An Improvement in Photographic Objectives.
Year: 1890

page_text: JUDIT LETN DI MUITICATION.
An Improvement in Photographic Objectives.
I, Dr. PAUL RUDOLPH, of Jena, in the Grand-Duchy of Saxe-Weimar, German
Empire, mathematician, 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 :-.
My invention relates to photographic objectives, and the improvement consists in
the construction of the objective of two separate systems of lenses, the individual
members whereof have, relatively to each other, particular refractive properties.
A main difficulty to be overcome in the construction of photographic objectives
beside realising the other numerous conditions subject to the purpose which these.
objective have to serve, consists in sufficiently compensating the astigmatism of the
obliquely incident pencils of light and in removing the indistinctness of the marginal
portions of the field resulting from such astigmatism. And, in fact, all the photo-
graphic objectives constructed so far, posses more or less this deficiency, that with an
approximately plane field the focus of a group of rays within a primary (meridional)
section of an oblique pencil of rays remains at a greater distance than the focus of
the group of rays within a secondary (sagittal) section of the same pencil.
After a course of investigations on the conditions whence results this anomaly and
the means of mitigating its effects, the photographic objective forming the subject
of this specification was arrived at. It is mainly a dissymmetrical combination of
lenses composed of two distinct systems of single lenses cemented together. This
combination of lenses is characterised by the concurrence of the following two
properties:
1.) That in one of the two distinct systems the positive member (collecting lenses)
has a smaller, in the other system, however, a greater refractive index than the
negative member (dispersing lenses) cemented thereto.
2.) That each system in itself is approximately achromatized i.e. that the chromatic
aberration of each of the systems, expressed by the difference of the reciprocals of its
focal lengths for two different colours, is small as compared with the chromatic
aberration of a single crown glass lens having the focal length of the entire objective,
the latter chromatic aberration being measured after the same manner as the
former.
A combination of lenses realising both these requirements simultaneously presents.
particular advantages. On the one hand it admits of the astigmatic differences being
removed without rendering the application of more than two distinct systems
necessary, and on the other hand it yields favourable conditions for satisfying all the
other requirements connected with the correction of photographic objectives..
The above defined contrariant combination of the two distinct systems composing.
the objective, furnishes the simplest means for compensating the astigmatic differences.
of oblique pencils. This is due to the fact that a system composed of lenses
cemented together and in which the collecting lens has a greater refractive index
than the dispersing lens, gives rise to astigmatic differences opposite to those caused
by a system of lenses the collecting lens whereof possesses a lower refractive index
than the dispersing lens.
With the mode of compounding both parts of the objective as indicated above; it
is therefore only necessary to choose the ratios of the single lenses in both these
parts in such a manner that the opposed astigmatic differences are of the same
magnitude, in view of attaining compensation of this astigmatism, or anastigmatic
correction of the entire objective.
But in order. that the realisation of this condition does not enhance the difficulty
of achromatizing the whole objective, it is essential that the objective possesses also
that second property as defined at the beginning of this specification; i.e. it is

page_text: necessary to effect the contrariant composition of the two separate systems in such a
manner that at the same time each system may be in itself achromatized. It is,
however, neither particularly advantageous nor in all cases practicable to perfectly
achromatize the individual members of a double objective; for the mere thicknesses
which must be given to the lenses may in each member of the objective necessitate
deviations from .achromatism amounting to about one fourth of the chromatic
difference of the reciprocals of the focal lengths of a single crown glass lens having
the focal length of the whole objective. But it is of practical importance that the
mode, of composing the systems of lenses does not in itself involve any limitations
even to their approximate achromatisation and that it does not render inevitable still
greater chromatic differences than are required to meet other conditions.
This last requirement may be satisfied by a suitable choice of the combinations of
glass which are employed to form the two parts of the objective. The following
observations may explain the principles which govern the selection of suitable glasses.
Those pairs of glasses (crown and flint) which are generally resorted to in the
construction of achromatic lenses (and which a few years ago constituted the sole
means of obtaining achromatism) possess this characteristic feature that a higher
refractive index invariably implies a greater relative dispersive power. (Under the
term dispersive power is to be understood, as usual, the quotient
i.e. the
n-1,
difference between the refractive indices for two different colours divided by one of
these indices, or by the mean index of refraction minus one.) A pair of glasses of
this kind, which may be said to be of a normal character-admits of the application
of positive (collecting) systems, if achromatism is to be attained, only under the
condition that the positive element has a lower refractive index than the negative.
An
Recent progress in glass-manufacture has, however, added to the list of optical
glasses other kinds of glasses which admit of such pairs being selected in which the
relation of the refractive indices and the relative dispersive powers is just the opposite
as in the other case, i.e. in which the glass having the higher refractive index does
not also possess the greater, but on the contrary the smaller relative dispersive
power. Such pairs of glasses may be said to be of an abnormal character, in
contradistinction to the first named class.
The application of pairs of glasses of the last named or abnormal character,
admits of the production of systems with positive focal lengths in which, without
prejudice to the attainment of achromatism, the more refractory medium acts as the
positive (collecting) element.
Accordingly, in an objective consisting of two distinct systems, the contrarious
elements required for compensation of astigmatic deviations, are obtained without
prejudice to full liberty as regards achromatisation of each single system, by
compounding one of the systems-presuming both to be positive systems-of a
normal, the other of an abnormal pair of glasses (the terms "normal" and "abnormal "
being taken in the sense of the definition as given above).
The elements of actual construction of objectives as specified in Tables I, II and III
furnish examples for the practical application of this invention under varying
conditions.
The opposite character of the pairs of glasses in the two separate members of a
doublet are, however, conditional to the desired effect only in case both these members
are required to be positive (collecting) systems, and if both are also required to assist
in a notable degree in the concentration of the rays of light or, in other words, in the
diminution of the focal length of the objective. For many purposes it may, however,
be sufficient or even advantageous to assign the function of the concentration of the
rays of light wholly or principally to one system (principal system), viz. in such a
manner that the other need not produce any notable refractory effects in the sense of
a diminution of the focal distance, it having simply and essentially to act as a
correcting system, which admits of a relatively large positive or even negative focal
length. According as in this particular case the principal system consists of a
normal or of an abnormal pair of glasses, the positive or the negative element of the
i

