Los Angeles herald., 1910-08-28
THE ART AND PAsTIME
OF PHOTOGRAPHY

of photography and one perhaps in
dulged in by relatively few workers is
telephotography, or long-distance pic
ture making. This field is open to any
one possessing a camera with from
twelVe-inch extension upward, and can
be enjoyed without incurring any
serious expense. The lenses fitted to
most cameras of the Century, Premo
or similar varieties are, ordinarily,
rapid rectilinears of the symmetrical
type; that is to say, having the front
and rear combination of equal foci.
These will permit of some degree of
long focus work by removing the front
lens, leaving the other half behind the
diaphragm. In this case distant objects
can be magnified about so per cent.
Very often great improvement in pic
lures of distant objects can be made
thus. 1t is possible to secure lenses of
unequal foci, so that the front lens is
of longer focus than the rear one, and
by placing the former behind the
diaphragm and using it alone about
100 per cent increase in size of image
is secured. thus bringing up ones sub
ject to one-half the actual distance.
Such subjects as mountain scenery,
sea views with vessels in the distance,
etc, are far more interesting when so
made. To obtain still greater magni-
fication specially constructed telephoto
attachments are necessary, and these
are of several types and makes If
the camera is fitted with an anastig-
mat lens telephoto lens can be fit
ted that will yield images of from two
to eight diameters enlargement with
Is to 20 inches bellows extension.
Telephoto lenses, such as made by
Beck, Dallmeyer, Ross, Goerz, Bausch
& Lomb, Voigtlaender, etc, are avail
able at a cost varying from SIS up
ward and can be applied to the regular
positive lenses very easily, and when
once fitted can be used by any care
full worker.
The principle of the negative .or
telephoto lens is simple It's function
is to magnify the image of the posi-
tive lens to the degree desired up TO
the limits imposed by the available
bellows extension. To obtain much
magnifcation of the image of very
distant object would require lens of
great focal length and a bellows eX-
tension of more than would be possible
in the compact cameras used by most
amateurs, and for this reason, that
the degree of magnification depends
entirely upon the focal length of the
lens. For instance. suppose we have
a building, a ship or some object I'M
possible to photograph satisfactorily
from any near position, and yet we
require as large an image as our plate
will permit, a lens of fiVe-inch focus,
at certain distance, will give an
image. say, of one inch high, one OF
ten-inch focus two inches, one of
twenty-inch focus four inches high ana
so on, it follows that the larger the
image desired the longer must our
camera extend, until the portability
becomes almost impossible; now, by
the use of telephoto lens we can
secure the same magnification within
very much shorter limits, and further,
we can vary the degree of enlargement
at will with no trouble by merely
varying the distance between the mega
tive and positive elements of the
lenses.
The negative element has its optical
center considerably forward of the
lens itself, hence outside of the camera,
and this obviates the necessity Tor
long extension of bellows behind. The
exposure must necessarily be increased
according to the magniication, and
this is calculated by multiplying the
diaphragm number of the positive
lens by the number of diameter OF
magnification; thus, suppose our posi-
tive lens is stopped to FIG and we are
magnifying four times, then our eX-
posure must be based on an F value
of F64 and requires 16 times the ex-
posure From this it will readily be
seen that a very solid tripod should be
used to insure perfect immobility OF
the camera.
Several of the leading lens makers
have introduced telephoto lens that are