For DoF scales DoF gets smaller as the format gets smaller.
In this case you're assuming constant distances while varying FoV/enlargement. So a smaller format has LESS DoF due to what you're keeping constant. Put another way, you're projecting exactly the same image, but cropping it and enlarging it more. So your CoC has to go down, and so does your DoF. For "equivalent image" comparisons the change in distance more than counteracts this, but that's not the comparison you want for DoF scales.
And the dpreview guy's crack about the scales being useless for APS because they're calculated for FF is especially idiotic. Just shoot a stop or so down from the line you're reading on the lens. Duh.
Getting back to the more useful "equivalent image" case, it may just be f-stop and magnification, but for different fields of view you can only match the magnification at a single plane. So with, say, a 15 vs. a 500 (assuming same format), if you keep the subject plane magnification the same (by changing distance), the magnifications some distance ahead of or behind it will be different. For short DoF this is trivial, but stop down enough at non-macro distances and there will be a large difference in DoF.
And I really don't see the point in arguing over the standard DoF definition. Yeah, yeah, it's right about as often as a stopped watch. But they needed some sort of standard, this is at least a marked-on-the-lens starting point to correct from to get to appropriate numbers for your 16 x 20 enlargements or 4x5 contacts or whatever else it is you're doing that isn't a mediocre 8x10.
Last edited by Roger Krueger; 07-21-2009 at 10:41 AM. Click to view previous post history.
Reason: added last paragraph