JPH02226212A - Rear conversion lens - Google Patents

Abstract

PURPOSE:To obtain a lens which is used suitable for a small-sized camera by arranging a positive lens which is convex on the image side, a negative lens which has large curvature on the object side, a biconvex positive lens, and a negative lens which has large curvature on the object side in order from the object side, cementing 3rd and 4th lenses, and satisfying five specific conditions. CONSTITUTION:The rear conversion lens consisting of four elements is arranged at an interval (d) with a main lens I. The astigmatism is held excellently by satisfying an equation I. With inequalities II and III, the refractive index of the positive lens is made small and the refractive index of the negative lens is made large to prevent the Petzval's sum from decreasing and hold it proper. An equation IV is for compensating aberrations generated by the cementing. The inequality V enables the distribution of refracting power for correcting the Petzval's sum properly while holding the spherical aberration and coma aberration excellent. The four lenses are put in a lens barrel from one side and the lens interval, coaxial extent, and eccentricity are held excellent to prevent the performance from deteriorating.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a rear conversion lens suitable for small cameras and the like.

<Conventional technology> Conventionally, a conversion lens is attached to the object side or image side of the main lens system, and an auxiliary lens system is used to change the focal length of the entire system while keeping the focal plane of the entire system constant. Many proposals have been made regarding lenses.

Of these, it is attached to the object side of the main lens system. The so-called front conversion lens has problems in that the conversion lens is large in size and is not easy to attach, resulting in a lack of maneuverability.

On the other hand, the so-called rear conversion lens, which is attached to the image side of the main lens system, is small and can be built into the camera body, and has a small number of elements and can be provided at low cost.

This type of proposal was proposed in Japanese Patent Application Laid-Open No. 59-67511. Tokukai 1986
-179712 etc.

These are composed of 2 to 6 sheets.

However, in all of these examples, the focal length when the rear conversion lens is attached is approximately 1.4 to 2 times the focal length of the main lens.

<Problems to be Solved by the Invention> An object of the present invention is to provide a rear conversion lens in which a lens system is attached to the image side of the main lens system, with a relatively small number of constituent elements, and a focal length of the main lens system when the conversion lens is attached. The object of the present invention is to provide a rear conversion lens that is large, compact, and has high performance with a focal length of 1.7 to 2.5 times, and is advantageous in terms of lens holding and component processing accuracy.

Means for Solving the Problems> In order to achieve the above object, the rear conversion lens according to the present invention extends the main lens system toward the object side and inserts the rear conversion lens on the optical axis to change the focal length to a long focal length. For rear conversion lenses that transition to the side, starting from the object side, a positive lens with convexity toward the image side, a negative lens with the stronger curvature facing the object side, a positive lens with biconvexity, and a negative lens with stronger curvature. There are 4 photos in total, with the side facing the object. The fourth lens is laminated and is configured to satisfy the following conditions.

■ 0.005< (d/F) <0.08■ nl<
1.65 ■ n2>1.75 ■ no T14 >0.05 ■ 0.2 < r s / f c < 1.2 where d: Distance between main lens system and conversion lens F: Focal length of the entire system nl : refractive index of the second lens n2 : refractive index of the second lens n3 : refractive index of the third lens n, : refractive index of the fourth lens fc : focal length of the conversion lens> In the above configuration, equation 0 is astigmatism If the lower limit of this formula is exceeded, the distance between the main lens and the rear conversion lens becomes narrower, making it difficult to attach mechanical parts. On the other hand, if the upper limit is exceeded, it becomes difficult to appropriately correct the Petzval sum. Moreover, if the distance between the main lens and the rear conversion lens becomes large, the rear conversion lens will also inevitably become large, making it impossible to incorporate it compactly into the camera.

In the above structure, formulas (1) and (4) are intended to prevent a decrease in the Petzval sum by making the refractive index of the positive lens relatively small and the refractive index of the negative lens relatively high. If this range is exceeded, it becomes difficult to maintain the Petzval sum at an appropriate value, and good correction becomes impossible.

In the above configuration, Equation 0 is used to satisfactorily correct aberrations caused by bonding.

This shows the difference in refractive index because it is necessary to lower the refractive index of the lens having a positive focal length and to increase the refractive index of the lens having a negative focal length in the bonded lens. Bonding the third lens and the fourth lens together is extremely advantageous in processing the lens holding component.

In the above configuration, Equation 0 makes it possible to distribute refractive power that can appropriately correct the Petzval sum while maintaining good spherical aberration and coma aberration.

If it is less than the lower limit, the curvature of the bonded surface will become small, causing problems in processing. Moreover, when the upper limit is exceeded, it becomes difficult to correct spherical aberration and coma aberration, especially coma aberration of the spherical image surface.

<Example> Hereinafter, an example of the rear conversion lens according to the present invention will be described in detail.

The rear conversion lens according to the present invention extends the main lens system forward, inserts the rear conversion lens on the rear optical axis of the main lens, and shifts the focal length to the long focal length side.

In the conversion lens, from the object side, the first lens has a positive focal length and is convex toward the image side, and the second lens has a negative focal length, with its stronger curvature facing toward the object side.

The third lens is a biconvex lens with a positive focal length, and the fourth lens has a negative focal length with the stronger curvature facing toward the object side.

The third lens and the fourth lens are made up of a total of four lenses.

FIG. 1 is a sectional view showing a first embodiment of a main lens combined with a rear conversion lens according to the present invention.

FIG. 2 is a sectional view showing a first embodiment of the rear conversion lens according to the present invention.

FIG. 3 is a sectional view showing a second embodiment of a main lens combined with a rear conversion lens according to the present invention.

