Quote Originally Posted by Rudeofus View Post
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).
Quote Originally Posted by Rudeofus View Post
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.
Well, it does add to the confusion when you say the DoF is the same, but different.

Below is an example of the first case you've given above. Claiming that a 300 f:2.8 has the same DoF as a 50 f:2.8 at the same magnification. It doesn't.

Your take often appears close enough to be valid at close distances, but doesn't hold at further distances. Read the Ctein article I linked to and run some examples.