05-19-2012, 12:53 PM
We seem to go over this frequently around here...
I'm still of the opinion that the majority of cases are simply a specific manifestation of LoCA, as there appears to be a correlation between PF and lenses showing LoCA. As there isn't a single focal plane for all colours, what appears in best focus still isn't entirely in focus, hence you can get the purple fringing if the conditions are right to show it. Of course I'm not saying that is the only cause, but likely to be the more common one in the majority of cases.
As to why it may appear stronger with digital than film, is harder for me to speculate since I don't have deep experience with film. One often cited reason for a generally different film look to digital are a softer non-linear nature as opposed to hard clipping over blown areas. Maybe this could impact PF since it tends to appear where bright areas contrast with dark ones. Another factor may be the increased depth of focus on film could mitigate some of the LoCA effect.
I'm still of the opinion that the majority of cases are simply a specific manifestation of LoCA, as there appears to be a correlation between PF and lenses showing LoCA. As there isn't a single focal plane for all colours, what appears in best focus still isn't entirely in focus, hence you can get the purple fringing if the conditions are right to show it. Of course I'm not saying that is the only cause, but likely to be the more common one in the majority of cases.
As to why it may appear stronger with digital than film, is harder for me to speculate since I don't have deep experience with film. One often cited reason for a generally different film look to digital are a softer non-linear nature as opposed to hard clipping over blown areas. Maybe this could impact PF since it tends to appear where bright areas contrast with dark ones. Another factor may be the increased depth of focus on film could mitigate some of the LoCA effect.
<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.