That's a different comparison. We both maintain same framing with different focal lengths, but I vary film size and keep subject distance the same, whereas you keep film size constant and change subject distance. In your case the COC has to stay the same for same DOF, whereas in my case COC must scale with frame size. In your case aperture number determines DOF (i.e. a 50mm lens at F/2.8 will have same DOF as a 300mm lens at F/2.8). In my case you must keep aperture diameter the same to maintain DOF (50mm F/1.4 on 35mm film has same DOF as 110mm F/2.8 on 6x7 film).
Originally Posted by Lee L
To add further to your confusion: in your case (constant film size) the 50/2.8 will have the same DOF as a 300/2.8, but far distance points will blur much more with the 300/2.8. It all boils down to the old saying: a COC is a bunch of photographers discussing about DOF