Black/Red text is softest K-50 + 50mm 1.8

Timbukto wrote:

Hi Mike,

Jim Kasson has done far more precise and thorough analysis and testing for certain lenses that show indeed its not entirely uncommon that the red channel differs a bit in optical performance -

http://www.strollswithmydog.com/chromatic-aberrations-mtf-mapped/

My test shots are very crude in comparison but one thing I can see by eye, is that in terms of MTF on some of the shots depending on focus, green was sharpest, then blue, then red was the worst, etc. But it appears you can put the red in focus at the cost of sharpness for blue/green.

In any case this pretty much confirms some subjective analysis of my 50mm 1.8 in that sometimes wide open regardless of getting focus optimized for blue/green, I can still see some softness. Anyways it is interesting to know that there is not only front/back focus of the image plane as a simple matter, but that all 3 color channels can have various front/back focus! And it is still the tendency that for best IQ you either have to stop down a bit, or get some expensive over-engineered/heavy lens for best wide open performances.

Anyhow it is one more possible measure of performance I might keep in mind when evaluating lenses in the future...i.e. that not only should a lens be sharp wide open, but each color might not focus the same as another. So I guess what happens when you look at black on white test charts is one color slightly differs in focus plane and thus its color fringe begins to show or the edges get smeared, etc?

Yes you've got it figured! In many cases its not a real visible issue or a problem, however at pixel level or in very specialized cases it can be an issue. in the realm of FF/BF i wouldn't worry about CA at pixel level, if FF/BF is going to be an issue it will be visible in the image as a whole.