01-19-2011, 05:28 PM
[quote name='PuxaVida' timestamp='1295448081' post='5646']
... and my question is, why is that so? How does being a slower lens helps correcting spherical aberrations? Is that because of the coating (which allows less but collimated light) or polishment applied to the glass etc...
[/quote]
A slower lens has a smaller aperture and hence light that's going through the corners need to bend less -> less chances for spherical aberration.
Longer lenses usually fair better because, due to their narrow FOVs, the glass used don't have very high refractive indices like fast/short lenses do. Also longer lenses get the good stuff - fluorite elements. Canon's 200 2, 300 2.8, 400 2.8, 500 4, 600 4, 800 5.6, 1200 5.6 all have expensive fluorite glass that's used for reducing CA. AFAIK, only the very expensive/special purpose 60mm f/4 UV-VIS-IR APO macro has fluorite elements in a non-tele lens (iirc, it has 4!).
Coatings have very little to do with CA.
... and my question is, why is that so? How does being a slower lens helps correcting spherical aberrations? Is that because of the coating (which allows less but collimated light) or polishment applied to the glass etc...
[/quote]
A slower lens has a smaller aperture and hence light that's going through the corners need to bend less -> less chances for spherical aberration.
Longer lenses usually fair better because, due to their narrow FOVs, the glass used don't have very high refractive indices like fast/short lenses do. Also longer lenses get the good stuff - fluorite elements. Canon's 200 2, 300 2.8, 400 2.8, 500 4, 600 4, 800 5.6, 1200 5.6 all have expensive fluorite glass that's used for reducing CA. AFAIK, only the very expensive/special purpose 60mm f/4 UV-VIS-IR APO macro has fluorite elements in a non-tele lens (iirc, it has 4!).
Coatings have very little to do with CA.