All right, I don't know what to make of this. My F 70-210 4.0~5.6 works
fine on the *ist-D and *ist-Ds and everything seems to be reported
correctly.
When I mount it on the 1.7x AF adapter the maximum aperture is reported
as f8, now I don't quite understand, because that just looks wrong I
was expecting it to report f 6.7. So I figure I'll calculate the
approximate f stop of that combination with fCalc. So naively I just
punch in the numbers. f4.0 returns 7.21. Hum That's only 1/3 stop off
of f8.0 so maybe that's right, but that's less than 2/10 from 6.7...
So I decide to test using another lens. I'd never use this combination
but it was available, so I put the FA 20-35 and FA 1.7x maximum aperture
is reported as f6.7 what? Well that's less than 2/10s stop off the
actual value in the other direction. Now I'm really curious so I try
the FA 28-70. That too reports f6.7. Next I try an A lens, lets see, I
have the A*300 f4.0 so I try that. Lo it too reports f6.7. Consistency
at least., reporting the nearest standard 1/2 stop to the actual value.
Maybe it's variable aperture lenses that give strange results so I
figure I'll try the F 35-70 3.5-4.5, now that reports 6.7??? That's
just about 2/10s of a stop off, and it's the next normal click stop...
<Aside> Now lets say right here that according to the information on
Boz's page there is no way for the camera to know that it's a variable
aperture lens mounted on the camera, that information is carried by the
digital data pin and there's no pass through for that on the AF 1.7x
converter, the converter substitutes it's own digital data. So it's
working from the maximum and minimum aperture data on the original A
specification.</aside>
But now I'm really curious. What about the A28 f2.8. Once again a
combination I'm unlikely to ever use. fCalc thinks it should be f5.05
the camera reports it as f4.5. Once again under 2/10 of a stop variance
and the next closest normal click stop once again consistency.
All right what about a 1/3rd party lens? The only one I've presently
got that will fill the bill is the Vivitar S1 70-210 f2.8~4.0. The
aperture at 70mm is the same as the A 28mm but it's a variable aperture
lens. Result is the same as the A 28mm, which is what I would have
expected if it weren't for the behavior of the F 70-210 with the 1.7x
converter.
So I decide to try the FA 43mm limited. I expect the combination to
show a maximum aperture of f3.5, and yes I'm not disappointed, once
again the nearest standard click stop to the actual, (well starting from
a base of f2.0 since there's no pin combination in the A specification
for a max aperture of f1.9), value.
So lets recap. The only lens that gives a result at variance with what
I would expect is the F 70-210. All the maximum apertures are a bit off
of actual, but all the lenses and the F 1.7x report consistently to the
nearest standard click stop, all on the high side except the F 70-210mm
f4.0~5.6. Which reports to the low side and a bit more than 1/3 of a
stop off.
This is actually bugging me, I know it shouldn't but it is. The next
nearest value should be f 6.7 for the 70-210. All the other f4.0
maximum aperture lenses and the F 1.7x adapter report a maximum aperture
of 6.7. All the other lenses report within 2/10 of a stop of their
actual maximum apertures. All the other lenses I tested reported values
constant with the f4.0 maximum aperture lenses, all quite predictable.
None of this seems to effect actual exposure either, just the reported
aperture. The only place this would be a problem would be with off
camera flash, and variable aperture lenses make hash of that anyway, so
it shouldn't bug me yet it does.
I'm wondering if anyone has an explanation, I certainly can't come up
with one that makes any sense. (Except that I'm obsessive compulsive and
I already knew I had those tendencies).
--
You get further with a kind word and a gun, than with a kind word alone.
--Al Capone.
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