Canon FD Lens, Manual Book

Page: 16 / 48

The brightness of a lens is expressed in terms
of its aperture ratio (the ratio of the lens's focal
length to the diameter of the lens opening).
Thus, the "1.4" in the FD 50mm f/1.4 standard
lens indicates the aperture ratio, an abbreviation of
1:1.4. The lenses with the smallest aperture ratios
are the brightest. Since the lens opening of such
lenses is large compared to their focal lengths, they are
capable of transmitting relatively large quantities of light.
For example, Canon produces two different 135mm
telephoto lenses, one with an aperture of f/2.8, the other,
with an aperture of f/3.5. The f/2.8 lens is the brighter of the
two. This can be confusing when purchasing a lens, but in
You need maximum
brightness for
theater photography.
Use of wide aper-
tures permits fast
shutter speeds.
A fast lens lets you
shoot without flash
even in a dimly-lit
museum.
general it is best to select the brightest lens which
your budget allows. Brighter lenses make night-
time hand-held photography easier, and permit
shooting under most low-light conditions. They're
also useful when flash is not permitted, such as
in art galleries or theaters.
The larger the lens opening, the brighter the lens — Aperture ratio.
Not For Resale – Free Download at http://www.joe-chan.com/manuals
14