But full frames have more DOF with same lens?

[Helen B]

The whole concept of traditional DoF (and DoF calculators) is really only valid for a viewed image - the intermediate image (the one recorded by the sensor) is not always important (unless the sensel pitch or film resolution is used as the basis of the MACC criterion, which can be a perfectly valid approach).

The usual base MACC criterion is 0.2 mm for an 8 x 10 print (which is then the MACC for 8x10 film of course). Everything gets worked from there - you vary the in-camera MACC in inverse proportion to the magnification required to get from the sensor/film image to 8x10. If you need to magnify 2x (ie use 4x5 film) you divide the MACC for 8x10 by 2. The more you magnify the camera image to get to the viewed image, the more you have to reduce the MACC used for the camera image. This means, of course, that the DoF markings on lenses are only applicable to one format - with the caveat that it is all an inexact science.