Originally posted by flyer You are implying that the APS-C sensor has less depth of field than FF sensor because you enlarge more to get to a given size. Then, can you tell us why P&S cameras with tiny sensors seem to have almost unlimited DOF? By the way, I just like to throw a wrench into what seems to be so obvious at first sight.
Short answer: because P&S cameras use lenses with very short focal lengths or, to put it another way, the magnification is small.
Long answer: as should be pretty clear now, there are a bunch of different factors that play a role in DOF. In this case, we can look at a few of them:
1. Magnification or focal length. P&S cameras use a very short focal length lens, like 5-10mm. The shots "look the same" as from an SLR since the sensor is so small. The small focal length makes the magnification low, giving a large depth of field. This is a big effect. You can see in the equations that the focal length squared comes into play. (all the talk about how focal length doesn't change DOF applies when you don't change the sensor size (and do change the subject distance), but not here)
2. How much you have to "blow up" the image on the sensor to get it to be "usual" sized. You have to blow up the image on the P&S sensor more, this decreases the DOF of the P&S more than the SLR.
3. Pixel size on sensor. The P&S sensors have smaller pixels and can therefore detect "out of focus" more easily. This will tend to decrease the DOF of a P&S more than the SLR.
So, two effects above decrease DOF on P&S compared to SLR and one increases it. It just so happens that the increasing effect is much larger and dominates, making P&S have a larger DOF than SLRs.
edit: looks like Marc beat me to the answer