Improvements in Photographic Lenses.
Year: 1895

page_text: Date of Application, 5th Sept., 1895
Complete Specification Left, 4th June, 1896-Accepted, 4th July, 1896
PROVISIONAL SPECIFICATION.
Improvements in Photographic Lenses.
I, HUGH LANCELOT ALDIS, Secretary of J. H. Dallmeyer, Limited, of
25 Newman Street, in the County of Middlesex, do hereby declare the nature of
this invention to be as follows:-
The lens forming the subject of this invention is made up of two compound
lenses, the front one being a deep strongly converging meniscus lens with strong
positive spherical aberration but being approximately correct for chromatic
aberration whilst the rear one is a diverging system with strong negative spherical
aberration to correct the positive aberration of the front lens and also approximately
correct for chromatic abberration, the whole forming a converging system correct
for spherical and chromatic aberrations and free from distortion.
The front lens is preferably made of two lenses cemented together as follows:-
The first a double convex lens of crown glass of high refractive index whilst the
other is a double concave lens of flint glass of about the same refractive index as
the crown glass. In some cases it is necessary to cement behind these a
meniscus crown glass also of about the same refractive index as the other two.
The rear lens consists of two lenses separated by a smali air space the front
lens is a thick nearly plano-convex of crown glass of high refractive index and
the rear lens a concavo-convex lens of flint glass of low refractive index.
Preferably however either or both of these lenses may be themselves double
cemented combinations of a double concave flint of low refractive index and a
double convex crown of high refractive index.
For some purposes the whole lens may be turned round end for end.
Dlhy of Santambar 1805
Improvements in Photographic Lenses.
I, HUGH LANOELOT ALDIS Secretary of J. H. Dallmeyer, Limited, of
25 Newman Street, in the County of Middlesex, 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 lens forming the subject of this invention is made up of two compound
lenses, the front one being a deep strongly converging meniscus lens with strong
positive spherical aberration but being approximately correct for chromatic aberration
whilst the rear one is a diverging system with strong negative spherical aberration
to correct the positive aberration of the front lens and also approximately correct
for chromatic aberration, the whole forming a converging system correct for
spherical and chromatic aberratious and free from distortion.
For some purposes the whole lens may be turned round end for end.
In order to obtain a flat field free from marginal astigmatism in an optical
system composed of a number of lenses ranged along a common axis it is

page_text: (P1 − P₂) + ª² —7——¹³ (p's — p'a) +---
-
which I call the "astigmatic
practically small.
constant " should be theoretically zero and
For this three conditions have to be observed as far as possible :-
:―
(1) The converging lenses of the system should be made of glass of high
refractive index and the diverging of low refractive index.
(2) The diverging components should be separated by a considerable interval
from the converging components.
(3) Thick meniscus lenses should be used.
Of these the first is the most important, the other two can only be employed
in a limited degree with good results.
There remains the consideration of the elimination of spherical and chromatic
aberrations.
In lenses hitherto constructed the spherical aberration is eliminated by cementing
together a converging lens of low refractive index and a diverging lens of high
refractive index. The objection to this method of correcting the spherical
aberration is that it ipso facto increases the astigmatism being contrary to the
first principle above cited.
Accordingly in the lenses here described a different method of curing the
spherical aberration is adopted, which is briefly as follows: One of the components
of the system is as a whole a diverging system and it is further composed of a
meniscus diverging lens and a meniscus converging lens, the two however not
being cemented together but separated by a convex meniscus air space: the
curvature of the surface of the diverging lens being greater than that of the
converging lens which is next to it. The converging lens is of glass of high
refractive index and the diverging of low. Correction of spherical aberration can
by this means be attained without any sacrifice of the flatness of field from
astigmatism and accordingly by this means I have constructed optical systems
with much shorter foci relatively to their apertures than any hitherto constructed
without sacrificing the correction of marginal astigmatism.
There remains the question of chromatic aberration. In a converging system
of converging components of high refractive index and diverging components
of low refractive index the converging components should be made of glass of low
dispersion, and the diverging components of high dispersion to obtain freedom from
chromatic aberration.
By the use of baryta salts as glass fluxes it has been found possible to
manufacture satisfactory glasses with a high refractive index and a low dispersion:
but up to the present no satisfactory glasses have been obtained with a low
refractive index and a high dispersion, consequently to obtain correction of the
chromatic aberration in the systems I and II hereinafter described one of the
diverging component lenses is made of glass of high refractive index and in
system III two of the diverging component lenses are of glass of medium
refractive index.
The drawings show three examples of lenses constructed in accordance with
this invention.
In the following tables R₁ R. &c. are the radii of curvature of the surfaces
and t₁ t₂ &c. their distances apart both being expressed as fractions of the focal length.
The lens shown in Figure 1 has ratio of aperture to focus of 1: 3.5.
The aperture of front and back lenses
The aperture of centre lens = .245.
= •291.

page_text: 4. A photographic lens substantially as described and shown in Figure 1 of the
drawings.
5. A photographic lens substantially as described and shown in Figure 2 of the
drawings.
6. photographic lens substantially as described and shown in Figure 3 of the
drawings.
HUGH L. ALDIS.