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Old 25-03-2012, 12:40 PM
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bojan is offline
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 6,412
Originally Posted by gregbradley View Post
I would have thought what you say is the case originally as those lenses should be better made and with exotic glass types (they often have ED elements).


All designs are a compromise.
It is VERY hard to design a F1.2 lens to perform in similar fashion like F1.8 at full aperture and in corners (the centre of the frame is relatively easy).
Stopping the lens down engages only the central parts of the glass, where the aberrations are negligible.
Because the F1.2 lens is much more carefully designed (hence more expensive) - to give acceptable performance in corners at full aperture, it will perform better than it's F1.8 counterpart at the same stop-down (because both are designed to have the same MTF at full aperture)
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