At 23:23 7/19/02, you wrote:
I would guess that your meter is fine. It may be that the SC lenses are
actually transmitting less light than the MC lenses. One of the benefits of
MC vs SC is supposedly better light transmission. As an example I have a
Soligor 200 f/2.8 that is really much closer to f/3.5 or so.
A better test would be to meter through the camera and shoot some slides
with the lenses to make comparisons. I suspect the slides will be similarly
exposed.
Jim Couch
The difference between SC and MC would not cause entire stops of
difference. To slow by an entire stop requires a 500ss in light
transmission! It's difficult for me to imagine a 1/6 stop difference, and
that's if the lens formulation is unchange. Lens formulations compensate
for losses in light transmission. Although the theoretical absolute
aperture diameter is focal length divided by the aperture f-number, in
practical lens design, absolute aperture diameters are adjusted to an
"effective" diameter to produce the required f-number.
Regarding aftermarket lenses, there was (and to some extent continues to
be) a "speed war" among them. Some aftermarket lens makers stretched the
truth about their lens speeds to claim as little as a silly 1/10th f-number
faster speed. All in the name of marketing and being able to claim it was
"faster" than another manufacturer's lens. 1/10th of an f-number makes so
little difference in exposure it's ridiculous, but nevertheless the
bragging rights that come with having an ever so slightly faster lens were
apparently enough to make a difference in sales, or so it was
perceived. The difference between f/1.7 and f/1.8 is about 1/6 stop at
most. So is the difference between f/1.5 and f/1.4, and it gets less as
the lenses get slower. f/3.5 -> f/4 -> f/4.5 are only 1/3 stop increments.
BTW, you may see some references in places to "t-stops." These are used
almost exclusively on cinema and scientific application lenses where
extreme accuracy is required. Even a very slight difference in exposure
between cinema camera lenses is unacceptable when shooting a motion picture
using several cameras simultaneously, each with a different lens, and the
result must be knitted together during editing. The "t" indicates
"transmission." It is the f-stop of actual light transmission as measured
for that lens in a laboratory.
-- John
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