Narrow-bandwidth Television Association
Choosing a lens for the camera
Although it is possible to use a single lens element as the objective lens on
the camera, a surplus photographic multi-element lens will generally provide a
more satisfactory optical arrangement with a noticeably better image.
When selecting a lens, the features to consider are as follows:
- f number
- image size
- focal length
- iris adjustment
- focus adjustment
- zoom facility
As described in the chapter on Optics and Lenses, the f number of a lens is a
measure of its light gathering power. In order to achieve adequate camera
sensitivity the f number should be as low as possible and preferably no greater
than about f4.
The size of the focussed image produced by the lens must be sufficient to cover
the scanned frame area without blurring towards the edges. The frame size on a
32-line, 12 inch Nipkow disc is approximately 28mm x 20mm, which rules out the
use of 8mm and 16mm cine lenses. On the other hand, lenses from 35mm still
cameras and slide projectors, which normally have an image area of 36mm x 24
are very suitable for this application.
The focal length of the lens will determine whether your camera has a 'wide
angle', 'normal' or 'telephoto' characteristic. As a guide, a 50mm focal length
lens will produce a head and shoulders shot with the subject about 1 metre in
front of the camera.
An adjustable iris is a very useful feature that allows the lens to be 'stopped
down' when there is plenty of light on the scene to be televised. This generally
results in a better depth of field (i.e. the range of distance over which the
subject remains in focus for a given focus setting). However, an adjustable iris
is not absolutely essential, especially in situations where the subject lighting
can be controlled (e.g. indoors). Also, adjusting the video amplifier gain will
to some extent provide a substitute iris action.
It is essential that the camera has a means of focussing the lens. If this is
built into the lens the camera construction is made easier. A lens without a
built in focusing adjustment, such as from a 35mm slide projector, will require
some mechanical arrangement to be devised to enable the distance between the
lens and the Nipkow disc to be made adjustable.
A zoom feature (that is, variable focal length) is very much in the
'nice-to-have' category, but not absolutely essential. Information on a
home-made zoom and focussing lens that can be constructed from two 2-inch
diameter lens elements can be found in Newsletter Vol. 25 No. 4.