Improvements in Photographic Lenses.
Year: 1891

page_text: Date of Application, 15th Dec., 1891
Complete Specification Left, 14th Sept., 1892-Accepted, 10th Dec., 1892
PROVISIONAL SPECIFICATION.
Improvements in Photographic Lenses.
I, THOMAS RUDOLPH DALLMEYER, of 25 Newman Street in the County of
Middlesex, Optician, do hereby declare the nature of this invention to be as
follows :-
My invention relates to a lens for the production of large primary images by the
employment of a powerful negative element, as a posterior lens, fractional in its
focus as compared with the auterior positive lens.
I employ a double combination negative element, the combinations being
preferably symmetrical and capable of being separated, more or less, from one
another.
This combination is corrected for actinism as well as for spherical aberration,
and is although an entirely negative system free from distortion.
Two forms suitable for the doubie combination negative element are represented
in the drawings annexed. In Figure 1 the exterior elements are of the higher
dispersive medium and of longer positive focus. The interior elements are of the
lesser dispersive medium and of shorter negative focus; the whole system being
aplanetic and actinic and of negative focus.
..
In Figure 2 the exterior elements are of the lesser dispersive medium of shorter
negative focus. The interior elements are of the more dispersive medium of longer
positive focus; the whole system being aplanetic and actinic, and of negative
focus.
Such systems are employed in connection with entire lenses composed of any
number of elements corrected centrally and marginally in themselves and of positive
focus. By the addition and proper adjustment of the new negative combination,
an enlarged image corrected throughout the field marginally as well as centrally is
produced.
Dated this 15th day of December 1801
:
COMPLETE SPECIFICATION.
Improvements in Photographic Lenses.
I, THOMAS RUDOLPH DALLMEYER, of 25 Newman Street in the County of
Middlesex, Optician, 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 a lens for the production of large primary images by the
employinent of a negative element as a posterior lens, fractional in its focus as
compared with the anterior positive lens. This negative element is a double com-
bination and is corrected for both actinic and spherical aberration. The annexed
drawing at Figure 1 shows a combination in accordance with my invention.
The anterior positive element represented in this figure is a well known portrait
lens, it is in four parts a¹ a² a³ and and of these the hindermost marked a can be
screwed out a short distance. The focal length of the combination is 8 inches and
its aperture 24 inches. The negative element is in four parts which con-
stitute a symmetrical double combination and are similar and so also and 4.
The parts
One of the pairs is shown separately and in full size in Figure 2.
and are of hard crown glass (refractive index 1.5179) and * and ³ are of heavy
flint (refractive index 1-6202). The anterior surface of 6' is convex, and the radius

page_text: of curvature is 4.396 inches. The posterior surface of 61 is concave and the radius
of curvature is 0.9 inches. The anterior surface of 6 has the same curvature and
surface of b¹.
is cemented to the posterior
The posterior surface of
has a curvature 1.375 inches.
There is an air space between the surfaces of 2 and 63 and these surfaces may be
separated by a considerable interval, but the study which I have made of the
subject since the date of my application leads me now to prefer that 12 and 1³
should be nearly in contact at their edges.
The negative focal length of the element is 2 inches and it is so
mounted that by a rack and pinion the distance between it and the anterior
element can be varied.
The correction of this negative element is such that it is complete for an
object 12 feet in front of the camera focussed at a distance of 12 inches. When
any deviation is made from these positions there is a resulting aberration in
the combination which can be corrected by screwing out the lens at a short
distance. The measurements which I have given above admit of variation.
For the positive element any (preferably large aperture and short focus)
photographic objective may be employed, but it is expedient that it should
admit of the spherical correction being varied.
The negative element is in all cases to be a double combination corrected
for actinism and spherical aberration; it is to be convex on its anterior and
posterior faces and its focal length is to be less than that of the positive
element.
The negative element may be constructed as indicated in Figure 1 of the
drawings accompanying my Provisional Specification, the heavy flint glass being
external in place of internal, but the arrangement above described is that which
I prefer.
In the use in the camera of the instrument represented by Figure 1, the
focussing glass is first set at any distance from the posterior face of the negative
element, according to the amount of the magnification it is desired to obtain.
Then the focussing is effected by moving the positive anterior element to or from
the negative posterior element by means of the rack work provided for the
purpose. Finally the sharpness desired is given to the focus by unscrewing the
posterior lens a' of the anterior element to the necessary extent.
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 :-
A photographic lens for the production of large primary images, and consisting
of an anterior positive element, and a posterior negative element the latter a double
combination convex on its anterior and posterior faces corrected for actinism as
well as for spherical aberration, and of shorter focal length than the anterior
positive element.
Dated this 14th day of September 1892.
THOMAS R. DALLMEYER.