Originally posted by HermanLee Steve, this is interesting. How can you tell if it's just a garden variety yellow/blue CA and not PF?
I'm trying to learn here.
Thanks,
This is going to sound sort of hokey, but given a back-lit tree branch, yellow/blue CA will generally show yellow (often vague) on one side of the branch and blue on the other. PF, on the other hand, will be blue or purple on both sides. CA is also visible as a color band extending a set distance (x number of pixels) from a light/dark boundary. PF is of inconsistent width and will span gaps. And finally...CA is generally correctable in Lightroom or ACR using the CA correction feature. PF does not correct except by de-saturating the blue and/or purple channels. (At this point it is important to note that CA is actually present throughout the frame, not just where you can see it. The result is a generalized decrease in sharpness both with color and b&w photos that improves when the CA is corrected in PP.)
The reason I made the earlier statement regarding PF as a myth is that I was involved in a fairly active thread several weeks ago where this assertion was emphatically and repeatedly made. At that time, I offered to do comparison film and digital images with my 77 limited on both film and the K10D. Unfortunately, I went out to try it and came back empty handed. I could not coax any PF from the FA 77.
I did go back through my film image scans looking for the illusive blue/purple artifact. Mind you, I have been shooting film since the late 1960s and have never seen anything like the PF thrown by my two digicams. CA, yes. Bluish halo around white shirts from flare, yes. But no PF. On the other hand, I have never looked for it either. After a short search, I came up with a couple of frames taken with my Olympus XA of high contrast subjects in bright overhead light. There it was...a wide bright blue swath of color where none should be.
"That sure looks like PF", I thought to myself and it really did. I did a higher resolution scan and started looking a little more closely. At that point I saw less intense, matching yellow bands on opposing borders. When I applied aggressive CA correction in Lightroom, the blue disappeared with a general improvement to the image as a whole. So, was it PF on film? I would say no. Others would likely disagree.
Steve