Originally posted by Class A An equivalence is an equivalence. It doesn't break down for some cases.
True, What I should have said, the equivalence can lead to misleading visceral reactions. Yes, assuming your math is correct, a 40/28 works out to a 60/4.3 in FF digital terms. But what are we to make of this? Unless we have actual experience with what a 60/4.3 can do on FF digital, how does this help us understand what the 40/2.8 can do on APS-C? So what actually happens in practice is, we see the f/4.3 and freak out, since our only experience is with APS-C digital. So appealing to a format few of us have experience with just ends up being misleading, even though of course completely correct.
Quote: I believe that many people mistake an APS-C f/2.8 with the FF f/2.8, i.e., think they can get less DOF than they can really get.
Could be - like I said, it depends on if you happen to have experience with FF. If you don't, than that comparison just misleads. I know what kind of DOF f/2.8 gives me on APS-C; comparing to another format I have little experience with doesn't clarify that.
Anyhow, my impression is that at close focus distance, I get as shallow a DOF as I might reasonably want from my DA40/2.8. But I am happy to have my manual 50/1.7 for the fairly rare situations when I want to get shallower DOF at longer distances. If you're a shallow DOF junkie, indeed, that will be an issue more often.
Quote: Scale this down to P&S sensor formats and then suddenly pretty much everyone knows how difficult it is getting thin DOF on such cameras even though the lenses often are specified as f/2.8 as well. Their FF equivalent f-ratio is much higher.
Right - the difference between APS-C and a typical P&S is far greater than the difference between APS-C and FF in this respect.