Improvements in or connected with Doublet Lenses for Photographic Purposes.
Year: 1892

page_text: Date of Application, 19th Dec., 1892
Complete Specification Left, 2nd June, 1893-Accepted 13th June, 1893
PROVISIONAL SPECIFICATION.
Improvements in or connected with Doublet Lenses for Photographic
Purposes.
We, CARL PAUL GOERZ, of 74 Haupstrasse, Schöneberg, and EMIL VON HOEGH,
of 60 Printzregentenstrasse, Wilmersdorf, both near Berlin, in the Empire of
Germany, Opticians, do hereby declare the nature of this invention to be as
follows:-
Our invention relates to doublet lenses for photographic purposes.
Within the last few years, as the result of the labours of the scientific glass
laboratory in Jena, there have been made available for optical industries new
kinds of glass, which by reason of their optical properties, differing from those of
the kinds of glass which alone were previously to be obtained, have given rise to
improvements in optical instruments, and especially in photographic objectives.
The conditions of the elimination of the astigmatic errors of pencils of rays
entering obliquely require that the crown glass should be of a refractive power
higher than that of the flint glass combined with it. Varieties of glass which
render combinations of this kind possible have been regularly manufactured since
1886, and since that time attempts have been made in various quarters to utilize
the new kinds of glass for the elimination of the astigmatic error in photographic
objectives. Thus for instance amongst others the "anastigmat of Hartnack,
calculated by Dr. Miethe, was introduced in the year 1887. This objective is free
from astigmatism when the image is approximately plane, but like all other
objectives manufactured by various opticians upon the same principle it has the
disadvantage that the spherical error cannot be eliminated, and hence the system
has an illuminating power too feeble for most of the needs of practical photo-
graphy, and is especially ill adapted for instantaneous exposures.
66
"
The reason for this fact is that in order that the spherical error may be com-
pensated for, the refractive power of the crown glass must of necessity be lower
than that of the flint glass which is combined with it. The construction of an
anastigmat" of good illuminating power, with the spherical error corrected, that
is to say, without injury to the sharpness of the image, thus appears to be prevented
by the interposition of two incompatible conditions, standing in opposition to each
other, and, as 'a matter of fact, it has been customary in the best informed scientific
circles for a long time to consider it impracticable to devise a method of construction
which should combine the two properties above referred to.
66
":
Dr. P. Rudolph of Jena, the inventor of the recently introduced anastigmat
of Zeiss, forming the subject of a German Patent No. 56109, was the first who
succeeded in bringing forward proof of the possibility of constructing "anastigmats
of good illuminating power.
"
We have endeavoured to attain greater approximation to the ideal form by a
method differing essentially from that adopted by Dr. Rudolph.
For this purpose we have made extensive calculations directed at first to the
single object of determining whether, as the fundamental principle of a new
objective, the astigmatic error could be reduced as far as possible, in the expectation
which appeared, humanly considered, to be justifiable that even if it should prove
to be impossible to eliminate the astigmatic error absolutely, it might yet be hoped
to effect important improvements in the direction indicated.
Starting from the above mentioned facts, well established theoretically, in
accordance with which a crown glass of lower refractive power than that of the
flint glass combined with it must be used to eliminate the spherical error, and a
crown glass of higher refractive power than the flint glass component for the

