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10-06-2009, 11:04 AM   #1
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aperture and zoom

I found this (M to A conversion) very cool "how too" and am left with two semi-related questions.

OK, I understand prime lenses but what about PENTAX-A _zoom_ lenses where the max aperture is, eg, 1:4 at 28mm and 1:6 at 200 mm?

How does the camera "know" that there is one aperture range at one end of the zoom and another at the other end? It seems that the travel of the lever doesn't actually change; it is just the effective ratio changing because of the length of the focal length. So, with a PENTAX-A lens mounted the auto-metering will be slightly inconsistent through the zoom range and the aperture won't change in the display as it does with a "modern" lens, correct?

10-07-2009, 07:56 AM   #2
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Stupid question or just inscrutably phrased?
10-07-2009, 08:43 AM   #3
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First off, I have not read through all of that article, but I will answer how zoom lenses with variable apertures work if that is what you are asking:

The camera knows because the lens tells the camera what aperture it is. In reality, the aperture does not jump half stops or whatever when you zoom with these lenses as it reports - every mm you change the FL, the f number also changes slightly, because it is defined as the ratio of the focal length and the opening. On these lenses with variable max apertures, the opening does not change, but the aperture is reported in discrete f-stops for practical purposes.

See here:
F-number - Wikipedia, the free encyclopedia

This doesn't really change metering because f-stops are relative and light is always variable anyway - it may have an effect on manual flashes or auto thyristor flashes though, but I'm sure there are always small differences in f-stops between lenses anyway.

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