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 Author
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Topic: Macro Aperature Shift (Read 9148 times)
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KeithB
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Just got the EF-S 60 mm, and that is a nice, cute lens. The manual mentions that the effective f stop increases as you increase the magnification. Is this just using the aperature as a handy way to measure light loss, or does the depth of field change, too?
(I bet I could answer this with the depth of field calculator...)
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KeithB
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I know that I put his in the wrong forum - this is more like technical optics, but do you have any response to this, Bob?
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Bob Atkins
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This is just "off the top of my head", but as far as I know most (if not all) of the DOF calculators automatically compensate for the smaller effective aperture, so if you set the lens to f8, you put f8 into the calculator, even if the "effective aperture" is f16 for exposure due to the amount of extension used.
The true DOF equations calculate everything based on the actual geometric path of the light and so any extension is taken account of. In fact there is a small amount of extension (and thus a small decrease in effective aperture) at any focus distance other than infinity. However it's usually negligible for anything other than macro work and TTL metering takes care of any exposure differences.
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