.

On the other hand, when increasing the refractive power of the front diverging lens group, it is advantageous to downsize the lens system, but in this case, coma and astigmatism worsen, so a large angle of view or aperture ratio is required. becomes difficult to obtain.

For this reason, it has been extremely difficult to obtain an ultra-wide-angle lens system that is bright, compact, has a long back focus, and has good aberrations.

The present invention solves the coma that occurs when the negative refractive power of the 0 group is strengthened by cementing the (1) group (L4, L5) with the same as the single lens L4 and making the refractive power of the cemented surface portion positive. It can sufficiently correct aberrations, astigmatism, and positive spherical aberration, has a bright aperture ratio of 1:2.8, and has an angle of view of 84.
The object of the present invention is to provide a compact, high-performance ultra-wide-angle objective lens that covers a wide range of angles, has excellent accuracy in various aberrations, has a large back focus of at least 1.55 times the focal length, and is compact and high-performance.

The gist of the present invention will be explained below with reference to illustrated embodiments.

The ultra-wide-angle objective lens according to the present invention has a four-group structure consisting of groups (1) to (3), the first group being a negative meniscus lens L1 with a convex surface facing the object, a positive lens L2, and a negative meniscus lens with a convex surface facing the object. It is composed of L3 and has a divergent effect.

The group (1) is composed of two single lenses L4 and L5 cemented together, and the cemented surface is convex or concave toward the object, so that the refractive power of the cemented surface portion is positive.

The (2) group is composed of a positive biconvex lens L6, and the (2) group is composed of three lenses: a biconcave negative lens L7, a positive meniscus lens L8, and a positive single lens L9.

Here, the total combined focal length is f, and the combined focal length of the I group is f.
I, the radius of curvature of the spherical surface is ri, the lens thickness and air gap on the optical axis are di, and the refractive index of the glass is ni, then the ultra-wide-angle objective lens according to the present invention satisfies the following conditions. This is a characteristic feature.

Next, the effects of each condition will be explained.

The condition is 1 group (L 1 + L 2 + L s )
It is related to the focal length of

In other words, it is advantageous to increase the refractive power of the first group in order to increase the back focus and reduce the diameter of the front lens, but if it is made too strong, negative distortion and coma aberration will increase, and the rear of these aberrations will increase. Since it becomes difficult to correct with the lens system, f<-fI
It is necessary to do so.

If the negative refractive power of the first group becomes weak, it is advantageous for correcting the above-mentioned aberrations, but miniaturization, which is one of the main points of the present invention, becomes impossible, so it is desirable that -fI<2.5f. .

Next, the condition (1) corresponds to making the first surface of the group on the object side concave, and therefore refracts oblique rays that are incident on the peripheral edge of the lens at a large angle of inclination in a direction that reduces the angle of inclination. I can do things.

As a result, the lens diameter of the first group can be reduced, and the occurrence of higher-order aberrations can be reduced.

In other words, when ≧0, it causes an increase in the lens diameter of the 71st group and an accompanying increase in higher-order aberrations, which is not preferable.

In group 2, which is made up of a cemented lens system of lenses L4 and L5, the condition is that the refractive power at the cemented surface is positive, and the negative refractive power that occurs when the negative refractive power of the first group is strengthened for miniaturization. This corresponds to attempting to correct distortion aberration, divergent coma aberration, and positive spherical aberration.

Specifically, the direction of the joint surfaces of L4 and L5 is n 4 <
If n is 5, make it convex toward the object (rB>0),
If n4>n5, it means to make it concave toward the object (rg<0).

If the radius of curvature of the cemented surface is made too small in order to provide a strong refractive power, high-order 5-n4 aberrations will occur, which is undesirable, so it is necessary to set it to 80.5 n5-n4-.

When becomes negative, f
rB It becomes impossible to correct the above-mentioned aberration of the first group, which is not preferable.

These conditions define the upper and lower limits of the optical length from the rearmost surface of the 1st group to the frontmost surface of the 1st group, which makes it possible to flatten the spherical image surface and increase the back focus. can.

If this upper limit is exceeded, it becomes difficult to achieve miniaturization, which is one of the objectives of the present invention, and this is not desirable.

If the lower limit is exceeded, not only will the spherical image plane at the intermediate angle of view be significantly undercorrected, resulting in poor image plane flatness, but also it will not be possible to lengthen the back focus.

Next, examples of the present invention and their respective Seidel coefficients will be shown using specific numerical values.

The Seidel coefficient is the
) and set f = 1.0,
B is a spherical aberration, F is a coma aberration, C is an astigmatism, P is a curvature of the image plane called the Petzval term, E is a coefficient representing distortion, and SUM is the sum of each term.

Since this coefficient is a value for a wide-angle lens, it does not necessarily indicate the essence of the aberration, but it does indicate that each aberration is small.

Example 1 The aberration curves of Examples 1 and 2 are shown in FIGS. 3 and 4.

These figures show that various aberrations are well corrected as an ultra-wide-angle lens with a ratio of 1:2.8 and an angle of view of 84 degrees, and coma aberration is extremely excellent.

[Brief explanation of the drawing]

1 and 2 are sectional views of Examples 1 and 2 of the lens of the present invention, respectively, and FIGS. 3 and 4 are sectional views of Example 1 of the lens of the present invention.
and 2 aberration curves. 1-■...Group, L1-L9...Lens, r1-r1□・°・°...Radius of curvature of spherical surface, d1-d
16...Lens thickness and air spacing.

Claims (1)

[Claims] 1. A negative meniscus lens L1 with a convex surface facing the object, a positive lens L2, and a negative meniscus lens L3 with a convex surface facing the object.
and the 0 group which has a divergent effect, and the two groups L4 and L5.
The first group consists of two single lenses cemented together, and the cemented surface is convex or concave toward the object, and the refractive power at the cemented surface is positive. It consists of a group consisting of three elements: a concave negative lens L7, a positive meniscus lens L8, and a positive single lens L9, and the total composite focal length is f, the composite focal length of the 0 group is fl, and the radius of curvature of the spherical surface A retrofocus type ultra-wide-angle objective lens that satisfies the following conditions: ri is the thickness of the lens on the optical axis and the air gap is di, and the refractive index of the glass is ni.