Los Angeles herald., 1910-06-26
Photography


indicator for distance and a pointer
under the frame carrying lens, and on
the indicator or scale are marked sew
eral lines labeled 100 so, % and so on
up to or 6. These numbers refer to
distance in foot measure for quickly
setting the lens at the right place to
give an image of objects at such dis
tances. Why should these be needed
and why do the markings vary? Our
question will soon be answered If we
go back to our focusing screen, for we
see that as we move our lens forward
from the 100-foot line that objects at
that or greater distances begin to lose
clearness or definition, while those
nearer begin to come out of confused
blur and assume more or less perfect
form and definition. Here then we
learn that lens will give certain and
definite image of varyirgly distant OBJ
jects at varying distances from our
ground glass. The act of doing this is
called focusing by the ope. ator. 1t
follows that the more carefully we adj
just our lens in relation to our object
the more faithful will be our resulting
picture. Another thing we may leaIn
is that the image can be varied in
sharpness by altering the diameter of
it's opening by means of the diaphragm,
or stops so-called, which are fitted into
the shutter between the front and rear
lens.
Let us focus on a landscape, for in
stance, in which we have many objects
lying between twenty-five feet and 100
feet or even more. Our lens, If we are
using q As camera, will have to be
drawn forward six or six and one-half
inches from the ground g.ass to give
definition anywhere and this point or
position is referred to as the universal
or in1nity focus, for the reason that
when so focused all rays of light from
extremely distant objects or infinity. so
to speak, are in sharp definition and
will be at their best. Nearer objects,
say at fifty or forty feet, will not be so
clear. and those at twenty-five feet
less still. This does not satisfy us, so
we will try to improve their appearance
by bringing into use our reduced
diaphragm, which we do by sliding a
little indicator just below the front
lens until the opening is about half the
full size. This indicator will rest over
a number stamped on dial plate read
ing 4, s, 16, 80, GA and perhaps 1913. We
probably have placed indicator over
No. 16, and on examining our screen
again find it less bright than before,
but that objects are all better defined
at closer range. Perhaps we are still
unsatisfied, so we will try smaller
opening and push pointer to No. 80.
shall find definition prefectly good from
even fifteen feet out to infinity. We
find therefore that we gain sharpness
or depth of definition at the cost of
illumination, therefore if we expose a
plate or film with the lens opening so
reduced we must increase the length of
our exposure. This increase is in exact
proportion to the decrease of illumina-
ton, For instance, If say lie second
would be right for our No. stop, No.
32 being one-half the diameter or one
quarter the area, would require four
times the length of time or 1-25 sec
onds.
1t is possible, if by reason of slight
movement in our subject this would be
ready, to reduce the exposure to 1-50
second, and we would accomplish this
by changing the position of our lens to
point little forward of the 100-foot
line to about fifty feet and open our
lens to No. 16. This would divide the
focus between near and distant points,
and while perhaps sharpness might be
slightly less the gain in rapidity woul1
warrant the compromise.
Most of the small cameras under the
AS size are usually of the type known
as fixed focus" and our experiments
have shown us how the focus may be
fixed" Practically any camera up to
this size can be used as a fixed focus
camera if we reduce the diameter of
lens sufficiently and expose according
ly. This explains why the little box
kodaks, etc., are not suitable for TAX
ing moving objects, 1. e., their lens are
stopped down" to give fairly univer-
sal definition and the shutters speeded
to about all the speed the small OPEN
ing will permit, about 1-10 seconds. One
should really serve an apprenticeship
with a focusing camera in order to in