c. Focal length controls image brightness, lens speed, and image size,
2-7. SPEED OF LENS. The speed of a lens refers to the intensity of the light
that forms the image on the film plane.
a. Light intensity.
The intensity of the light transmitted by a lens
depends upon the diameter of the aperture (diaphrahm opening), the number of
lens elements, the number of reflecting surfaces, and the focal length of the
lens, (fig 2-9).
With other factors remaining constant, a large aperture
admits more light, or is faster, and a longer focal length decreases the light
intensity, or is slower, (fig 2-10). In the same way that a card gets dimmer
as a flashlight is moved further away from it, a photographic image becomes
dimmer as the focal length is increased, (fig 2-11).
This is because light
must travel a longer distance to reach the focal plane.