Quote Originally Posted by RalphLambrecht View Post
Correct, but who works in actual aperture dimensions? Aren't we all more comfortable with f/stops (aperture values)?

Yes, nobody works in aperture dimensions, but you could use an empirical method using the ratio between the focal length of the lenses and adjust the aperture value (f/stops) accordingly.

Let's say that a 160mm lens on 4x5 gives you the same AOV of a 320mm lens on 8x10, then the ratio 320/160=2. You have to multiply by 2 the aperture in the 320mm to get the same DOF as the 160.

In the case of the OP, to get the same DOF in 35mm using a 50mm lens and 6x4.5 using a 80mm lens, you need to multiply by 1.6 the aperture in the 80mm lens. Here the results are approximate since the film proportions and angle of view between the two are not the same.

In the attachment there are two examples with data obtained from the online DOF calculator http://www.dofmaster.com/dofjs.html