Originally posted by Class A Not to be pedantic, but the P&S lens (not the sensor) produces the large DOF. At the long end, the H55 lens is at FF-equivalent f/20 wide open. You could get the same large DOF if you found an APS-C lens that can be stopped down as much as the H55 lens.
You're right that it's the lens that makes biggest difference in the DOF, but the key factor in P&S lenses and DOF is not the aperture, but the
focal length. Ignore all this "equivalent focal length" nonsense. Lenses have a true focal length that is a physical property of the optics, and is usually marked on the front of the lens. For example, the Canon SX20 IS superzoom has an advertised "equivalent" focal length of "28-560", but its
actual focal length is 5-100mm, as marked on the lens itself. Imagine what the DOF on your DSLR would look like if you used a 5mm lens, if one existed.
It is a well known phenomenon that for a given magnification (absolute terms, not scaled to sensor size) DOF will appear roughly the same (with some exceptions at the extreme ends). Magnification is a ratio between actual object size and absolute size of the image projected into the plane of the sensor (regardless of size of sensor). Magnification is dependent on two things: focal length and distance from subject. For a P&S, the absolute magnification requirements (size of the image) are far less because the image sensor is much smaller.
Thus you can look at it two ways:
- The focal length on the P&S is far shorter than the focal length of a lens used to produce the same frame on a larger sensor at the same distance.
or
- The distance between the P&S and the subject is far greater than that needed to produce the same frame on a larger sensor with the same focal length.
Either way, it means that that P&S produces far less magnification, and thus far greater apparent DOF for the same frame. There are other factors involved, such as the "circle of confusion" (which not only depends on the sensor, but how closely you examine the final image), but fundamentally the focal length is the biggest difference.
Last edited by Cannikin; 02-14-2011 at 10:30 PM.