Movies for TV - Early Television Foundation

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LENSES 137
In addition the older film emulsions and exposures were quite in-
accurate so that the error in the / value was not important. Now,
however, emulsions are controlled very precisely and exposures are
determined by photoelectric means so that the margin for error
is much less. Therefore the / value error is much greater by con-
trast, and a better means of measurement was requested.
Probably the greatest error occurred in the assumption that
there was 100 per cent light transmission in every lens with the
same / stop. Every time there is an air to glass surface, light is lost
by reflection; some glasses absorb more than others, and the more
elements there are in a lens, the more light will be lost. The Bell
& Howell Company brought out the T system:
it seems to the
author coincident with the upsurge of television in 1948. The T
stop is a pure measurement of the amount of light
transmitted and
is calibrated by an electronic measurement at the time the lens is
manufactured so that the real transmission is recorded. Because
the T system takes losses into account by the very action of meas-
uring the light transmitted, it is obvious that it will be very slightly
slower than the / rating for the same lens. Probably a new generation
of photographers who have never heard of the / stop and
rely only on the T figures will arise. What does it matter? They
will still forget to remove the lens cap on occasion!
One last note on / and T stops; for every aperture there is a
minimum distance that must be maintained between the subject
and the camera. This varies according to the setting of the speed-
controlling diaphragm; as this opens so does this minimum distance
increase together with a decrease in the depth of field. This
means that in conditions of poor light when it is necessary to use
a wide-open lens, focussing of the lens is more critical and greater
care must be used to ensure that all areas of the frame are in
focus.
These are all problems which will apply more to the cameraman
than the producer and there are any number of excellent
books written on the use and operation of cameras and lenses
which explain in greater detail exactly what to do about these
effects.