JPH02162310A - Rear converter lens - Google Patents

Abstract

PURPOSE:To obtain a small-sized rear converter lens whose aberrations is compensated sufficiently by providing negative combined focal length, employing two-group, two-element constitution of positive and negative lens elements in order from the object side, and satisfying specific conditions. CONSTITUTION:This rear converter lens has the negative combined focal length and consists of two elements in two groups, i.e. a 1st positive lens which has a convex surface on the image side and a 2nd negative lens which has a concave surface on the object side in order from the object side. Then inequalities I, II, and III hold, where ni is the refractive index of an (i)th lens from the object side, nui is the Abbe number, and ri is the radius of curvature of the (i)th element. Consequently, this rear converter lens is small and inexpensive and has the aberrations compensated excellently, and the lens is incorporated suitably in a lens shutter camera.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rear converter lens that expands the focal length, and particularly to a small rear converter lens suitable for being built into a lens shutter camera.

(Prior Art) A rear converter lens is a lens that can be used in conjunction with the main lens as a lens with a focal length different from that of the main lens by being mounted on the optical path on the image side of the main lens for photography. It has been used to change the focal length of a photographic lens by converting it into an interchangeable lens. Recently, by simplifying the structure of the lens system and making it more compact, it has become possible to easily expand the photographic range by incorporating it into a lens-shutter camera, and it has become widely used. Converter lenses used for such purposes have a negative focal length and are
Although many products have been commercialized that are composed of about 100 lenses, it is desired to reduce the manufacturing cost by reducing the number of constituent lenses.

An example of a two-element rear converter lens is the lens system described in Japanese Patent Publication No. 61-45207, but since a relatively expensive glass material is used for the negative second lens, the cost is lower. It was hoped that

(Problems to be solved by this invention) This invention has the following problems:
．． The objective is to obtain a rear converter lens that has a magnification of about 5 times, is small enough to be built into a camera, is low in cost, and has various aberrations well corrected.

(Means for Solving the Problems) The rear converter lens of the present invention has a negative composite focal length and includes, in order from the object side, a positive first lens having a convex surface on the image side, and a concave surface on the object side. It is composed of two lenses in two groups of a negative second lens having a surface, where the refractive index and Atsube number of the first lens from the object side are n;, vi, and the radius of curvature of the i-th lens is r+, respectively. ,,65 < n2 < 1.75
... ■１０〈乍2-v〈25
・・ ■0.60 < r, / r2 <
0.95 - Constructed to satisfy the condition (2).

Further, when the focal length of the rear converter lens is fC1 and the distance between the first refractive surfaces from the object side is dl, it is desirable that the following condition be satisfied as a secondary condition.

0.02 <d2/ fC< 0.05 ■0
．． 35 <"1±厖"<-1,2■2-r 1 (Function) In this type of combination lens, aberrations are corrected for each of the main lens and converter lens, so that the aberrations do not worsen as a whole even when combined. It is being done.

The lower limit of condition (2) is a condition for keeping the Petzval sum at a positive or small negative value by using a glass material with a relatively high refractive index for the negative lens.

If this condition is violated, the Petzval sum becomes negative and large,
The sagittal image plane is tilted to the over side.

The upper limit of conditional expression (■) is a condition for suppressing glass material costs.

This negative lens needs to have low dispersion, as shown in conditional expression (■), so if the refractive index increases beyond the upper limit of conditional expression (■), glass materials that meet this condition become expensive, and an inexpensive converter lens is needed. It becomes unsuitable for the purpose of obtaining it.

Condition (2) relates to correction of chromatic aberration, and uses a glass material with large dispersion for the positive lens and a glass material with small dispersion for the negative lens to correct longitudinal chromatic aberration and lateral chromatic aberration. If this lower limit is exceeded, the chromatic aberration of magnification becomes significantly positive.
If the upper limit is exceeded, the axial chromatic aberration becomes significantly undervalued.

Conditional expression (2) is a condition for correcting various aberrations on the opposing surfaces of the positive lens and the negative lens. If the lower limit of this condition is exceeded, the spherical aberration will be greatly exceeded. On the other hand, if the upper limit is exceeded, the spherical aberration will be significantly undervalued, and if this is attempted to be corrected with another refractive surface, the astigmatism will become negative and large.

Conditional expression (2) and conditional expression (2) are necessary conditions regarding correction of coma aberration, astigmatism, etc. when used in combination with a photographic lens having an angle of view of about 60 degrees.

