Movies for TV - Early Television Foundation

138
FUNDAMENTALS
Focussing the Lens In all types of lenses, focussing
is accom-
plished by rotating the lens barrel. This has the effect of changing
the spacing between two or more lenses and thus bringing all the
rays of light to converge at one point and produce a sharply de-
fined image on the film.
Quite obviously focussing is even more important than film
movement speed for if a fuzzy out-of-focus image is produced it
is of absolutely no use whatever, while a series of frames taken too
fast or too slowly may be usable by the employment of one or an-
other of the methods used to change frame speed. Probably the
most important effect of focussing is the effect on depth of field.
This is the "working distance" of the lens. In other words, it describes
the limits of closeness to, and distance from, the camera
between which the set will be in focus. This figure changes with
every change of / opening: the larger the opening, the more critical
and smaller it becomes. Thus for close work it is always ad-
visable to "stop down" to increase the depth of field so that all
featuress of a face may be in focus. It is quite possible when using
close shots and large apertures for the tip of a nose to be in focus
and the rest of the face blurry due to extremely short depth of
field.
There are three general methods of focussing in use; the last
one to be described is the best, but it requires a more costly
camera. For normal use, the calibration on the lens barrel or
focussing ring is sufficiently accurate, so the distance from the
lens to the subject is measured with a long steel tape. The meas-
ured distance is then set on the lens barrel. This is quite satisfac-
and cameramen.
tory and is used by many excellent producers
However, it does not produce the exact focus demanded by per-
fectionists and sometimes through carelessness gives very poor
results.
The second method is the rack-over system in which the
camera body slides to one side and a viewing eyepiece comes into
position so that the picture which will be recorded can be seen
and lens which will be used for film-
through the same aperture