Improvements in Photographic Objectives.
Year: 1896

page_text: Date of Application, 4th Dec., 1896
Complete Specification Left, 5th Apr., 1897-Accepted, 5th June, 1897
PROVISIONAL SPECIFICATION.
Improvements in Photographic Objectives.
I, PAUL RUDOLPH of Jena in the Grand Duchy of Saxe-Weimar, in the
German Empire, Doctor of Philosophy, do hereby declare the nature of this
invention to be as follows:-
The object of my invention is to provide a photographic objective which, while
having a great intensity of light and over a wide field of view being well corrected
as to astigmatism (that is to say producing an anastigmatically levelled image)
meets even exceptional requirements, as regards chromatic and spherical correction.
This object is attained by rendering the principle of correction embodied in Gauss'
telescope-objective, serviceable for the purposes of the present invention.
As is well known Gauss has demonstrated that a double lens objective enables
both chromatic and spherical aberration to be corrected with great accuracy for a
comparatively large aperture, if such compound object lens consists of a convex
crown glass meniscus and a convexo-concave flint glass lens, the two surfaces
whereof which are turned towards each other are separated by air, differing
materially in their curvatures, so that they may not be cemented together. Unlike
the types of object glasses of Fraunhofer, Littrow and others, in which the faces,
turned towards each other, have practically equal curvatures and are generally
cemented together, the type of objective devised by Gauss is liable to undergo on
the basis of a proper distribution or arrangement of the curvatures a correction of
the spherical aberration for two different colours; so that the so-called "chromatic"
difference of the spherical aberration is done away with and both aberrations are
effaced over an area corresponding to a large aperture. Objectives of this
description have been repeatedly and successfully applied to astronomical telescopes,
both the arrangements of lenses that are practicable, viz. crown first and flint first,
having been tried. But before the present invention no advantageous application
of this type of compound lens to photographic objectives has to my knowledge
been either made or deemed feasible. Such application is attained in the first place
by imparting to Gauss' objective tbe property of producing an anastigmatically
levelled image-which is so desirable for photographic lens systems but has hitherto
not been realized in the said objective-and secondly by achromatizing such
anastigmatic objective in a novel manner satisfactory for photographic purposes.
In attempting to attain this result considerable difficulty has been experienced
owing to the narrow range of suitable sorts of glass. It has been found that in
an objective of great aperture a perfect anastigmatic level of the image may be.
secured only with lenses either of considerable thickness or placed at a great
distance apart.
Furthermore a sufficient chromatic correction-while being
attainable with any given pair of crown and flint glass in the astronomical objective,
which needs not produce an anastigmatically levelled image, and which has hitherto
been constructed with lenses of but little thickness and placed at a short distance
from each other-in an anastigmatic objective of a comparatively large aperture
may be ensured only where the dispersive power of the flint glass (as expressed by
be formula) has a comparatively great value, viz. ; about double the value
of the dispersive power of the crown glass. Such a pair of lenses, however is:

