Movies for TV - Early Television Foundation

142
FUNDAMENTALS
it is very similar to the human eye in its interpretation of perspec-
tive and general size in 16 mm work.)
The next size is a 50 mm lens which corresponds to 2 inches.
This has a speed of / 1.6 in the Cine Special range. At the dis-
tances previously mentioned, the field covered would be halved;
this means that at 15 feet the field is now only 2 feet 10 inches by
2 feet 1 % inches, and at 1 8 inches it would be a little more than
half the previous size. It will at once be seen from this that an
object photographed with a two-inch lens will be twice the size
of one photographed with a one-inch lens at the same distance.
These two are often sufficient to take care of all studio re-
quirements, but on occasion it is very nice to have a really longfocus
or telephoto lens available. This is generally a 152 mm, or
six-inch lens with an / value of about 4.5. (All the figures
mentioned are for Kodak lenses ;
other makes may differ slightly. )
There is a four-inch lens with a rating of / 2.7, but this is hardly
more than a very long-focus lens without the advantages of a
telephoto lens.
It will be noticed that as the focal length increases, the speed
decreases. A moment's consideration will show that as the number
of elements increases, so does the amount of light
lost in trans-
mission so the T rating will be higher together with the / value.
Since / is the ratio of diameter to focal length, it is obvious
that when the focal length increases, the ratio is bound to be- ;
come larger. This also accounts for the fact that telephoto lenses
and all long-focus lenses are larger in diameter than the smaller
ones.
There is another type used very often in the studio for shots
where there is not much room to get far back from the subject and
yet
it is essential to obtain a wide view of it. These are called wide-
angle lenses, and one is often included in place of one of the types
previously mentioned ;
the one discarded depends on the purpose
at hand. The Kodak 15 mm or %-inch lens has an opening of f\
2.7 and at 18 inches has a field of 10% by 8 inches. This is almost j
twice that of the one-inch lens. It will be noted that this is not^
exactly in proportion ; the wide-angle feature necessarily intro- 1