At 07:45 PM 10/5/2000 -0500, Don Gaikins wrote:
[snip]
>I have four lenses. Two 50mm 1.8 Zuiko, a Kitstar f135mm 1:2.8 and a
>Vivitor 28mm 1:2.8. I have seen posts on the list (and adds on eBay)
>that talk about fast and slow lenses. Could someone explain to me what
>a fast lens means? What slow lens is?
>
>I thought that the second number just meant the widest aperture that the
>lens could achieve. What does that have [to do] with speed?
Don:
Think of it this way. If you own two lenses of identical focal length (say,
two 50mm Zuikos), but different maximum apertures, one will be "faster" than
the other.
Example: You own a 50/2.0 macro and a 50/1.4 "standard" (one close-focuses,
the other doesn't, but ignore these differences for a moment). You take a
picture of a scene with the 50/2.0 and find that the maximum shutter speed you
can coax out of it is 1/60th of a second. Then you switch to the 50/1.4. If
you take a picture of exactly the same scene, you now find that the maximum
shutter speed you can obtain is 1/125th of a second. It's a *faster* shutter
speed (holding everything else constant). Thus, the 50/1.4 is a "faster" lens
than the 50/2.0, by exactly one F-stop.
Smaller maximum apertures (denoted by smaller F-numbers) are "faster" than
larger maximum apertures. Hence the reference to "speed."
Garth
P.S.: If you actually *do* happen to own the above two lenses, you're also a
lucky bastard. ;-)
"A bad day doing photography is better
than a good day doing just about
anything else."
The Unofficial Olympus Web Photo Gallery at:
http://www.taiga.ca/~gallery/, or
http://www.enable.org/~gallery/
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