page_text: elimination of the astigmatic error, it was sought to determine by strict calculation,
how far a doublet, cach half of which was built up of three lenses, would be able
to fulfil the conditions required for the elimination of the two aberrations, the
spherical and the astigmatic. The separate components of such a system might
with reference to the result be conceived to be so made up that they should consist
either of a negative flint glass enclosed between two positive crown glasses in such
a way that one of the two latter, should have a higher refractive index, and the
other a lower refractive index, than that of the enclosed flint glass, or, be made
up of two negative flint glasses enclosing a crown glass, in which case one of the
two flint glasses, should have a refractive index higher, and the other one lower
than that of the enclosed positive crown glass.
It was found to be possible, in both methods of construction, whilst compensating
for the spherical and chromatic aberrations, not only to reduce the astigmatism but
to eliminate this last in a manner theoretically perfect. Of the two methods of
construction under consideration, the latter (crown glass between two flint glasses)
is found to involve unfavourable forms for the separate components, and the
employment of such kinds of glass as have their practical applicability limited by
disadvantageous peculiarities, so that the former combination (flint glass between
two crown glasses) was exclusively adopted in the further working out of the
problem and the determination of the constants of an objective capable of being
carried out in practice.
The lens may be symmetrically constructed, but the combination of two
absolutely identical halves is not to be regarded as a special characteristic of the
objective system under consideration, the only characteristic mark of objectives of
the new kind is rather the combination of separate systems independently corrected
for spherical, chromatic and astigmatic error, each made up of three lenses, the
glass of these last having refractive indices properly graduated, indeed one system
of the objective might have an external form differing from that of the other, or
in the same objective two differently built up systems might be employed. Any
auch modifications may be undertaken by a calculating optician without difficulty,
but from a practical standpoint they are to be regarded as purposeless, since the
more simple symmetrical form satisfies all the conditions herein taken into account
to a sufficient extent. Furthermore, since each of the two separate systems
constitutes in itself an objective corrected for spherical chromatic, and astigmatic
errors, each may be employed alone as a simple photographic objective.
Dated the 19th day of December 1892.
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicants.
COMPLETE SPECIFICATION.
Improvements in or connected with Doublet Lenses for Photographic
Purposes.
We, CARL PAUL GOERZ, of 7ª Haupstrasse, Schöneberg, and EMIL VON HOEGH,
of 60 Prinzregentstrasse, Wilmersdorf, both near Berlin, in the Empire of
Germany, Opticians, 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 :-
Our invention relates to doublet lenses for photographic purposes and is illustrated
in the sectional views, Figures 1, 2 and 3 of the accompanying drawing.
Within the last few years, as the result of the labours of the scientific glass
laboratory in Jena, there have been made available for optical industries new kinds
of glass, which by reason of their optical properties, differing from those of the
kinds of glass which alone were previously to be obtained, have given rise to
improvements in optical instruments, and especially in photographic objectives.
The conditions of the elimination of the astigmatic errors of pencils of rays

page_text: entering obliquely require that the crown glass should be of a refractive power
higher than that of the flint glass combined with it. Varieties of glass which
render combinations of this kind possible have been regularly manufactured
since 1886, and since that time attempts have been made in various quarters
to utilize the new kinds of glass for the elimination of the astigmatic error in
photographic objectives. Thus, for instance, amongst others the "anastigmat" of
Hartnack, calculated by Dr. Miethe, was introduced in the year 1887. This
objective is free from astigmatism when the image is approximately plane, but like
all other objectives manufactured by various opticians upon the same principle it
has the disadvantage that the spherical error cannot be eliminated, and hence the
system has an illuminating power too feeble for most of the needs of practical
photography, and is especially ill adapted for instantaneous exposures.
The reason for this fact is that in order that the spherical error may be com-
pensated for, the refractive power of the crown glass must of necessity be lower
than that of the flint glass which is combined with it. The construction of an
"anastigmat" of good illuminating power, with the spherical error corrected, that
is to say, without injury to the sharpness of the image, thus appears to be pre-
vented by the interposition of two incompatible conditions, standing in opposition
to each other, and, as a matter of fact, it has been customary in the best informed
scientific circles for a long time to consider it impracticable to devise a method of
construction which should combine the two properties above referred to.
Dr. P. Rudolph of Jena, the inventor of the recently introduced "anastigmat"
of Zeiss, forming the subject of a German Patent No. 56109, was the first who
succeeded in bringing forward proof of the possibility of constructing “anastigmats"
of good illuminating power.
We have endeavoured to gain greater approximation to the ideal form by a
method differing essentially from that adopted by Dr. Rudolph.
For this purpose we have made extensive calculations directed at first to the
single object of determining whether, as the fundamental principle of a new
objective, the astigmatic error could be reduced as far as possible, in the expec-
tation, which appeared, humanly considered, to be justifiable that even if it should
prove to be impossible to eliminate the astigmatic error absolutely, it might yet be
hoped to effect important improvements in the direction indicated.
Starting from the above mentioned facts, well established theoretically, in
accordance with which a crown glass of lower refractive power than that of the
flint glass combined with it must be used to eliminate the spherical error, and a
crown glass of higher refractive power than the flint glass component for the
elimination of the astigmatic error, it was sought to determine by strict calculation,
how far a doublet, each half of which was built up of three lenses, would be able
to fulfil the conditions required for the elimination of the two aberrations, the
spherical and the astigmatic. The separate components of such a system might
with reference to the result be conceived to be so made up that they should consist
either of a negative flint glass b, Figure 1, enclosed between two positive crown
glasses a and a in such a way that one of the two latter, say a, should have a
higher refractive index, and the other, say a', a lower refractive index, than that of
the enclosed flint glass b, or, be made up of two negative flint glasses b, b', Figure 2,
enclosing a crown glass a, in which case one of the two flint glasses, say b, should
have a refractive index higher, and the other, say b', lower than that of the enclosed
positive crown glass a.
It was found to be possible, in both methods of construction, whilst compensating
for the spherical and chromatic aberrations, not only to reduce the astigmatism but
to eliminate this last in a manner theoretically perfect. Of the two methods of
construction under consideration, the latter (crown glass between two flint glasses)
is found to involve unfavourable forms for the separate components, and the employ-
ment of such kinds of glass as have their practical applicability limited by
disadvantageous peculiarities, so that the former combination (flint glass between
two crown glasses) was exclusively adopted in the further working out of the

