[Brightcolours]

Now my question concerns the 75 F/1.8  It is a truly luscious lens, beautiful and smooth to operate.  I haven't done much with it thus far but I did do some star images, star tests at F/2 and the results leave me a bit at a loss.  Most everything about the results are fantastic - sharpness across the field, NO Coma, great resolution etc.  However, there is a lot of chromatic aberation - purple mostly but some blue fringing around bright stars.  The tests herein show very little CA.  I also tested the 12-40 at 40mm and see virtually no CA with that zoom, which is in line with the tests results here. 

So I am wondering - do I perhaps have a bad sample of the lens?  I think I might be able to swap it for another as I bought direct from Olympus and they always treat me well.  But I don't want to send it in just yet.

I wonder if I could post the test image, might someone here give a look and an opinion?  I can say that the CA I saw with the 75 is on a par with my old Zuiko 90/2 macro in the same test and actually worse than an old 50/1.8 MC Japan lens but that one had abundant COMA so is fairly useless for this application.

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...