Improvements in Photographic Lenses or Objectives.
Year: 1893

page_text: COMPLETE SPECIFICATION.
- Improvements in Photographic Lenses or Objectives.
I, PAUL RUDOLPH of Carl-Zeiss, Strasse Jena in the Grand Duchy of Saxe-
Weimar 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 :-
•
these
This invention relates to photographic objectives and its purpose is to remove
the indistinctness of the marginal portions of the image by reducing as much as
possible the " 'astigmatic" aberrations. As indicated by their name
aberrations are caused by "astigmatism" that is to say the property possessed.
by the rays of an oblique pencil of light transmitted through an objective of not
uniting in one and the same point the rays lying in the primary or meridional
section. of such pencil uniting in one point and the rays lying in its secondary
or sagittal section uniting in another point. Or in other words the focal length
of the rays in the primary section differs from the focal length of the rays in the
secondary section. Thus are formed two partial focal points. The distance of these
two partial focal points or the difference of the two focal lengths is called the
"astigmatic difference." The latter is termed positive when the focal length of
the rays, in the primary section is greater than that of the rays in the secondary
section and it is called negative when the focal length of the rays in the secondary
section is greater than that of the rays in the primary section."
.
"
As it is possible to so compose or construct lenses as to cause them to manifest
a predetermined astigmatic difference either positive or negative this astigmatism
supplies in itself a means for obviating its prejudicial effects. With this object
the objective can be constructed with two distinct or separate lenses or systems of
lenses for example one of which is so composed as to produce a positive astigmatic
difference whilst the other is so arranged as to give rise to a negative astigmatic
difference the said two differences being caused to neutralize each other. Such a
double objective: or "doublet as it is called is described in the Specification
annexed to British Letters Patent No. 6028 dated 21 April 1890. In this
double objective the correction of the said astigmatic aberrations is obtained by
combining an achromatic lens the positive member of which being the collecting
lens is made of glass having a higher refractive index than the glass of its
negative member (that is to say the dispersing lens to which it is cemented) with a
second achromatic lens the refractive indices of the two cemented members of
which are contrarily graduated. The neutralizing effect obtained by this reversed
or inverted composition of the members of a doublet relatively to the astigmatic
aberrations is principally dependent on the fact that in virtue of the said com-
bination the inner surface or "surface of union" in the one member that is to say
the surface on which the component lenses of the member are united together is
caused to act as a collecting lens whilst in the other member the surface of union
is made to act as a dispersing lens and so with regard to the influence upon the
focal length as well as upon the spherical aberration of the respective member.
Now the present invention consists in a new organisation or combination of
achromatic lenses whereby the same neutralization or correction is brought about
within a single system of lenses cemented together. For this purpose the system
is composed of three individual lenses in such a manner that the inner member is
a collecting lens and has cemented to it on each side a dispersing. lens one of the
latter having a greater and the other a smaller refractive index than the collecting
or intermediate lens or that the said inner member is a dispersing lens and the said
two outer members cemented thereto are collecting lenses one of which has a
greater and the other a smaller refractive index than the dispersing lens situated
between them.

