DE1020193B - Three-lens apochromatic objective - Google Patents

Description

The invention relates to an improvement of three-lens apochromatic objectives, consisting of two converging lenses enclosing one diffusing one. Apochromatic objectives made up of three lenses have been known for a long time. This is understood to mean lenses that correct for the following image errors are: spherical longitudinal deviation, sine condition, chromatic longitudinal deviation for two colors of a finite Spectral range and also for a third color, the so-called secondary spectrum. Under The theory provides neglect of the thicknesses as conditions for achieving the chromatic correction the following known relationships for the elimination of longitudinal chromatic aberration for two Colours:

V ₁

J ?? _ = 0

V _S

therein mean ψ ^. ■ .φ ₃ the refractive powers of the individual lenses and V ₁ ... v _s the so-called Abbe numbers, ie the relative reciprocal values of the chromatic longitudinal deviation for the individual lenses I, II and III with respect to two colors.

The index 1 and 3 denotes values for the converging lenses I and III, 2 those for the diverging lens II, N the resulting Abbe number of the two converging lenses I and III

N =

Ψι , Ψ3

= Ψι +

the resulting power of these two lenses, then can be written (1)

+ ■

= 0.

For the correction of the secondary spectrum, i.e. the elimination of the longitudinal color deviation for a third one In addition to equations (1) and (4), color must also be fulfilled:

Θ =

Ψι

Ψζ

Here ϋ _ν $ ₂ and # ₃ mean the relative partial dispersions of lenses I, II and III. If one refers to this relative three-lens apochromatisch.es objective

Applicant:
Carl Zeiss, Heidenheim / Brenz

Helmut Knutti, Oberkochen,
has been named as the inventor

Partial dispersions on the color G 'as the third color are defined as follows:

n _e , - n _c njf - n _c

if nc, nj ?, ng are the refractive indices for Fraunhofer lines C, F, G ' . The resulting relative partial dispersion of the two converging lenses I and III is denoted by Θ in (5). A prerequisite for the fulfillment of equation (5) is the requirement that the relative partial dispersions of all three lenses do not have the same linear relationship of the shape

= A + Bv

may suffice, ie for one of the three lenses the relative partial dispersion & as a function of ν must be represented with a different constant than it is valid for the other two lenses.

So-called "short shotguns" have been used in combination with crown glasses to meet this requirement. The refractive indices of all these glasses, which were previously used for apochromats, were below 1.600 and, according to equation (2), resulted in a v-value N that differs only slightly from v _% . As next to equation

(4) now also the "power equation"

+ Ψ2 = ^Φ

(φ = total refractive power required by the lens), the use of the previously common types of glass for apochromats means that the absolute values of the refractive power 9J ₁ ... ψ _{Ά must be} very large compared to Φ . As a result, these apochromatic objectives exhibited a strong Gaussian error (chromatic difference in the spherical aberration) and a strong zone error in the spherical longitudinal aberration. Usable apochromats of this type have therefore only become known for aperture ratios less than 1:10. Investigations of the optical glasses melted in recent years have now shown that particularly

70S 807/130

the more recent "heavy shotguns" with a refractive index greater than 1.61 and a v value .ldein.er than 35 also have ■ & - values whose course deviates from the normal linear function A + B ■ ν of the other optical glasses. The invention uses such glasses in a three-lens apochromatic objective, consisting of two converging lenses which enclose a diffusing lens, in such a way that glasses are used for both the converging lens in the first place in the direction of incidence of light and for the diffusing lens Abbe number of mine than 35 and their relative

Partial dispersions &

ng, - n _c

are greater than 1.61 than can be achieved with the previously known combination was.

In the case of the objectives according to the invention, the greater effective ^ difference results in connection with the high Refractive indices much smaller curvatures. As a result, with the glass combination after Invention achieve a better correction of the zone error and the Gaussian error if in a known manner by corresponding deflections of the individual lenses, the spherical longitudinal deviation and the sine condition Getting corrected. As a result, in contrast to the known lenses, according to the invention, such lenses can also be used produce a larger aperture ratio, namely up to about 1: 3.

Moreover, in most cases it is sufficient if equation (5) is only approximately fulfilled. aside from that must when transitioning to lenses with finite thicknesses Anyway, from equations (1) to (5), a correction generally to be determined empirically is applied in a known manner will.

It is also advantageous for correcting the image errors, and at the same time it serves for further shortening the back focus if, according to an additional idea of the invention, the object-side radius of the front

and that furthermore the diverging lens has a spherical-chromatically converging cemented surface with a difference in the refractive indices of less than 0.05 for the yellow helium line d and against the other converging lens a dispersing cemented surface with a difference in the refractive indices of more than 0, 1 forms for the yellow helium line d .

