I'm sorry Tomasz, but you've missed something in your logic. You say you are doubling focal-length F, but keeping M constant. But you said M=F/f, which means you have to double your distance. This is why you get the same DOF for the same magnification and same focal ratio. You then confuse things further with your comparison of 35mm and medium format standard lenses. The magnification is not the same for the 50mm and 80mm lenses even though the field of view is the same, because the image in the medium format camera is larger (i.e. larger magnification).
On the other hand, your thoughts about smaller digital formats are nearly right, for the same reason that things cancelled out between 35mm and medium format - you have confused magnification with field of view. The DOF you'll get with the Sony will be the same as a 35mm camera with a 200mm lens set to F/11, but the brightness of the image is the same as at f/2.8. This could be viewed as a big advantage of digital. Your calculations are right about the DOF compared with the 85mm at f/1.4 though - to get so slim a DOF with a 21mm lens is practically impossible.
I hope this helps.
Mark
On the other hand, your thoughts about smaller digital formats are nearly right, for the same reason that things cancelled out between 35mm and medium format - you have confused magnification with field of view. The DOF you'll get with the Sony will be the same as a 35mm camera with a 200mm lens set to F/11, but the brightness of the image is the same as at f/2.8. This could be viewed as a big advantage of digital. Your calculations are right about the DOF compared with the 85mm at f/1.4 though - to get so slim a DOF with a 21mm lens is practically impossible.
I hope this helps.
Mark