page_text: problem and the determination of the constants of an objective capable of being
carried out in practice. As an example, there is depicted in Figure 3 of the
appended drawing an objective of the kind described, of an equivalent focal
length of 240 mm., and a free aperture of 36 mm., of the natural size, and in axial
section.
The maximum effective aperture is 30 mm., corresponding to one eighth of the
focal length.
The constants required for the construction of the objective are as follows :—
Radii of curvature.
R1
=
-
R2
R8.45-835
R754-324
R3 = R6 = = 19.853
R4
-
R5 = 49.088
Thickness of lenses.
nD nG
Kind of glass.
dl
= d6 = 7.334 LL 161310 1.62683
d2d5 = 1.833 L'L' 1.56804 1.58182
d3d4: = 4.584 L3L 151497 1-52663
A (distance apart) = 11.00.
All measurements, radii, thickness, distance, etc. are expressed in millimetres,
the kinds of glass are designated by their exponents of refraction for the line D
(nD) of the solar spectrum and for the line H, (NG¹) of the spectrum of
hydrogen.
8
If a principal ray inclined at an angle of 300 to the axis be calculated through
the system, and the position upon it of the image point of the sagittal rays, and
those of the meridional rays be determined, a measure of the residual astigmatism
is found in the distance of the two points. This distance amounts, in the case of
the objective chosen as a specimen, to 1.2 mm, when the image formed by the
meridional rays is flat; and this residual amount of astigmatism has no greater
unfavourable effect upon the sharpness of the image formed by the marginal rays
than that caused by the secondary spherical aberration in the axis.
The lens just referred to as an example, based upon strict calculation is
symmetrically constructed, but the combination of two absolutely identical halves
is not to be regarded as a special characteristic of the objective system under
consideration, the only characteristic mark of objectives of the new kind is rather
the combination of separate systems independently corrected for spherical,
chromatic and astigmatic error, each made up of three lenses, the glass of these
last having refractive indices properly graduated in the manner described, indeed
one system of the objective might have an external form differing from that of the
other, or in the same objective one system might be built up as in Figure 1 and
the other as in Figure 2. Any such modifications may be undertaken by a
calculating optician without difficulty, but from a practical standpoint they are to
be regarded as purposeless, since the more simple symmetrical form satisfies all
the conditions herein taken into account to a sufficient extent. Furthermore, since
each of the two separate systems constitutes in itself an objective corrected for
spherical, chromatic, and astigmatic errors, each may be employed alone as a simple
photographic objective.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed, we declare that what we
claim is :-
:―
1. A photographic doublet, the two component systems of which are each
corrected for spherical, chromatic, and astigmatic errors, being to this end made
up of three lenses, as follows:-either
(a) of a negative lens enclosed between two positive lenses, one of which has
a higher and the other a lower refractive index than the enclosed negative
lens, or
(b) of a positive lens, enclosed between two negative lenses, of which one has a
higher, and the other a lower refractive index than that of the enclosed
positive lens.

page_text: 2. The employment of a single system of the kind described in Claim 1 as an
independent photographic objective, substantially as herein before described.
Dated this 31st day of May 1893.
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicants.