Chromatic Aberration should be measured as focus shift across the spectrum from the primary point of focus. Measured at the out-of-focus areas on a blown out Digital image is meaningless, except to decide if you like a lens or not.
APO lenses are best used on a Monochrome Camera or Black and White film. This is where using Color Filters has a big impact on focus.
Pentax 85/4.5 Ultra-Achromatic lens on the M Monochrom, UV and Red filters.
Pentax 50/1.4 Syper-Takumar, converted to M-Mount and RF coupled, on the M Monochrom.
Yellow filter:
Red filter.
Color pencils representing the Spectrum. Roy G Biv. Similar "debate" on LUF 5 years ago- I took these images.
Full res images here:
https://ibb.co/album/tmmWP1
What's the point? Chromatic Aberration causes focus shift similar to Spherical Aberration. On a color camera, some can be eliminated in Post. On a Monochrom camera, it must be eliminated Optically. Most lenses shift towards infinity with deep color filters.
So- this was a 15 minute test of the 50/1.4 Pentax that runs ~$50 and the Pentax 85/4.5 that today cost about the same as the Leica APO-Summicron.
Sidenote: Purple Fringing on a Digital Camera- some can be attributed to IR contamination. I use IR cutfilters on my faster lenses with the M9 to greatly reduce purple fringe.