page_text: It is evident that by means of either of these two combinations one of the inner
surfaces or "surfaces of union" produces a positive refraction that is to say the
effect of a collecting lens and the other union surface a negative refraction that
is to say the effect of a dispersing lens.
In the annexed drawings examples of the two new arrangements of triple lenses
are represented in section in Figure 1 and in Figure 2 respectively.
In both figures L, L, L, denote the three lenses cemented together, 1, 2 the
inner surfaces or "surfaces of union "and B the diaphragm.
Figure 1 illustrates that type in which the inner chamber L, consists in a
collecting lens being biconvex, whilst the outer members L, and I, are dispersing
lenses, the former being biconcave and the latter concavo-convex. In Figure 2
the inner member L, consists in a biconcave dispersing lens and the
members L and L are formed by collecting lenses being respectively concavo-
convex and biconvex. In either case the lens L₁ being the member nearest to the
diaphragm B is assumed to have the smaller refractive index.
outer
The letters rir, ", " denote the radii of curvature of the surfaces of the lenses
and di do d, the central thickness of the lenses. 6 indicates the distance between
the lenses and the diaphragm B.
There is now no difficulty in obtaining glass of suitable quality for making
achromatic lenses of the kinds required for carrying out the present invention.
The practical question as to whether in a combination of the above description it
is possible to obtain a sufficient astigmatic correction and at the same time to
comply with the other conditions required in photographic objectives has been
answered in. the affirmative by theoretical and practical investigation of both types
of the new triple lens.
Guided by the annexed drawing and the following tables in which are given
the elements of construction of two typical examples an optician skilled in the
construction of lenses or objectives will be enabled to numerically determine or
calculate according to known methods those elements of construction (radii, thick-
ness of lenses, and distance between the lenses, and the diaphragm B) which are
necessary to properly compensate astigmatic aberrations.
The letters employed in the following tables correspond with the letters marked
on the drawings. The kinds of glass to be employed are determined by the indices
of refraction n D and n G, relating respectively to Fraunhofer's D. line and to the
Hz line of the spectrum of hydrogen. All dimensions are expressed by
proportional numbers, the focal length of the whole objective being taken as unity.
A simple multiplication of these numbers with the focal length actually required
will suffice for obtaining the dimensions of any objective wanted.

page_text: By correspondingly modifying the elements of construction lenses having the
same or approximately the same effect as the typical lenses herein before described
can likewise be obtained with the aid of other kinds of glass provided that the
latter enable the conditions necessary for achromatisation to be complied with
and also enable suitable differences of the refractive indices to be established on
both sides of each of the two union surfaces of the triple lens. The external shape
of the latter must in every case be either that of a meniscus or that of a biconvex
levs with very unequal external curvatures, and the surface having the least curva-
ture must be turned towards the diaphragm B. The outer member next to the
diaphragm may be composed of glass having the smaller refractive index as
assumed in Figures 1 and 2 or of glass having the greater refractive index.
An achromatic lens of the kind hereinbefore described may be advantageously
employed for the construction of objectives comprising but one composite lens such
as a landscape lens or for making doublets and in the latter case it may either be
combined with a triple lens of the same kind or with an ordinary achromatic lens.
By combining two equal lenses constituted according to this invention so as to
form a symmetrical doublet of the ordinary aplanatic type, objectives are obtained
in which the advantages of the double objective described in the hereinbefore cited
Specification No. 602800 are united with the peculiar advantage of the symmetrical
systems of ensuring the most accurate resemblance between the object and the
image. According to the rules governing the construction of symmetrical system,
such double objectives are obtained without any change in the elements of con-
struction set forth in the preceding tables, by oppositely arranging two equal triple
lenses, the place of the diaphragm indicated in the tables being made the
geometrical centre of the doublet.
By the combination of a triple lens constructed according to this invention, with
a second lens of the same description but of different focal length and other curva-
tures or with an ordinary achromatic lens, two valuable modifications of the
unsymmetrical (anastigmatic) double objective described in the aforesaid Specifica-
tion are obtained. The compensating effects which are brought about in the said
unsymmetrical system by the cooperation of its two members are obtained in the
modified system in each member separately or in one member alone for the entire
system.
In employing the new triple lens in either of these unsymmetrical double
objectives it is evidently necessary to modify the elements of construction herein-
before given in the tables, because in such systems the best result is obtained not
by correcting each member separately, but by compensating a rest of aberration in

page_text: one member by an opposed rest of aberration in the other member. The alterations
to be made must depend on the requirements of each special case and can be
ascertained by means of methods of calculation well known to opticians.
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
An achromatic photographic lens or objective composed of three lenses cemented
together the refracting index of one of the outer lenses being smaller and the
refracting index of the other outer lens being greater than that of the middle or
intermediate lens substantially as hereinbefore explained.
Dated this 3rd day of March 1893.
J. H. JOHNSON & Co.,
47, Lincoln's Inn Fields, W.C., Agents.