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04-06-2010, 04:45 AM   #31
Lowell Goudge's Avatar

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QuoteOriginally posted by Arpe Quote
I've never "corrected" my exposure when using TCs using my istDs, K20D, and K-7. All my thousands of TC pics come out fine. The camera can see how much light is coming through and meters for it.

Surely if the camera thought it was 2.8, but with the TC it was 5.6 (so less light), the pics would be underexposed, not over?
It depends on the TC. The sigma feeds the aperture info through without modification. I believe kenko TCs have a small chip that reassigns the pins to be correct for the new aperture, as does the pentax SMC-F-1.7x AF TC.

with the sigma TC there is a definite issue on exposure on the K10D. K7 is pretty good.

the K10 thinking F2.8 needs to add exposure, this is why it over exposes. the camera is trying to take a grey value of 80 and make it 120. i.e. adding at least one stop.

My belief is that the camera meters the light it sees, it doesn't matter what f stop the camera thinks it's using.
this is sound thnking, if the camera metered correctly across all F stops, BUT the K10D specifically, and to a lesser extent the K7D, have some non linearity in the metering, where at large apertures, it thinks there is more light than there is, causing it to under expose, and at small apertures it thinks there is less light than there is, causing it to over expose. That is what the graph tells us, because this shows the resulting greyscale value at each aperture on the same lens, tested with 5 varients, (3 different cameras, one camera with 3 different focusing screens)

For the K7 fortunately the really irregular spot is beyond F5.6 and as a result, most lenses, even with TCs will meter correctly on the K7.

Last edited by Lowell Goudge; 04-06-2010 at 05:26 AM.
04-18-2010, 12:04 PM   #32
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Need help on determining the price of a used Sigma 2x EX APO TC
I check the prices on BH it was 299

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