02-06-2014, 12:26 AM
Quote:Like Klaus points out, the CA measurements are about lateral CA. Lenses refract light of different wave lengths to varying degrees. This can result on some colours being projected larger than others, which leads to for instance the red channel being "bigger" in the projection than the red and green channel. This most visible in the resulting coloured edges around contrasty areas, near the borders of the image. They colours always coma in pairs (green/magenta, yellow/blue, red/green-blue).
This is not what you are seeing.
There is another kind of CA, LoCA. This happens when in OOF areas light lets "focussed" on different planes. Usually one gets magenta in front and green behind the focal plane. This probably is also not what you are seeing.
Most likely you are seeing purple fringing. PF happens when a lot of light hits the sensor, and part of it reflects back. When you stop down, it will lessen and disappear. PF is lens dependent, as you have noticed (your 75mm f1.8 and 90mm f2 show more than your 50mm f1.8). This probably has to do with the shapes of the lens elements, the 50mm f1.8 will diverge the reflected light more and less will end up concentrated. Lens manufacturers have developed coatings for lenses to reduce PF, Sigma started to call lenses with the new coatings "DG" (digital generation) and later when they introduced APS-C lenses "DC". And Tamron started to call lenses with the new coatings "Di" (digital integrated or something like that) and "Di II" for APS-C.
You can't do anything about camera/lens combinations which show PF, other than get a lens which is designed not to show any PF problems. I am not aware of any lens test site which does test for PF showing up, so it will have to be trial end error...
I do not think (as opposed to know) that PF is a reflection effect caused by the lens. It feels highly unlikely that reflections create a halo effect. (Re-)Reflections would be distributed across the image field - essentially re-reflections are a freak effect.
If I had to GUESS, this may be caused by the micro-lenses that sit in front of the photodiodes but not by the lens. These micro-lenses are certainly NOT coated. It feels viable that sensor-surfaces reflections are distributed to adjacent micro-lenses. This would also explain the halo effect.
Just guessing.