page_text: unsuited for photographic purposes, inasmuch as it would have to. comprise a very
heavy flint glass.
According to the present invention this drawback is obviated in the following
manner. In the lens system either one lens or the other lens, or both, consist each
of a positive (biconvex) lens and of a negative (biconcave) lens cemented to the
first and formed of glass having the same or nearly the same refractive power as
the glass of the positive component but different dispersive power.
A lens so composed will, as regards all the effects dependent upon refraction
(viz. focal length, the location of the cardinal points and spherical aberration of
every description) be exactly identical with a single homogeneous lens of like
external curvatures and equal thickness while as regards chromatic dispersion, the
compound lens is equivalent to a single lens with like curvatures and of a like
thickness but made of glass having a dispersive power different from those which
actually occur in either part of the combination. In particulars the resulting
dispersive power of the compound lens is less than the least dispersive power of
the component lenses-that is to say, the compound lens is achromatic or partially
achromatic-if in the case of the compound lens being a positive lens the positive
component and in the case of it being a negative lens, the negative component
consists of glass of less dispersive power; and the resulting dispersive power of
the compound lens is greater than that of either component-that is to say the
compound lens is hyperchromatic-if in the case of the compound lens being a
negative lens the positive component is formed of glass of less dispersive power
and the negative component of glass of greater dispersive power and the negative
component of glass of greater dispersive power. The former fact agrees with the
well-known rules of achromatization of leuses and the latter with the demonstrations
respecting hyperchromatic dispersing lenses, contained in the Specification of my
Patent No. 12219 of 1896. The value of the resulting dispersive power of the
compound lens in all these cases depends upon the values of the dispersive power
of the glasses employed in forming the two components and upon the ratio of the
total curvatures or of the focal lengths of both components; it depends
consequently, if the outer curvatures of the compound lens are given, upon the
radius of curvature commion to the inner surfaces united by cement. Such value
may therefore be calculated in each individual case by means of well-known
formulæ.
By the stated manner of composing the lenses of a lens system of the Gauss
type every required ratio of their dispersive powers may be realized, without
causing the lenses to differ from non-composite lenses of glass of the same thickness
and outer curvatures, as regards the effect of refraction and the spherical
aberrations by which refraction is attended. The outer curvatures and thicknesses
of the lenses A and B may therefore be determined beforehand, regardless of the
dispersive powers of the particular kinds of glass to be employed, but in view to
produce the correction of spherical aberration and to anastigmatically level the
field of the image; and subsequently, by properly determining the inner radius
of curvature of the components to be cemented of lens A or of lens B or of both
lenses, the ratio of the resulting dispersing powers of the lenses A and B, which.
is requisite for the proper correction of chromatic aberrations, may be easily
established. In the practice of this method values of the said ratio result, which
in the case of non-composite lenses, could only be obtained by employing a flint
glass of a very high dispersive power, but which with the lenses constructed as
above explained are forwarded by crown and flint glasses, exhibiting but slightly
different dispersive powers, provided such glasses possess an equal or approximately
equal refractive power.
Pairs of glasses, which meet these requirements, in sufficient range of gradations
form part of the kinds of optical glass, which are constantly for sale at the Jena
Glass Works. The condition of identical refractive power needs only, of course,
be fulfilled approximately, inasmuch as slight differences in the indices of refraction
between the cemented components, even where the curvature of the inner surface

page_text: is very marked, influence the corrections but to an extent, which is comparatively
insignificant and may be easily introduced into the computations. Indeed, a little
difference in the refractive power, say a few units in the third place after the
decimal point, may even prove advantageous, since, owing to such a difference, a
more perfect equalization of the so-called "zones" of the spherical aberration may
become feasible.
Apart from this, no essential variation, or departure from principle, is involved
in the construction of object pieces of the new type, whenever some medium other
than air, be interposed between the constituent parts of such object piece; the one
important condition being, that such separating (or spacing) medium should possess
an uncommonly small exponent index of refraction, in comparison with the
exponents of the different kinds of glass employed.
The objective forwarding an anastigmatically levelled image as herein described
should, preferably, be corrected by placing the diaphragm in front of the objective.
But it will still display a noticeable "coma" and, like any an astigmatic objective
with the diaphragm in front of it, it will be attended with the defect of orthoscopic
distortion of the image. These two drawbacks may without difficulty be obviated,
however, by constructing a double object glass (with a central or intermediate
diaphragm), the objective herein before described being utilized as one of the
constituent elements thereof.
Dated this 4th day of December 1896.
JOHNSONS & WILLCOX,
47 Lincoln's Inn Fields, London, W.C., Agents.
I, PAUL RUDOLPH of Jena in the Grand-Duchy of Saxe-Weimar, in the
German Empire, Doctor of Philosophy, do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
The object of my invention is to provide a photographic objective which, while
having a great intensity of light and over a wide field of view being well corrected
as to astigmatism (that is to say producing an anastigmatically flattened image),
meets even exceptional requirements, as regards chromatic and spherical correction.
This object is attained by rendering the principle of correction embodied in Gauss'
telescope-objective, serviceable for the purposes of the present invention.
Fig. 1 represents the telescope-objective of Gauss,
Fig. 2 represents an objective constructed according to the present invention.
Fig. 3 represents a doublet-objective composed of two object lenses of the
improved kind.
As is well known Gauss has demonstrated that a double leus objective enables
both chromatic and spherical aberration to be corrected with great accuracy for a
comparatively large aperture, if such compound object lens be constructed as
illustrated in the section Figure 1 of the accompanying drawing, that is if it
consists of a convex crown glass meniscus A and a convexo-concave flint glass
lens B, the two surfaces whereof which are turned towards each other are separated
by air differing materially in their curvatures, so that they may not be cemented
together. Unlike the types of object glasses of Fraunhofer, Littrow and others,
in which the faces, turned towards each other, have practically equal curvatures
and are generally cemented together, the type of objective devised by Gauss is
liable to undergo on the basis of a proper distribution or arrangement of the