FIG. 4 is a sectional view showing a second embodiment of the rear conversion lens according to the present invention.

In each figure, R1 is the radius of curvature of the lens constituting the main lens, D+ is the interval, Nt is the refractive index, I is the Atsube number, r
I represents the radius of curvature of the lens constituting the rear conversion lens, di represents the distance, and nl represents the refractive index. Also, F is the focal length of the main lens, FNO is the F number, fc
is the focal length of the rear conversion lens, and β is the magnification.

The numerical values of each example are shown below.

(Figure 1) N, 1.7433 ν,, 49.2 First example of main lens R, 9,3270,3,5 R215,017020,8 R, -20,01031,6N, 1.6889R ,9,
847040,7 R5+15.797 052.2 R6-15,797Da 1. O R7■ F=26.8 FNO=3.3 N51.7859 νd331.2 νds 43.9 First example of rear conversion lens (Fig. 2) d
l, 5 d+1.8 620.3 d30.8 d, o, 5 d52.0 d, 0.8 ns 1.5927 νas 35.5n61
．． 7725 νas 49.6 shi,, 36.
6 shi,, 42.8 131.8554 n+ 1.5673 17, 05 10, 01 10, 78 11.19 10, 02 7.06 48, 85 fc=-24,6 β=2.5 Example 2 (Figure 3) R, 9, 9D, 2.4 Il, 1.7433R2
25,94020,8 R, -69,70,1,5'N31.6477R,9,
5D, 1.2 R5+21,95 D52.7 L 1.713
R6-10, 68060, 85N, 1.51R7-2
80,7071,4 ν,, 49.2 shi, 333.8 νd553.9 νd6634 Second embodiment of rear conversion lens (Fig. 4) d
2,3 r, -126, Od+1.8 n, 1.5407
vat 47.2r210.5 d20.6
B rs 10.46 dso, 75 r+++
1.7725 1/d3 49,6r4 726.9
d40.8 rs 41.16 ds2. Onsl, 575
! 'ds 41.5rs 13.86
d60.75 nal, 7725 Va649.
6r7-131.0 fc-50,4β=1.8 <Effects of the Invention> Since the rear conversion lens according to the present invention is configured as described above, the following effects can be obtained.

The air spacing, coaxiality, and eccentricity of the 3rd lens, 2nd lens, and 3rd lens greatly contribute to performance deterioration, but by bonding the 3rd and 4th lenses together and inserting all the lenses into the lens barrel from one side. Prevents performance deterioration.

Also, the aberration caused by bonding the two lenses is F=3
6 FNO=3.6 Compensated by making a difference in refractive index.

Generally, when the main lens system alone performs good aberration correction, the Petzval sum of the entire system becomes small when a conversion lens is attached, making it difficult to maintain good image plane performance.

In particular, the degree of difficulty increases as the magnification increases and the focal length of the entire system increases.

However, in the present invention, the positive and negative lenses are positive and negative, and the 3rd and 4th lenses are bonded together, so it is possible to maintain a good Petzval sum.

A high magnification of about 1.7 to 2.5 times can be obtained.

Fig. 5 is a graph showing the aberration characteristics of the main lens shown in Fig. 1, and Fig. 6 is a graph showing the aberration characteristics when the rear conversion lens shown in Fig. 2 is attached to the main lens shown in Fig. 1. It is a graph showing characteristics.

Fig. 7 is a graph showing the aberration characteristics of the main lens shown in Fig. 3, and Fig. 8 shows the aberrations when the rear conversion lens shown in Fig. 4 is attached to the main lens shown in Fig. 3. It is a graph showing characteristics.

As shown in each graph, good aberration characteristics are obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a first embodiment of a main lens combined with a rear conversion lens according to the present invention. FIG. 2 is a sectional view showing a first embodiment of the rear conversion lens according to the present invention. FIG. 3 is a sectional view showing a second embodiment of a main lens combined with a rear conversion lens according to the present invention. FIG. 4 is a sectional view showing a second embodiment of the rear conversion lens according to the present invention. FIG. 5 is a graph showing the aberration characteristics of the main lens shown in FIG. 1. FIG. 6 is a graph showing aberration characteristics when the rear conversion lens shown in FIG. 2 is attached to the main lens shown in FIG. 1. FIG. 7 is a graph showing the aberration characteristics of the main lens shown in FIG. 3. =12 FIG. 8 is a graph showing aberration characteristics when the rear conversion lens shown in FIG. 4 is attached to the main lens shown in FIG. 3. R1... Radius of curvature DI of the lens constituting the main lens
... Distance N + ... Refractive index νdl ... Atsube number F ... Focal length of main lens FNO ... F number r, ... Radius of curvature di of the lenses constituting the rear conversion lens ... Interval ni...Refractive index fc...Focal length β of rear conversion lens...
·magnification

Claims (1)

[Claims] In a rear conversion lens in which the main lens system is extended toward the object side and a rear conversion lens is inserted on the optical axis to transition the focal length to the long focal length side, the positive lens is sequentially moved from the object side to the image side. The third and fourth lenses are a convex negative lens with the side with the stronger curvature facing the object side, a positive lens with biconvex sides, and a negative lens with the side with the stronger curvature facing the object side. A rear conversion lens that is laminated and meets the following conditions. [1] 0.005<(d/F)<0.08 [2] n_1<1.65 [3] n_2>1.75 [4] |n_3-n_4|>0.05 [5]0. 2<r_6/f_c<1.2 d: Distance between main lens system and conversion lens F: Focal length of main lens n_1: Refractive index of first lens n_2: Refractive index of second lens n_3: Refractive index of third lens n_4: Refractive index of the fourth lens f_c: Focal length of the conversion lens