Will the DOF ration shift as aperture changes?

[Bob Keenan - (bobkeenan)]

I just finished a cal on my Canon 7D with a new 70-200mm 2.8. I set the focus front to back ratio at 50/50. I think checked the focus at 2.8, 4, and 8. The DOF expanded as expected but the ratio kept shifting backward and was about 20/80 at f8. Is that expected?

[Michael Tapes - (MichaelT)]

No, the front/back ratio should stay the same as indicated by the LensAlign Distance Tool. Link is here.

Could be focus shift (where the wide open focus is different than stopped down focus), but focus shift in my experience is generally a step function and not gradual. So not sure what is happening here.