I'd expect the AF1.7x converter to perform a simple translation
based entirely on the "maximum aperture" pin combination on the
attached lens (using the KA-mount conventions).
For any given pin encoding on the attached lens the converter
will report a (different, fixed) maximum aperture to the body.
Your results bear this out. Lenses with a maximum aperture of
f2.0 converts to f3.5
f2.8 converts to f4.5
f4.0 converts to f6.7
What does the pin encoding on the back of the F70-210 indicate?
I'd guess it doesn't show f4.0 - perhaps it claims f4.5?
On Thu, Dec 04, 2008 at 12:11:10PM -0600, Bob Sullivan wrote:
> PJ,
> Here's what I get for minimum apertures in the K20D viewfinder...
> F70-210 f4-5.6 reports f8 with AF1.7 converter.
> F28/2.8 reports f4.5 with converter.
> FA43/1.9 reports f3.5 with converter.
> FA77/1.8 reports f2.8 with converter.
> Regards, Bob S.
>
> On Thu, Dec 4, 2008 at 1:13 AM, P. J. Alling <[EMAIL PROTECTED]> wrote:
> > 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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