05-19-2012, 11:15 PM
[quote name='wim' timestamp='1337466232' post='18343']
LoCA are caused by spherical aberrations (hence the proper name of spherochromatism). They are therefore not visible within the DoF or in-focus zone.[/quote]
I think we're drilling in on a subtle semantic here.
LoCA could occur on a nominal focal plane, unless the lens is exceptionally well corrected. Even a true APO lens has three wavelengths in common focus, with the goal of minimising the deviation from ideal at others. For lesser lenses it will be worse, which is a mechanism for CA on the apparent focal plane. Of course, the better corrected the lens is, the harder it will be to provoke the CA to appear.
Bringing in the depth of field as opposed to the focal plane, if it is great enough, then LoCA is significantly reduced. So logically if my argument is correct, LoCA based PF should diminish in strength as a lens is stopped down. Again I'll welcome clear examples to the contrary. But as a side caution, a pre-requisite is extreme contrast typically requiring bright/blown areas. If it is strong enough, it could still "punch through" stopping down benefits. The other side of depth of field is depth of focus, and as mentioned previously I speculate that may be a mechanism for a reduced effect on film compared to digital.
I guess to clarify what I'm saying, what might be not perceptible levels of focal plane LoCA in normal use could be bought out by a sufficiently strong contrasting area.
LoCA are caused by spherical aberrations (hence the proper name of spherochromatism). They are therefore not visible within the DoF or in-focus zone.[/quote]
I think we're drilling in on a subtle semantic here.
LoCA could occur on a nominal focal plane, unless the lens is exceptionally well corrected. Even a true APO lens has three wavelengths in common focus, with the goal of minimising the deviation from ideal at others. For lesser lenses it will be worse, which is a mechanism for CA on the apparent focal plane. Of course, the better corrected the lens is, the harder it will be to provoke the CA to appear.
Bringing in the depth of field as opposed to the focal plane, if it is great enough, then LoCA is significantly reduced. So logically if my argument is correct, LoCA based PF should diminish in strength as a lens is stopped down. Again I'll welcome clear examples to the contrary. But as a side caution, a pre-requisite is extreme contrast typically requiring bright/blown areas. If it is strong enough, it could still "punch through" stopping down benefits. The other side of depth of field is depth of focus, and as mentioned previously I speculate that may be a mechanism for a reduced effect on film compared to digital.
I guess to clarify what I'm saying, what might be not perceptible levels of focal plane LoCA in normal use could be bought out by a sufficiently strong contrasting area.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.