Improvements in Instruments for Calculating Photographic Exposures.
Year: 1894

page_text: Date of Application, 22nd Dec., 1894-Accepted, 9th Feb., 1895
COMPLETE SPECIFICATION.
Improvements in Instruments for Calculating Photographic
Exposures.
I, ALFRED WATKINS, of The Imperial Flour Mills, Hereford, Master
Miller, do hereby declare the nature of this invention and in what manner
the same is to bo performed, to be particularly described and ascertained in and
by the following statement :-
My invention has for its object an improved construction of the instruments for
calculating photographic exposures described in the Specification to my Pateuts
Nos. 1388 of 1890 and 14364 of 1893, whereby such instruments are rendered of a
more convenient shape and size, there being only one moveable part and the
instrument being by preference made similai in form to a watch, and the scales
being modified to suit the altered form, and the long narrow stip of sensitive papei
or film being coiled within the inner circumference of the case and arranged to
appear at an opening as will be presently described Also a pendulum is provided
in the shape of a watch cham of suitable length for swinging half seconds.
The factors involved in the calculation and indicated on scales on the instrument
are the same as those described in the Specification to Patent No. 14364 of
1893, namely P the speed of the plate, D the relation of the diaphragm of the lens
to its focal length, A the actinic value of the light falling upon the subject as
indicated by the sensitive paper, and lastly E the exposure scale
I will describe the arrangement of these scales upon the improved instruments
with reference to the accompanying drawing, in which Fig. 1 shews to an
exaggerated scale, for the sake of clearness a back view of the same when
constructed in the form of a watch, Fig. 2 shows a front view, Fig. 3 a similar
VIOW with the front cover removed and Fig. 4 a back view with the back plate
removed and Fig. 5 an inside view of the front cover.
a is a casing, similar in form to a watch, having a handle b. The back plate is
formed of the fixed outer annular part c, and the central revolvable part d. On
the periphery of the latter are marked two scales namely the diaphragm scale D
and the actinometer scale A while on the meeting edge of the fixed part care
marked the other two scales P and E, these being so aanged that the numeral 1
of the one scale coincides in position with that of the numeral of the other scale,
by which aangement the scale P can also be used as a continuation of the
E scale for all values below one second, such values being expressed as the
denominator of a vulgar fraction of which the numerator is 1, so that 2 repre-
sents, 3, and so on.
For making a calculation with the scales so arranged the disc d 15 turned until
the diaphragm value D is opposite the proper division on the speed scale P, then
the division of scale A representing the actinic force of the light as determined
by the actinometer will be opposite the division on scale E indicating the proper
time of exposure, this division being to the right of the numeral 1 if the time is
more than one second, and to the left of 1 if less than a second
In the above arrangement of the scales it is assumed that the subject that is
being photographed is of average color value (as is usually the case). It it should
not be of average color, an allowance will have to be made from the indicated
exposure.
Thus an open on panoramic landscape only requires half the indicated exposure,
now scenes of white objects one quarter and clouds one-tenth,

page_text: On the romoveable front cover e of the instrument is provided the actinometer
This is formed in a similar manner to that described in the Specification to my
Patent No. 14364, Damely consisting of a mask by preference of black colo
having a cential opening one half of which is filled by a piece of paper having
the standard tint while through the other half appears the sensitive strip /, this is
coiled 1ound in an annular space formed in the periphery of the case, the end
being led through a slit 7' in the central cavity and after hoing twisted one quai ter
round it is passed through a second slit at k to the outer side of the plate I
formning the bottom of the cavity, whence the strip is led to the outside through
The hont
a slit at in, so that it can be drawn past the opening of the actinometer
glass protecting the actinometer is by preference of a blue shade as described in
my Patent No. 14364. On the inner side of the mask I provide a second piece
paper or film n Fig 5, having a tint preferably of such a depth that the sensitive
strip will darken to match it in one half or one quarter the time required to darken
to the hue of the full standard tint
Thus when the darker standard tint is in use, the lighter one is out of sight and
when the lighter one is requined, the mask ŕ is turned over so as to bring it
outside, the darker one being then out of sight The object of this mangement
19 that it is of great importance that only one tint should be presented to the eye
at one time, in order not to impair the accuracy of judging the tint of the sensitive
strip. The lighter tint is used in such cases when, for photographing interiors it
is desirable to use such a diaphragm that the camera exposure shall be equal to the
actmometer exposure, in which cases the diaphragm proper for use with a light
tint is often of a more convenient size than that propei for use with the usuaļ
standard tint.
The light tint may be distinguished by a different form or by some distinctive
mark on the face of the mask.
1
I make the double reversible tint by cementing sheets of the light and dark tints
back to back, and culting this up into pieces of the required size, one of which is
then enclosed between two exactly similar dark discs with cential opening, of which
the tint occupies exactly one half.
The ordinary watch chain o Fig. 1 is made of such a length, that when held
suspended it will swing in half seconds.
It will be evident that the position of the four scales arranged as above described
might be reversed, P and E, being on the moveable part d, and D and A ou the
casing, also that they might be used on a straight slide rule, either as indicated at
Fig. 6, or with the scales P and E ariauged on opposite sides of a slide carrying
the scales D and A, as at Fig. 7, or the scales may be arranged in the last described
manner on a circular slide rule as at Fig. 8, which may be on a case containing
the actinometer in a similar manner to Fig. 1.
With the straight arrangement the actinometer would be in a separate box
Having now particularly described and ascertained the nature of my said inven
tion and in what manuer the same is to be performed, I declare that what
claim 18 -
1. In an instrument for calculating photographic exposures arranging four scales
representing respectively the plate, the diaphragm the actinometer and the exposure
in such manner that the first and fourth named scales are on one part of the
instrument while the second and third nemed ones are arranged on another part, the
one part having a sliding motion clatively to the other pait substantially a
described.
2. In an instrument for calculating photographic exposures, arranging the plate
scale in such position relatively to the exposure scale that the position of the
unit numerals coincide, so that the plate scale divisions can also serve as the
divisions of the exposure scale representing vulgar fractions below one second
substantially as described.
3. In an actinometer for photographic purposes in which a ribbon of sensitive

page_text: paper or film 18 arranged to pass behind an exposure opening, arrauging the coil
of the sensitive ribbon with its axis at right angle to the plane of the opening, and
causing it to pass with a quarter twist from the coil to the exposure opening
substantially as described.
4 In an actinometer for photographic purposes constructing the mask cauying
the usual standard tint for comparison so as to be reversible and providing on the
back side thereof a tint of a lighter or darker hue substantially as and for the
purposes described.
5. An instrument for calculating photographic exposes, combined and operating
as described with reference to Figs. 1 to 5 of the drawings.
Dated this 22nd day of December 1894.
ABEL & IMRAY.
Agents for the Applicant.