Improvements relating to Photographic Lenses.
Year: 1899

page_text: Date of Application, 12th Dec., 1899-Accepted, 17th Feb., 1900
Improvements relating to Photographic Lenses.
I, THOMAS RUDOLPHUS DALLMEYER, of 25, Newman Street, Oxford 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 :-
The object of this invention is to provide an optical means of increasing the size
of the image formed by any ordinary converging lens, without necessarily diminishing
the intensity of the illumination.
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Throughout the specification I employ the terin lens as including compound
lenses or lens systems.
In all optical systems hitherto constructed for photographic purposes, direct
enlargement of the image has been brought about as follows:-
(A). By "primary" magnification, in which case a converging lens is combined
with a diverging lens, forming what is known as a "telephotographic lens," the
image is direct and inverted, its illumination being decreased in proportion to the
area of the enlarged image as compared to that given by the converging lens alone,
that is to say, as the square of the linear magnification.
(B). By "secondary" magnification, in which the primary inverted image formed
by an ordinary converging lens is subjected to enlargement by a second converging
lens, re-inverting the image, and forming an erect enlarged image. Here again the
illumination of the latter is decreased in proportion to the square of the linear
magnification.
In both cases it has hitherto been customary to employ a converging lens of given
initial intensity as a basis for subsequent conversion into an enlarging system by the
addition of a posterior enlarging system as described under paragraphs (A) or (B)
where the initial intensity of the converging lens rapidly diminishes with the degree
of enlargement.
My invention consists in an adjunct or addition of an optical enlarging system to
the anterior portion of any converging lens, whereby:
(1). I avoid the proportionate loss of illumination in the enlarged image necessi-
tated by the old methods.
(2). I may even maintain the initial intensity given by the converging lens alone
at its focal plane, when the enlargement is not great.
My present object in particular is to assist the user of the "hand camera," and the
smaller sizes of cameras, to avoid "exaggerated" perspective by enabling the operator
to be at a greater distance from the foreground of his subject; and yet maintain a
sufficiently large image; also to obtain, by the addition of the enlarging system to
his lens, enlarged details in subjects of general interest included by the ordinary lens
used alone; distant objects &c.
The invention may, nevertheless, be applied to any ordinary lens to attain the ends
of a telephotographic system with greater mechanical convenience: the additional
system to screw into the place of the "hood" of the lens.
Figures 1, 2 and 3 are diagrams illustrating one way of carrying out my invention.
Figure 4 illustrates a modification.
A is an ordinary photographic lens, and B and C are the lenses inaking up the
enlarging system. B and C are mounted in tubes D and E sliding one within the
other so that the distances apart of B and C can be varied as shown in Figures 1, 2

page_text: and 3. Figure 4 is similarly constructed but it has not been thought necessary to
show more than one position.
The lens R should be of the highest intensity possible, that is to say, of the largest
diameter and shortest focal length convenient, so that when combined with C for a given
linear magnification, the initial intensity of the lens A to which it is added, may be
maintained, or at any rate diminished in a less degree than would be the case with
any posterior attachment to the original converging lens A.
The lens C should be of shorter focal length than that of the lens A, again
maintaining the largest diameter and shortest focal length possible, so as to maintain
in combination with B the initial intensity for a given linear magnification when this
is feasible.
The power of the diverging lens C (Figures 1, 2 and 3) should preferably be
equal to or greater than the sum of the powers of the converging lenses A and B.
According to the degree of magnification required, I decide the ratio of the
focal length of B to that of C.
When the lenses B and C are placed at the normal separation, that is at a distance
equal to the difference of their focal lengths as shown in Figure 1, parallel rays
incident upon B will, after refraction at C, emerge parallel. Now if we place this
supplementary system in front of any ordinary lens A, which has been focussed for
parallel rays (forming an image at F in its focal plane), it is obvious that the addition
of this system, which transmits parallel rays, will not alter the plane of the focus,
but will increase the linear dimensions of the image in a ratio equal to the ratio of
the focal lengths of B and C.
If the effective diameter of the aperture of the lens B is greater than that of A,
it is evident that the loss of illumination, due to magnification may be compensated
for partially or entirely.
If the ratio of the effective diameters of B and A is equal to the ratio of the focal
lengths of B and C the illumination is identical, if less the illumination is diminished,
supposing always that the full pencil of rays incident upon B is transmitted by C,
which can be accomplished.
If the lenses composing the supplementary system are brought nearer together as
shown in Figure 2 it becomes a system of variable diverging focal length, transmitting
divergent rays from any object to the lens A, thus permitting a still more enlarged
image to be received at F any distance beyond the focal plane of the lens A
depending upon the separation between B and C for the time being.
If the lenses B and C are separated by any interval greater than the normal
separation as shown in Figure 3, it is obvious that it becomes a system of variable
converging focal length. Parallel incident rays upon the supplementary system now
emerge convergent, causing the image to be received upon a plane nearer to the
lens A than its focal plane. In this case it will be seen that the linear magnification
will be less than the ratio of the focal lengths of B and C, but the illumination may
be greater than that of the lens A used alone. With the lenses separated as in
Figure 3, it is evident that any near object may transmit parallel rays to the
lens A with an enlarged image at its focal plane.
It will be evident that near objects may be photographed with B and C arranged
as shown in Figure 2, but the plane of the image must be beyond the focal plane of
the lens A.
The lenses B and C are each preferably aplantic. The form ascribed to the lens B
will determine that to be ascribed to C. Figures 1, 2 and 3 illustrate one of many
forms convenient for carrying out the object of this invention; the details of the
lenses are as follows:-
Bis a cemented combination
Dinmotor 1.5" hain trian that
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page_text: 2. The combination with a photographic lens of an enlarging system placed in
front of it and consisting of two lenses.
3. The combination of a photographic lens, an enlarging system placed in front of
it and consisting of two lenses and means for varying the distance apart of these two
lenses.
4. Optical means for increasing the size of the images formed by photographic
lenses substantially as described and illustrated in the drawings.
Dated this 4th day of December 1899.
THOMAS R. DALLMEYER.