Improvements in Certain Photographic Lenses.
Year: 1899

page_text: Date of Application, 24th Jan., 1899-Accepted, 25th Feb.,
1899
Improvements in Certain Photographic Lenses.
I, HAROLD DENNIS TAYLOR, Buckingham Works, Bishophill, York, Optician and
a Director of the Company named "T. Cooke & Sons Limited," also of the above
address, 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 :-
It is often of the greatest importance to photographers to have the power of
In the case of rectilinear lenses,
altering at will the focal lengths of their lenses.
each half of the lens may itself be used as a lens of about double the focal length
of the complete combination. But there are lenses of other types, especially that
known as the "Cooke" lens, in its various series, whose constitutent lenses cannot
be separately employed for photographic purposes as lenses differing in focal length
sufficiently from that of the complete combination, and this invention is for the
purpose of supplementing this deficiency in a sufficiently simple and effective
manner. In the case of all the "Cooke" lenses described or comprised in the
Patent Specifications No. 22607, of 1893 and No. 15,107 of 1895, the back lenses of
the combinations are elementary or simple, and the essence of this my invention is
the device of making these simple back lenses interchangeable with one or more
supplementary simple back lenses made of glasses differing in dispersive power and
varying in focal length substantially as their dispersive powers, thereby keeping
the state of achromatic correction practically constant, while permitting of very
considerable alterations in the focal lengths of the whole combination. These inter-
changeable back lenses may be either mounted each into a separate cell to screw
into the back of the lens mount, in place of the normal back lens or may be mounted
in a sort of wheel, sector or swivel, all equally distant from the centre of such wheel
etcetera, each lens in turn being brought into correct position behind the permanent
parts of the Cooke lens by means of rack or lever and a light spring detent. Or
they may be mounted two or three together in a straight slide working to and fro.
Various other methods of mounting or interchanging these lenses may be devised.
The changed equivalent focal length may be expressed by the following formula
where
F:
-lens.
-
2=
the equivalent focal length of the whole combination-with-its normal-back
the distance from the back lens of a point on the optic axis to which or from
which the axial rays are converging or diverging before entering the
back lens: u is positive if the_rays, are divergent--and--negative -if-they-are
convergent.
bis the focal length of the normal back lens.
c is the focal length of the supplementary back lens.
x is the required focal length as modified by the use of the supplementary lens
(whose focal length is c).
-b
-26-
C

page_text: 25th
If the rays entering back lens are parallel and therefore u = infinity, then X is
simply F. The rays are diverging before entering the back lens of the Cooke lens
series 111ª described in Patent Specification No. 2260793 pages 10 and 11 and the
back lens thereof is made of a borosilicate crown, having the very low dispersive
1
power of
for rays D to G. This back lens may be changed for one of extra
50.5
dense flint glass, which may be of double the focal length of the normal back lens
since its dispersive power is double that of the borosilicate crown, or for
the same
rays D to G. In this case doubling the focal length of the
back lens more than doubles the focal length of the whole combination,
as may be seen below. I will now give a list of schedule indicating
the curves of several interchangeable back lenses for use in conjunction with various
Cooke lenses. All the radii of curvature are expressed in terms of the equivalent
focal length of the normal combination or Cooke lens to which they are applied.
Therefore to obtain the actual radii of curvature of such interchangeable lenses, the
figures given for radii must be multiplied by the equivalent focal length of the
normal combination for which they are intended. All refractive indices are for
the D ray and all dispersive powers for the rays D to G.
curves +1.23 and + 1.23 (equi convex).
This lens in place of the normal back lens will increase the normal focal length
by about 16 times.
2. Another interchangeable lens may be inade of extra dense flint glass having a

page_text: Refractive index
1.6527 and
1
Dispersive power ==
25
curves +3.06 and +2.04.
This lens, replacing the normal back lens will increase the normal focal length
by about 15 times.
The above are the interchangeable lenses likely to be the most useful. Of course
other interchangeable lenses may be worked out which will give more moderate
changes of focal lengths should they be desired. It would scarcely answer, how-
ever, to resort to a flint glass at all denser than that above specified, of which to
make an interchangeable lens for a still greater elongation of focal length.
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 :-
The application to photographic lenses of the character herein described,
possessing simple back lenses, of interchangeable simple back lenses made of various
glasses and varying in their focal lengths approximately as their dispersive powers
for the spectrum interval D to G, whereby considerable variations in the equivalent
focal lengths of the whole combinations may be obtained; such interchangeable
lenses being either mounted in separate cells for screwing or otherwise fitting into
or onto the back of the compound lens mount, or else mounted on a-slide, wheel;
sector or other mechanism whereby they may be quickly interchanged.
--
Dated this 12th day of January 1899.