I used to drive a VW bug back then.... It didn't even get to 60mph... it was struggling to reach 55mph even...
Hey, I can hijack my own thread!
Enough OT, let's go back to DoF, or is everyone satisfied so far?
Another way to see it (keeping it simple) is that when comparing two different formats and same AOV (angle of view), then the DOF is approximately the same for a given aperture diameter (not the aperture ratio).
4x5 using a 105mm at f/8 will have approximately the same DOF as 8x10 using a 210mm at f/16, since the aperture diameters for 105mm at f/8 and 210/mm at f/16 are the same
As the focal length of a lens increases the DOF decreases..I believe that stands for the most part with all formats. Switching between formats and lenses one might notice a difference, but if using the appropriate lens for a particular format things should fall into place. You have to test this with the same subject, same distance different formats, same f-stop. As always there is some interesting technical information here but again I believe understanding the basics of this is key.
Just my thought.
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