When the lower limit of conditional expression (■) is exceeded, a large coma of extroversion occurs. Outside this upper limit, astigmatism becomes large and lateral chromatic aberration also becomes large. In addition, the lens diameter becomes large, which impairs the functionality of the lens when it is built into a camera. If the lower limit of conditional expression (■) is exceeded, a large coma of extroversion will occur. Further, if this upper limit is exceeded, a large positive astigmatism will occur in this positive lens.

(Example) Examples of the lens of this invention will be shown below.

In addition, each symbol in the table is as follows: r is the radius of curvature of each refractive surface, d is the distance between the refractive surfaces, n is the refractive index of the lens material, y is the Abbe number, f is the focal length of the entire lens system, and 2ω is the The angle of view, F is the F number, and fc is the focal length of the rear converter lens.

In addition, the * mark in the table indicates an aspherical surface. The aspherical shape has an orthogonal coordinate system (xyz
), the apex radius of curvature is r, the conic constant is K, the aspheric coefficient is A, and the aspherical surface number is Pl.

An example of a main lens combined with the rear converter lens of the present invention is as follows.

f: 10 Surface number 6 * Aspheric coefficient 3rd surface k = -1,08850X102 A1 = 7.12519X10-' A2 = -6.07537 X 10-' A3 = 2.4
3738 X 10-”A4=-3,95736X10
-13 4th surface k = 1.17140 A□ = 5.38402 X 10-'A2 = -1.2
0998X10-7 A3= 5.55883 X 10-100 F:4
．． 1 r d 26.065 8.55 84.664 2.28 *-142,5513,71 * 20.002 4.56 86.666 7.70 37.651 2ω :64.8°1.74400 44.7 1.58500 30.0 1.49200 58.3 P work = 4.0000 P2 = 6.0000 P3 = 8.0000 P 4 = 10.0000 P = 4.0000 P2 = 6.0000 8.0000 A4 = -2,45705X10-” P4=1
0.0000 6th surface k: 6.17120 A work = -8,53020X 1O-IIP□ = 4.0
000A2= 9.34451X10-1lP2= 6
．． 0000A3=-1,04179X10-9P3=
8,000OA, = 3.40340X10-”
P4=10.0000 A sectional view of this main lens is shown in FIG. 1, and an aberration diagram thereof is shown in FIG.

Examples 1 and 2 of rear converter lenses that satisfy all of the above-mentioned conditions are shown, and lens cross-sectional views and aberration diagrams are shown for a composite system with the above-mentioned main lens.

Example 1 Focal length of synthesis system f: 153.5 F: 6.42ω
: 4.2.4″ fc knee 1.67.7 surface number r d n**7.95 1 -262.418 3.71 1.64769
33.82 -56.198 4.56 3 -43.631 2.00 1.696
80 55.54 -2183.967 The ** mark in the table indicates the reference distance from the main lens.

A cross-sectional view of the lens of this example is shown in FIG. 3, and an aberration diagram is shown in FIG. 4.

Example 2 Focal length of synthesis system f: 152.2 F: 6.42ω
:43.0° f, knee 168.6 surface number
d n **9.12 1 -348.190 3.71 1.620
04 36.32 -53.212 3.9
93 -42.633 2.00 1.71
300 53.94 -2477.775 A cross-sectional view of the lens of this example is shown in FIG. 5, and an aberration diagram is shown in FIG.

(Effect of the invention) As shown in each example and aberration diagram, this invention (7
) The IJ aconverter lens is small and inexpensive, and its aberrations are sufficiently corrected, making it suitable for being built into a lens sectional view camera.

[Brief explanation of the drawing]

Figures 1 and 2 are a lens cross-sectional view and aberration diagram of an example of the main lens to which the rear converter lens of the present invention is attached, and Figure 3 is a lens cross-sectional view and aberration diagram.
Figures 4 and 4 are lens sectional views and aberration diagrams when the first embodiment of the rear converter lens of the present invention is attached to the main lens shown in Figure 1, and Figures 5 and 6 are when the second embodiment is attached. FIG. 2 is a cross-sectional view of the lens and an aberration diagram when

Claims (1)

[Claims] Two groups of lenses having a negative composite focal length, in order from the object side: a positive first lens having a convex surface on the image side, and a negative second lens having a concave surface on the object side. It consists of two lenses, and the refractive index and Atsube number of the i-th lens from the object side are n_i and ν_i, respectively.
, where the i-th radius of curvature is r_i, the following conditions are satisfied: 1.65<n_2<1.75 10<ν_2−ν_1<25 0.60<|r_3/r_2|<0.95 rear converter lens.