05-20-2012, 10:03 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.
Chromatic aberrations one will see in the focus area are lateral CAs. PF is visible in DoF and in OOF areas, and doesn't show the typical dual chromatic characteristics of chromatic aberrations proper.
Check the sample shot by Klaus taken with, e.g., the Canon 50L. Proper PF.
Kind regards, Wim
[/quote]
Hmm, not sure if this statement "LoCA are caused by spherical aberrations" is right. As far as I was able to figure out, they are two separate things, which are difficult to separate though in practice.
And LoCA could possibly show up within the Dof or in-focus zone. No lens is perfectly corrected. So, if a lens is mainly corrected for 2 wave-lengths (there are few apochromats (3 wave-length corrected) out there), those wavelengths would usually be in the main visible range. It means the far red and the deep blue (purple) on the UV side would be least corrected wavelengths. Thus, a black spot on white background, or vice versa could be surrounded by a purple halo because those wave-lengths are not in focus at the particular focal length where green etc. is in focus.
However, it is noted that the purple fringing essentially seems to occur mainly in very high contrast situations. I was now wondering if possibly IR, which is not completely blocked by the camera filter, can contribute - another reason for film and digital differences. IR would be even more out of focus in a LoCA situation and gives magenta casts if not filtered properly. Given the high-contrast situation - reflected sunlight- the IR contribution could be strong enough to be become visible?
Regarding the Sony 135mm purple fringing, I think it's not representative - most of the purple fringes there seem to occur in areas where out of focus leaves in the foreground are sitting. - I guess that's what Klaus called out of focus fringing.
LoCA are caused by spherical aberrations (hence the proper name of spherochromatism). They are therefore not visible within the DoF or in-focus zone.
Chromatic aberrations one will see in the focus area are lateral CAs. PF is visible in DoF and in OOF areas, and doesn't show the typical dual chromatic characteristics of chromatic aberrations proper.
Check the sample shot by Klaus taken with, e.g., the Canon 50L. Proper PF.
Kind regards, Wim
[/quote]
Hmm, not sure if this statement "LoCA are caused by spherical aberrations" is right. As far as I was able to figure out, they are two separate things, which are difficult to separate though in practice.
And LoCA could possibly show up within the Dof or in-focus zone. No lens is perfectly corrected. So, if a lens is mainly corrected for 2 wave-lengths (there are few apochromats (3 wave-length corrected) out there), those wavelengths would usually be in the main visible range. It means the far red and the deep blue (purple) on the UV side would be least corrected wavelengths. Thus, a black spot on white background, or vice versa could be surrounded by a purple halo because those wave-lengths are not in focus at the particular focal length where green etc. is in focus.
However, it is noted that the purple fringing essentially seems to occur mainly in very high contrast situations. I was now wondering if possibly IR, which is not completely blocked by the camera filter, can contribute - another reason for film and digital differences. IR would be even more out of focus in a LoCA situation and gives magenta casts if not filtered properly. Given the high-contrast situation - reflected sunlight- the IR contribution could be strong enough to be become visible?
Regarding the Sony 135mm purple fringing, I think it's not representative - most of the purple fringes there seem to occur in areas where out of focus leaves in the foreground are sitting. - I guess that's what Klaus called out of focus fringing.