The inventive choice of the refractive indices for the lenses, such that the converging lens with the higher refractive index in the first place, based on the direction of incidence of light, is arranged, and in that the

Putty surface with the dispersing effect is between 25 lens smaller than the Ö, 7 times the amount of the lens burner included diffusing lens and the image-wide is made.

laterally arranged converging lens, the focal length of the lens can be shortened. In this way the lens has the effect of a telephoto lens, in that the main planes are opposite to the direction of incidence of light relocate. In the opposite case, namely when the converging lens with the higher refractive index in the last place, based on the direction of incidence of light, the main planes would be in the direction be shifted to the focal point of the lens, resulting in a relatively large focal length and thus a relatively large overall length of the telescope in which the lens is used results. These The main reason for the effect would then be because the cement surface with the strongly dispersing effect Located in the front of the lens, giving it the effect of an inverted telephoto lens receives.

The invention thus makes use of the property of a series of so-called>.> Heavy flint,> glasses, which consists in the fact that these have a deviation from the above-mentioned linear function ■ & - A + B · ν . Such a deviation is also present in the above-mentioned »short flinto glasses, but the latter have 9- values that are smaller than corresponds to the general formula A + B ■ ν. As a result, the »Kurzflintc. -Glass used exclusively in the diffusing lens, which was combined with two converging lenses of normal dispersion position. The glasses used in the subject matter of the invention, on the other hand, have 9 values, which are greater than what corresponds to the general formula A + B · ν. According to the invention, such glasses are also used in one of the converging lenses, namely in the first converging lens, calculated in the direction of light, and combined with a further converging lens of normal dispersion position, while for the included diverging lens according to the invention also a high-refraction glass with low dispersion , ie also a so-called heavy flint glass is used. In combination with an ordinary crown lens as the converging lens III, an N can be generated according to equation (2), for which a glass for the diverging lens II can be found, the v _{% of which has} a greater difference compared to N , while observing equation (5) Drawing shows an embodiment according to the invention. The following table shows the associated values for radii, refractive indices and glass thicknesses, based on a focal length of f = 1. However, the invention is of course not limited to this exemplary embodiment.

In the drawing and the following table are indicated

with r the radii of the refracting surfaces, with d the thicknesses of the individual lenses, with s the back focal length from the image-side vertex of the

last lens to the focal plane, with n _c the refractive indices for the C-line, with n _a the refractive indices for the d-line, with np the refractive indices for the F-line, with ng the refractive indices for the G'-line of the spectrum , with v _a the Abbe numbers,
with $ the relative partial dispersion, based on the refractive indices
and G '.

for Fraunhofer lines C, F

n _c Lens I. example Lens III 50 n _d 1.7536 1.5497 np 1.7618 1.5523 55 W <j 1.7823 1.5584 V 1.8007 1.5632 ■ & 26 5 63.5 1.640 1.559 d _x = 0.098 d _a = 0.067 Lens II 1.7473 1.7552 1.7747 1.7920 27.5 1.631 d _ä = 0.023

= 0.48697 τ- ,, = - 0.29427 r _% = + 0.24760 r ₄ = -3.16230 s = +0.8544.

Claims (3)

Claims 1. Three-lens cemented apochromatic objective, consisting of two converging lenses which enclose a diffusing lens, characterized in that for both the converging lens (Li) and the diverging lens (L ₁₁ ) which are in the first place - counted in the direction of incidence of light Glasses are used whose Abbe numbers are less than 35 and whose relative partial dispersions α ng · - ι P - nc _ß Diverging lens (Z _n ) against the converging lens (Zj) a spherical-chromatically converging cement surface with a difference in the refractive indices of less than 0.05 for the yellow helium line d, and against the other converging lens (Z ₁₁₁ ) a dispersing cement surface with a difference the refractive indices of more than 0.1 for the yellow helium line d . 2. Three-lens cemented apochromatic objective according to claim 1, characterized in that the object-side radius of the front lens is selected to be smaller than 0.7 · f , where f means the focal length of the objective. 3. Three-lens cemented apochromatic objective according to claims 1 and 2, characterized in that the surface powers (An / r) by at most ± 0.02 / f and the thicknesses by at most ± 0.02 · f from the following numerical example values to be taken differ: Radii d-i Thick f 1.7618 d Δη / r T ₁ = + 0.48697 * f + 1.5644 / f d ₂ = 0.098 f 1.7552 26.5 ^ ₂ = -0.29427 · / · + 0.0224 / f d _s = 0.023 - f 1.5523 27.5 r _s = + 0.24760. f + 0.8195 // · = 0.067 63.5 r ₄ = - 3.16230 · f + 0.1747 // · 1 sheet of drawings © 709 807/130 11:57