processed under a red light and the rate
of development checked.

One of the points most often mis-
understood in photography is that, except
in special branches, it is the art of repro-
ducing reflected light. In other words,
the amount of light reflected from the
subject and the surroundings is the con-
trolling factor in the amount of exposure
required. The general idea that a bright
day indicates a certain exposure is not
necessarily a fact. A bright area with
high reflectivity will cut down required
exposure, regardless of the general illum-
ination, and of course, vice versa. This
point is important in photographing areas
that have very poor reflectivity. Dark
colours tend to absorb light or to reflect
less light than bright colours and detail
in dark areas will be lost altogether unless
compensation is made for this fact. On
a bright sunny day, if the view includes
shadowed parts which are important, the
exposure should be calculated for the
dark parts, disregarding the bright por-
tions of the picture. This “high reflect-
ivity” angle should be especially studied
in relation to photographing the interior
of rooms and buildings or in any con-
fined area. Since most police photography
is concerned with the latter, a good deal
of attention should be paid to the colour
of the walls and ceilings, the furniture
and the type of illumination.

Licut Source HigHiy IMporTANT

The light source is a frequent trouble
maker. Pictures of the interior of rooms
are sometimes attempted with the camera
facing the light source, such as the
window or a table lamp. Generally speak-
ing, if the camera faces the light source
it causes halation on the film which, in
effect, is like a sunburst. This will block
out a large part of the detail on the
negative and usually renders it useless.
This same effect must be avoided when
using a flash gun. Care should be taken
that there is no brightly polished object
in front of the camera which will direct
the flash back into the lens. Place the
camera at an angle to any bright sur-
faces so that the light is reflected to one
side or the other, and when using room
illumination have the light source at the
side or back of the camera. If it is
necessary to include a window, block out
the light with a blanket and use what-
ever is available in the room for light.

Any lighting may be used, such as
gasoline lamps, electric lights, etc. An
interesting article on the art of “painting”
a room with a flashlight, is contained in
the 13th issue of THE SHOULDER STRAP,
under “Police Science”, page 30. It must
be borne in mind that the camera records
only a portion of the light available to
the human eye and the most important
sections should receive the most favour-
able lighting. Reflectors such as white
sheeting, large blotters, etc., can be used
to advantage to highlight certain spots.

SEVENTEENTH EDITION

Fundamentally, the plan to photograph
a scene or an object should start by
deciding the area to be photographed, the
light source and the amount of light
reflected from the walls, ceilings, etc.

The importance of depth of field is
not realized until one has to work at
short camera distances. As this feature
arises in all cases where pictures are
taken in rooms, buildings, etc., a schematic
drawing is attached to assist in explain-
ing the principle. The top illustration
shows a lens with the diaphragm at its
widest aperture. The farthest and nearest
objects on the right, being at different
distances from the lens produce a fuzzy
image on the film plane, while the ball in
the centre, being the object the lens is
focused on, produces a clear image. The
lower diagram shows what happens when
the diaphragm is closed down. The depth
of field increases to include the far and
near objects so that all are now in focus,
on the film plane. The importance of
this feature in ordinary picture-taking
may not be manifest, but it is essential
to use care in this regard in police work,
since objects in a room which may have
escaped the notice of the officer when
making the original investigation, may
become obvious later when examining
the picture and in order to be of any
value, must be in focus enough to render
detail identifiable.

The general rules of depth of field
follow:

(1) With a given focal length and
object distance, the smaller the diaphragm
opening the greater the depth of field.

! plane

Faca
°F pyramid.

Focal plane

(2) At a given object distance and
diaphragm opening, the shorter the focal
length the greater the depth of field.

(3) With a given focal length and
diaphragm opening, the greater the dis-
tance up to infinity the greater the depth.

Let us examine these rules with regard
to confined areas. Depth of field is not
a problem where the object 1s some dis-
tance from the camera so that we can
pass over the last rule, No. 3. Focal
length has a direct bearing on the size of
the film negative and generally speaking
the smaller the negative on a camera, the
shorter the focal length. The exception
to these rules are the telescopic lens,
the wide angle lens, etc. At one end of
this band, as it were, is the miniature
camera, the smallest being the 35 MM,
both movie and still. Depths of focus
rarely bothers this type of camera since
the focal length of the lens is so short
that it will give acceptable depth of field
in all but very close work. It is usually
advisable to use fine grain film and take
special care in the processing in order to
obtain the best possible prints. Since
fine grain film does not come in the fast-
est speeds it is necessary to lengthen the
exposure to allow for this, but the fact
that the lens can be used at the widest
aperture overcomes this disadvantage.

With the larger cameras and the con-
sequently longer focal length lenses used
in these cameras, depth of focus becomes
more of a problem and the larger the
camera the more the lens diaphragm
should be closed down to ensure adequate
depth. This will lengthen the exposure

of recrane\c. IRIS
FILM SEEN
OEP
PLANE ILLUSTRATION Pe orev ee

OF “DEPTH OF FOCUS”
(Schematic ~ Not to scale)

“STOPPED
DOWN”

DEPTH OF FIELD.

Page Seventy-five