page_text: curvatures a correction of the spherical aberration for two different colours; so
that the so-called "chromatic" difference of the spherical aberration is done away
with and both aberrations are effaced over an area corresponding to a large aperture.
Objectives of this description have been repeatedly and successfully applied to
astronomical telescopes, both the arrangements of lenses, that are practicable viz.
crown first and flint first, having been tried. But before the present invention not
advantageous application of this type of compound lens to photographic objectives
has to my knowledge been either made or deemed feasible. Such application is
attained in the first place by imparting to Gauss' objective the property of
producing an anastigmatically flattened image-which is so desirable for photo-
graphic lens systems but has hitherto not been realized in the said objective-and
secondly by achromatizing such anastigmatic objective in a novel manner satisfactory
for photographic purposes. In attempting to attain this result considerable
difficulty has been experienced owing to the narrow range of suitable sorts of glass.
It has been found that in an objective of great aperture a perfect anastigmatic
flatness of the image may be secured only with lenses either of considerable
thickness or placed at a great distance apart. Furthermore a sufficient chromatic
correction while being attainable with any given pair of crown and flint glass in
the astronomical objective, which needs not produce an anastigmatically flattened
image and which has hitherto been constructed with lenses of but little thickness
and placed at a short distance from each other-in an anastigmatic objective of a
comparatively large aperture may be ensured only where the dispersive power of
the flint glass (as expressed by the formula
Δη
has a comparatively great value,
viz. about double the value of the dispersive power of the crown glass. Such a
pair of lenses, however, is unsuited for photographic purposes, inasmuch as it would
have to comprise a very heavy flint glass.
According to the present invention this drawback is obviated in the following
manner. In the lens system (shown in Figure 1) either one lens A, or the other
lens B, or both, consist each of a positive (biconvex) lens and of a negative
(biconcave) lens cemented to the first and formed of glass having the same or
nearly the same refractive power as the glass of the positive component but
different dispersive power.
A lens so composed will, as regards all the effects dependent upon refraction
(viz. focal length, the location of the cardinal points and spherical aberration of
every description) be exactly identical with a single homogeneous leus of like
external curvatures and equal thickness, while as regards chromatic dispersion, the
compound lens is equivalent to a single lens with like curvatures and of a like
thickness but made of glass having a dispersive power differing from those which
actually occur in either part of the combination. In particulars the resulting
dispersive power of the compound lens is less than the least dispersive power of
the component lenses-that is to say, the compound lens is achromatic or partially
achromatic-if in the case of the compound lens being a positive lens A. the
zitive component and in the case of it being a negative lens B, the negative
component cousists of glass of less dispersive power; and the resulting dispersive
power of the compound lens is greater than that of either component-that is to
say the compound lens is hyperchromatic-if in the case of the compound lens
being a negative lens B, the positive component is formed of glass of less dispersive
power and the negative component of glass of greater dispersive power. The
former fact agrees with the well known rules of achromatization of lenses, and
the latter with the demonstrations respecting hyperchromatic dispersing lenses,
contained in the Specification of my Patent No. 12219 of 1896. The value of
the resulting dispersive power of the compound lens in all these cases depends
upon the values of the dispersive power of the glasses employed in forming the
two components and upon the ratio of the total curvatures or of the focal length
of both components; it depends consequently, if the outer curvatures of the

