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. > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
