02-18-2011, 03:57 AM
[quote name='PuxaVida' timestamp='1297947635' post='6190']
And just out of curiosity... how can we distinguish residual spherical aberration (focus shift at closed aperture) from field curvature?
[/quote]
At full aperture, field curvature should be at its highest and focus shift would be zero.
At a small aperture, field curvature is reduced and focus shift would be at its highest.
[quote name='PuxaVida' timestamp='1297947635' post='6190']
Is the field curvature effected by the closing of the aperture?
[/quote]
Ignoring the increase of DOF, field curvature should reduce when stopping down (e.g. imagine a lens with a smaller aperture and glass) because light needs to refract less.
[quote name='PuxaVida' timestamp='1297954061' post='6192']
Thanks... What about the DOF dependencies? With the closed the aperture (lets say @ f/10) the focus shift arises during exposure but at the same time the DOF is increased. So depending on the shifted distance, it can happen that we cannot recognize the focus shift on the final image at all, right?
[/quote]
In a word, yes... in practice I see very little of this. For example the 24L II has a strong field curvature but I rarely shoot flat subjects (at 1.4) or focus-recompose with this lens. When I'm stopping down, it's almost always for landscape shots... so f/11 at 24mm gives a hyperfocal distance after a few feet and field curvature is not an issue.
[quote name='PuxaVida' timestamp='1297954061' post='6192']
Regarding the field curvature: Is it constant on the edges regardless of the aperture value?
[/quote]
Like I said earlier, I can't see how this can be the case, even if we don't account for the DOF increase. Field curvature is a result of refraction, as the aperture gets smaller, refraction is less... therefore field curvature should be less.
GTW
And just out of curiosity... how can we distinguish residual spherical aberration (focus shift at closed aperture) from field curvature?
[/quote]
At full aperture, field curvature should be at its highest and focus shift would be zero.
At a small aperture, field curvature is reduced and focus shift would be at its highest.
[quote name='PuxaVida' timestamp='1297947635' post='6190']
Is the field curvature effected by the closing of the aperture?
[/quote]
Ignoring the increase of DOF, field curvature should reduce when stopping down (e.g. imagine a lens with a smaller aperture and glass) because light needs to refract less.
[quote name='PuxaVida' timestamp='1297954061' post='6192']
Thanks... What about the DOF dependencies? With the closed the aperture (lets say @ f/10) the focus shift arises during exposure but at the same time the DOF is increased. So depending on the shifted distance, it can happen that we cannot recognize the focus shift on the final image at all, right?
[/quote]
In a word, yes... in practice I see very little of this. For example the 24L II has a strong field curvature but I rarely shoot flat subjects (at 1.4) or focus-recompose with this lens. When I'm stopping down, it's almost always for landscape shots... so f/11 at 24mm gives a hyperfocal distance after a few feet and field curvature is not an issue.
[quote name='PuxaVida' timestamp='1297954061' post='6192']
Regarding the field curvature: Is it constant on the edges regardless of the aperture value?
[/quote]
Like I said earlier, I can't see how this can be the case, even if we don't account for the DOF increase. Field curvature is a result of refraction, as the aperture gets smaller, refraction is less... therefore field curvature should be less.
GTW