Movies for TV - Early Television Foundation

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FUNDAMENTALS
thus the greater the diameter of the lens, the smaller the number
representing / and the more light that is admitted.
Each / value or is stop ("stop" a derivation from the early lenses
which actually used stops to prevent the lens' adjustment from
being changed inadvertently), progressing in decreasing numerical
value increases the amount of light passed by 100 per cent, so that
the smaller the / number, the larger or faster the lens. About the
smallest aperture used in normal operation is / 22 and at the other
end of the scale we find lenses as large as / .9. A standard lens may
be called an / 2.8, i.e., maximum opening, and be calibrated in a
series of stops as follows: / 2.8, / 3.5*, / 4.5*, / 5.6, etc. Note that
some of these numbers are half stops: that is, they do not increase
the light by 1 00 per cent. The half stops are marked with an asterisk
(*). There are, of course, many of these but those shown are perhaps
the most used. The speed varies inversely as the square of the
about four times slower
stop number. This means that / 8 is than / 2.8.
just
Thus it is seen that for any type of lens, be it telephoto, close-up,
or wide-angle, the amount of light passed by the lens may be ex-
pected to be the same for the same / number. This makes matters
meters since it
very easy for the photographer and maker of light
eliminates a variable. However, as usual, people became tired of the
good old system of / numbers which served for so long and with the
advent of television it was decided that the light transmission figure
was of more value than the ratio of diameter to focal length. There
is a lot of merit in this proposal, and though it is always difficult to
change to something new and not very well known, it is likely that
in the future T stops will come into general use. At present many
lenses are made with both calibrations marked on the barrel.
Since the T series is essentially a light transmission measure,
it is based on the amount of light that will pass through the various
elements of the lens. Quite briefly, the idea behind the new
system
was so much error introduced due to irregular light transmission
and refraction and reflection losses that the ratio no longer was
is that with the advent of new multi-element lenses there
a reliable indication of the amount of light which passed through.