Ben, one way to get the hang of photography terms like "fast" or "faster" is
to think in terms of light sensitivity and shutter speed.
The more light sensitive the film (i.e., "faster"), the faster the shutter
speed you use can be.
The more light sensitive the lens (i.e., the larger the maximum diameter of
the aperture), the faster the shutter speed can be with the lens wide open
(set to maximum aperture).
The "faster" the lens (e.g., a lens with a maximum aperture of f/1.2 as
compared with one of f/2.8; and an f/2.8 is faster than an f/4, etc.), the
brighter the image you see through the viewfinder, making it easier to
compose in dim light and to shoot, if desired, in dim light.
Everything involved in exposure - film speed (or, more accurately,
sensitivity to light), shutter speed, aperture, and variations thereof such
as exposure compensation - is related to two factors: the duration of
exposure to light; and the brightness of the light.
But...a faster lens does not change the rules of exposure. For example, if
you have a 50mm f/1.4 lens and a 28mm f/2.8 lens and are using ASA 400 film
in very bright conditions, the required exposure may be 1/1000 at f/5.6. In
such a case it makes no difference which lens you use, the aperture will be
f/5.6 for either one.
On the other hand, the faster lens can give you an advantage in low light,
tho' it still does not change the rules of exposure. With the same two
lenses and the same film used indoors in dim light possible exposures might
be 1/15 at f/2.8 or 1/30 at f/1.4. Since the faster shutter speed might be
preferable to prevent blurring either from movement of the subject or your
own body, you must choose the 50mm f/1.4 lens to shoot at 1/30. The
exposure values are identical, tho'.
What you will gain with a faster lens, regardless of film, lighting
conditions, etc., is a brighter image through the viewfinder which is a
greater advantage to composing and focusing in lower light. Even autofocus
SLRs need adequate light to focus, which is why manual focus is often
superior in low light situations.
Lex
===
Date: Thu, 1 Feb 2001 10:01:27 -0000
From: "Frootcake@xxxxxxx" <frootcake@xxxxxxx>
...Can you tell me what you mean when you say the lens is faster.
Ben Ashton
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