Movies for TV - Early Television Foundation

CHAPTER
LENSES
'n this chapter the principle of the lens and its use will be covered
together with the need for it. Discussing the last point first, it is
seen that the purpose of the lens is to focus the rays of light and
picture information on to the sensitized film in the camera (or,
in the case of a projector, to direct the rays of light from the film
in the light gate onto the screen) .
Strictly speaking, a lens as it is known today, consisting of
two or more pieces of curved glass, is not needed. The first camera
made use of the pinhole lens. This was merely a hole pierced by
a pin in a sheet of metal foil a few inches in front of the photographic
plate. The extremely small size of the hole allowed the
rays of light to enter the film compartment and impinge on the
plate and still remain in their respective positions.
Perhaps some readers may remember the direction given in
old children's wonderbooks on how to make a camera? The in-
struction read "pierce a hole in one end of a box and cement a
piece of very thin tracing paper over the other end. An inverted
image of the scene in front of the lens will be seen on it."
As a matter of fact, this type of lens gives excellent definition
and sharpness of picture, but unfortunately it suffers from the un-
pardonable sin of being slow. The matter of speed
nothing
in a lens has
to do with actual movement but refers to the amount of
light admitted at its widest opening. As will be seen later, the speed
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