Improvement in Photographic Objectives.
Year: 1890

page_text: of Application, 21st Apr., 1890-Accepted, 24th May, 1890
COMPLETE SPECIFICATION.
Improvement in Photographic Objectives.
We, Dr. ERNST ABBE, Professor at the University of Jena, and Dr. PAUL RUDOLPH,
Mathematician, both residing at Jena, in the Grand-Duchy of Saxe Weimar, German
Empire, do hereby declare the nature of our 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 photographic objectives, and the improvement consists
in the construction of a system of lenses in which the function of collecting the
rays of light forming the image, and that of correcting the spherical and chromatic
aberration are allotted to distinct parts of the system.
In constructing photographic objective-glasses in which the advantage of an
extended image-field is to be coupled with a large effective aperture, particular
difficulties are encountered in attaining the desired degree of absence of spherical
aberration of the pencils of light that produce the image. In view of obtaining
the extended field aimed at, objectives of the usual types require to be provided with
lenses of high curvature, and these always give rise to certain residual spherical
aberrations (the so-called "zones") whereby the distinctness of the image is
impaired when the aperture is fully open.
For the purpose of obviating this defect and simultaneously facilitating, an
improved correction of the chromatic aberration, we compose the objective of two
uncorrected single collecting-lenses, which, in view of the aplanation of the field,
have their convex, or more convex sides turned outwardly, and a correcting-system
possessing an infinite, or a large positive or negative focal length, so that it does not
at all, or but inconsiderably alter the focal length of the two collecting-lenses com-
bined the said correcting-system being constituted by two or three single lenses
cemented together and arranged between, and separate from, the collecting-lenses.
By means of this disposition it is possible to considerably diminish the aforesaid
residual spherical aberration or zones."
At the same time it gives a wider scope in
the selection of the glasses to be employed in the manufacture of the lenses composing
the correcting-system. In particular it furnishes favorable conditions for the appli-
cation of those kinds of glass which have the property of almost completely
uniting three colors of the spectrum, or with other words, which neutralize the
secondary spectrum and thus produce a superior degree of chromatic correction.
The construction therefore yields comparatively ready means for the manufacture of
such highly achromatic objectives as are called " apochromatic" according to the
terminology introduced by Professor Abbe.
"
:
1.
In the annexed drawing an objective carried out according to our invention is
shown partly in outside view and partly in section. L₁ and L, are the two collecting-
lenses made in this case of concavo-convex form. The correcting-system consists in
the biconvex lens L, and the concavo-convex dispersing-lenses L, L, the two lattor
being composed of glass having such power of chromatic dispersion as is necessary to
compensate the chromatic aberration of the lenses L₁ Land L
According to the kinds of glass used in the construction of the described objectives
and according to the special conditions to be realized by the total result obtainable
from the system, a perfectly symmetrical arrangement of the lenses, such as is shown
by the figure, may, in certain cases, suffice; in others, where a wider range of means
for corrections or for removing objectionable reflexions is desired, a dissymmetrical
arrangement may be preferable.
In the following tables we append three numerical examplifications of the manner
in which lenses of given kinds of glass have to be ground for carrying our invention
denote the radii of
into ionaration
In those tables the letters »
2° 2°

page_text: curvature of the lens-surfaces marked by the said letters in the figure. d₁d, d, d. d.
are the central thicknesses of the lenses, and b₁ b₂ the respective air-distances between
the lenses L L and L. L. All dimensions are expressed as fractions or multiples of
the focal length of the entire system taken as unity. To obtain the dimensions of any
particular objective all that is necessary is to multiply the numerals given in the
tables, by the focal length required. The kinds of glass to which the tables refer, are
determined in the first example by the refractive indices n n and nai corresponding
to Fraunhofer's spectral lines D and F and the line Hy of the spectrum of hydrogen,
respectively, whereas in the second and third examples the indices no and np only are
employed.

page_text: 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.
In a photographic objective, the combination of two single collecting-lenses and a
compound correcting-system composed of single lenses cemented together, the said
correcting-system having a large focal length and being placed between the
collecting-lenses, substantially as described.
Dated this 21st day of Anvil 1800
J. H. JOHNSON & Co.,
Agents.