Canon AT 1 - Manual (Page 50)

Lens Aperture
The adjustment of the aperture is used
with the shutter speed to get the correct ex­
posure. The amount of light reaching the film
is controlled by the aperture's size.
On the aperture ring are a series of mark­
ings which indicate the proportion of the light
allowed to pass through the lens, which are
known as f/numbers. When the aperture ring
is set to the next larger f/number on the scale
of the ring, the amount of light passing
through the lens is decreased by 1/2. The
lens's brightness is based on the smallest
numerical aperture value for the lens.
With a f/2 serving as the standard, the com­
parative brightness at each f/stop will be as
indicated below.
Biighiness
(f/stop) 1.2 2.8 4 5.6 8 11 16
Ratio 3
2
1
J1/2 1/4 1/8 1/16 1/22 1/64
The aperture ring can be set at positions
between the settings on the scale.