page_text: correcting system must be made to have the higher refractive index; the nature of
the pair of glasses of which the correcting system is composed, i.e. whether it is of a
normal or of an abnormal character, is, however, non-essential; provided its focal:
lengths be taken very great. If, however, a negative focal length which is not very
great as compared with the focal length of the principal system,, be given to the
correction system, the latter, in order to insure the conditions of achromatisation,
must be composed of a pair glasses of the same character as the principal system;
i.e. either both of pairs of glasses of the normal or both of pairs of glasses of the
abnormal character.
The elements of construction as specified under IV, serve to illustrate an
anastigmatic combination of lenses of this last kind, in which one of its members
constitutes such a correcting system of a negative focal length, both menibers
consisting of normal pairs of glasses.
The new type of photographic doublets thus set forth, admits, as is shown by the
preceding explanations and the examples appended to this specification, of many
modifications of detail, which modifications do not, however, introduce any novel
features. The focal lengths of both parts of the double objective may have any ratio
whatever the system consisting of an abnormal pair of glasses and likewise the
correcting system-assuming this latter case to enter into consideration-may
constitute either the anterior or posterior member of the combination; finally a great
variety of glasses may be employed for carrying out the constructive principles as
stated above, provided they leave sufficient scope for the selection of such pairs as
are prescribed in each particular case by the foregoing directions.
All these special alternatives of practical construction are intrinsically dependent
upon the particular purpose which such an objective has to serve and the corre-
sponding conditions regarding the aperture, the size of the field and the degree of
perfection required with respect to correction. Whenever these particular conditions
on the one hand and the optical constants (refractive index, and dispersive power) of
the available kinds of glasses on the other hand, are given. numerically, any optician
versed in the treatment of problems of this kind is enabled, guided by the preceding
remarks and the examples appended at the end of this specification, to numerically
determine according to known methods those elements of construction (radii,
thicknesses of lenses, air-distances) which will sufficiently compensate astigmatic
deviation beside furnishing all the other usual corrections. In particular, no novel
constructive feature is introduced by making one (vid. Table III) or both of the
cemented systems of the doublet to consist of three instead of two lenses, which may
be particularly advantageous in such cases where the objective is to have a large
aperture. For in- as much as the use of three single lenses has no other aim in
view but to distribute the effect otherwise attained by a single lens, over two, made
of the same or similar kinds of glass, the substitution of a triple for a double lens
belongs to those expedients which are ordinarily made use of by practical and
'theoretical opticians in order to attain an increased number of elements affording
means for optical correction.
In the annexed drawings three different arrangements of objectives carried out
according to my invention, are shown in full size partly in elevation and partly in
section. According to fig. 1 the anterior system of lenses is composed of the
dispersing lens L, and the collecting lens L,, while the posterior system consists in
the collecting lens L. and the dispersing lens L, all these 'lenses' being concavo-
convex. B is the diaphragm. In the objective represented by fig. 2, the anterior
system L, L, is composed alike to the preceding one, whereas the posterior
system consists in the biconcave dispersing lens L. and the biconvex collecting
lens L. Fig. 3 again shows a like system L, L. while the posterior system
comprises two concavo-convex dispersing lenses, L and L, and the concavo-convex
collecting lens L. The letters r, r,
, to be referred to hereafter, denote
the radii of curvature.of the lens surfaces to which they are applied, 7, d. d. d. d, the
central thicknesses of the lenses, and b, b, the distances between the diaphragm B and
the lenses.

page_text: Ι
In conclusion I append to this specification of my invention four examples which
serve to illustrate the manner in which the principles set forth in the preceding
pages may be applied to the construction of different types adapted to various
purposes. All dimensions (radii, thicknesses of lenses and air-distances), 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.
The letters of the following tables correspond to the letters marked on the
drawings. The different kinds of glasses are determined by the indices np and nal
relating respectively to the D line of the spectrum and to the Hy-line of the
spectrum of hydrogen. In order to render obvious the character of the pairs of
glasses used in each system I have appended to each kind of glass the value of
where has been calculated for the
interval from the D to the Hy-line of the spectrum, while the value of ?D
taken for n.
the relative dispersive power (凹)

page_text: Having now particularly described and ascertained the nature of this inven
tion and in what manner the same is to be performed, I declare that what I
claim is.
In a photographic objective, the combination of two distinct systems of lenses, each
composed of single lenses cemented together, the positive element of one system
having a higher, the positive element of the other system a lower refractive index
than the respective négative elements cemented thereto, and each system being in
itself approximatively achromatic, substantially as described.
Dated the 31st day of Amil 10