02-05-2014, 04:11 AM
I do not think that these are LATERAL CAs in the image taken with the Oly 75. Lateral CAs would have different inbound/outbound colors from the image center (things are radial on lenses).
You mentioned that you used it at f/2 here ?I suspect that you are seeing purple fringing. Honestly I am not sure about the origins of this. I haven't seen any good explanations out there at least. The best bet points towards a blooming effect on the image sensor due to the angle of light at large aperture settings combined with a overly harsh contrast. You may notice that the effect is only visible around very bright stars thus around extreme contrasts but not around the fainted ones. The effect is also more pronounced on smaller sensors (pixel density ?).
I suspect that if you use the Oly 75 at f/2.8 and f/4 the effect will be gone or at least largely reduced.
FWIW, I am aware of 3-4 CA types:
- Lateral CAs (which is what we test)
- Axial/Longituadal CAs
- Bokeh fringing (I am not 100% sure about the relationship of bokeh fringing and axial CAs - there seems to be a relationship but it doesn't seem to be identical). During earlier tests we referred to LoCA (axial CAs) but lately we are only using the term bokeh fringing.
- Purple Fringing (as discussed)
You mentioned that you used it at f/2 here ?I suspect that you are seeing purple fringing. Honestly I am not sure about the origins of this. I haven't seen any good explanations out there at least. The best bet points towards a blooming effect on the image sensor due to the angle of light at large aperture settings combined with a overly harsh contrast. You may notice that the effect is only visible around very bright stars thus around extreme contrasts but not around the fainted ones. The effect is also more pronounced on smaller sensors (pixel density ?).
I suspect that if you use the Oly 75 at f/2.8 and f/4 the effect will be gone or at least largely reduced.
FWIW, I am aware of 3-4 CA types:
- Lateral CAs (which is what we test)
- Axial/Longituadal CAs
- Bokeh fringing (I am not 100% sure about the relationship of bokeh fringing and axial CAs - there seems to be a relationship but it doesn't seem to be identical). During earlier tests we referred to LoCA (axial CAs) but lately we are only using the term bokeh fringing.
- Purple Fringing (as discussed)