page_text: b
mon to the mner surfaces
united by cement. Such value may therefore be calculated in each individual case
by means of well-known formula.
By the stated manner of composing the lenses A and B of a lens system of the
Gauss type every required ratio of their dispersive powers may be realized, without
causing the lenses to differ from non-composite lenses of glass of the same thickness
and outer curvatures, as regards the effect of refraction and the spherical aberrations
by which refraction is attended. The outer curvatures and thicknesses of the
lenses A and B may therefore be determined beforehand regardless of the dispersive
powers of the particular kinds of glass to be employed, but in view to produce the
correction of spherical aberration and to anastigmatically flatten the field of the
image, and subsequently by properly determining the inner radius of curvature of
the components to be cemented of lens A or of lens B or of both lenses, the ratio
of the resulting dispersing powers of the lenses A and B, which is requisite for
the proper correction of chromatic aberrations, may be easily established. In the
practice of this method values of the said ratio result, which in the case of non-
composite lenses, could only be obtained by employing a flint glass of a very high
dispersive power, but which with the lenses constructed as above explained are
induced by crown and flint glasses, exhibiting but slightly different dispersive
powers, provided such glasses possess an equal or approximately equal refractive
power.
Pairs of glasses, which meet these requirements, in sufficient range of gradations
form part of the kinds of optical glass, which are constantly for sale at the Jena
Glass Works. The condition of identical refractive power need only, of course, be
fulfilled approximately, inasmuch as slight differences in the indices of refraction
between the cemented components, even where the curvature of the inner surface
is very marked, influence the corrections but to an extent, which is comparatively
insignificant and may be easily introduced into the computations. Indeed, a
little difference in the refractive power, say a few units in the third place after the
decimal point, may even prove advantageous, since, owing to such a difference, a
more perfect equalization of the so-called "zones" of the spherical aberration may.
become feasible.
Apart from this, no essential variation, or departure from principle, is involved
in the construction of object pieces of the new type, should some medium other
than air be interposed between the constituent parts of such object piece; the one
important condition being, that such separating (or spacing) medium should
possess an uncommonly small exponent index of refraction, in comparison with the
exponents of the different kinds of glass employed.
The objective producing an anastigmatically flattened image as herein described.
should, preferably, be corrected by placing the diaphragm in front of the objective,
as illustrated, by way of example, in Figure 2 of the drawings. But it will still
display a noticeable "coma" and, like any anastigmatic objective with the diaphragm
in front of it, it will be attended with the defect of orthoscopic distortion of the
image. These two drawbacks may without difficulty be obviated, however, by
constructing a double object glass (with a central or intermediate diaphragm),
the objective hereinbefore described being utilized as one of the constituent
elements thereof. An example of this modified form of the improved object lens
is given in Figure 3.
In the following examples given by way of illustration of some forms of the
new objective, represented in Figures 2 and 3, I have introduced the following
symbols:-
The letters ri, T₁₁ T3, T4 r. designate the radii of the spherical ground lens-
surfaces; d₁, da, d, are the central thicknesses of the lenses; b indicates the central
distance between the diaphragm and the lens, do the central distance between the
main parts of the lens system, L1, L2, L3 the lenses themselves and D the diameter
of the lenses. In the following tables radii, thicknesses and diameter of the lenses
are expressed in relative numbers, the focal length of the complete object piece

page_text: "being taken as a unit. By simply multiplying these numbers by the focal length
required in any given case, the dimensions of an objective, possessing the requisite
.focal length, will be found.
In order to characterize the different sorts of glass employed, there are given the
-refractive indices no and no', relating respectively to the D line of the solar
spectrum and to the H y line of the hydrogen spectrum. Besides, for each kind of
glass, the value of the dispersive power (^_^) is mentioned, An being calculated
for the interval between D and H-y, while for n the value n p is adopted.
EXAMPLE 1.
An astigmatically, spherically, and chromatically corrected object lens with a
front diaphragm as shewn in Figure 2. The relative aperture of the objective is
equal to of the focal length. The objective consists of two components separated
from each other by air, the negative component being a single lens L, while the
positive part is made up of a dispersing lens L, and of a collecting leus L., both
cemented together. The refractive indices of both sorts of glass employed in
making the lenses L, and L, are approximately equal, while the glass of which the
lens L, is formed is the one possessing the higher dispersive power of the two.
Dimensions for a focal length = 1
Maximum relative aperture
= 111
Radii:
•1164
r1
*2215
ゲス
-
1-6097
Thicknesses & Distances
d₁
-0320
d2
=
⚫0172
d₁
*0222

page_text: Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is:
1. A compound photographic objective which gives an anastigmatically flattened
image, consisting of two lenses separated from each other by a medium of little
refractive power, viz. one positive lens and one negative lens, one or both of which
lenses being made up of two constituent parts united by cement and formed of two.
different sorts of glass of unequal dispersive power but nearly equal refractive
power, essentially as shown and described.
2. A photographic doublet-objective, one or both main parts of which are
objectives of the kind claimed by the preceding claiming clause, essentially as.
shown and described.
Dated this 5th day of April 1897.
JOHNSONS & WILLCOX,
47 Lincoln's Inn Fields, London, W.C., Agents.