Russ Butler wrote:
> I've noted that different lenses call for different exposures/f-stops
> when aimed at the same subject, e.g. the sky.
>
> Here are a few examples using an OM-1n set at 1/1000:
>
> lens f-stop @ match
> toyo 28-80 2.8-4
> 28 8-11
> 80 5.6-8
>
First of all, how are you reading the aperture settings? I don't happen
to have any of the specific zooms you name, but I have several variable
aperture zooms. Some have a second aperture indicator line for each
extreme focal length setting. For example, the Tokina AT-X
28-200/3.5-4.5 has a white line indicating f3.5 @ 28mm and a thinner
reddish line pointing at f4.5 wide open for 200 mm. Most of the variable
aperture zooms I have have such an arrangement, but not all.
So, if you read the aperture on this lens using the same reference mark
at both 28 and 80 mm, you have, in fact read the same exposure at both
ends, as the lens is one stop slower at the long end and the difference
between f5.6 and f8 is one stop.
> z 200 4 11-16
> z 50 1.4 11-16
> tam 28-200 3.8-5.6
> 200&100 5.6-8
> 28 8-11
> z 28-48 f4 28&48 11
> viv 17-28 4-4.5 @28 5.6
> tok 80-200 3.5-4.5
> 80 11-16
> 200 8-11
> z 100 2.8 11-16
If the above problem of reading aperture is true for all the variable
aperture lenses, then al the prime lenses and all the zooms when at
their shortest focal length read essentially the same, as they should. A
certain amount of sample variation and the relatively casual set-up for
measurement make the modest variation from just over through just under
f11 normal. The variable fl lenses require opening up as they go to
longer focal lengths.
> This is probably well known by you all, but puzzles me a bit.
>
If the above doesn't explain it, there is more to explore.
> Is it caused by difference in transmission?
Certainly not.
> If so, a 2.5 stop range seems huge to me.
As indeed it would be. If the zooms were not multi-coated, differences
between them and the primes might be measurable with your set-up.
However, they are MC and they match pretty closely with the primes at
their widest fls.
> The zuiko's match well and are the 'fastest' but of
> course all the others are zooms.
>
> Offline replies are welcome if this is too 'old hat' for the list.
>
Actually, it's nice to get back to some basic questions once in a while.
:-)
Moose
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