Quote Originally Posted by ultima16888
by thinner dof , do you mean the area in focus will seem smaller?

Yes. If we set aside bellows factor, resolution, and post-processing differences, the only factor that affects the DOF is the lens (or, you could think of it as "lens, sensor size, and print size", but I prefer to think of it as just the lens).


It's important to remember that aperture is not the same as f-number. Aperture is the focal length divided by f-number, so that a 100mm f/4 lens has an aperture of 25mm (100/4).


There are several ways this principle can be stated:
  1. The lens with the widest aperture has the thinnest DOF, no matter what camera or focal length.
  2. When aperture is the same, the lenses have the same DOF, no matter what camera or focal length.
  3. When f-number *and* composition are the same, DOF gets thinner with larger sensors.



What I mean by "composition" is: field of view, perspective, and focus distance.


It's possible to compare two different cameras (e.g. 5D and XSi) using two different compositions. For example, if you put a tripod in a certain spot and don't move it, then put the 50mm f/1.4 on the XSi, set it to f/1.4, and have a focus distance of 5 meters, then you have a certain 25-degree horizontal angle of view. If you do the exact same thing with the 5D, then everything is the same except for the angle of view (field of view), which will be 45 degrees instead of 25.


The DOF will be the same, but the angle of view will be different. I don't think it makes sense to compare different angles of view, because that's a fundamental component of the photographer's composition, and he isn't going to start shooting wider just because he upgraded to full frame: instead, he's going to find a new lens that will give him the same capability that 50mm did on his XSi: such as the 85mm f/1.8.


100mm f/4 on the XSi has the same DOF as 160mm f/6.4 on the 5D. Both have the same composition and both have an aperture of 25mm.


160mm f/4 on the 5D has the same composition as 100mm f/4 on the XSi, but it has thinner DOF. Another way to look at it is this: the 5D will give all your full frame lenses a wider field of view and thinner DOF, unless you stop down to the same aperture for a given composition.


By the way, DOF isn't the only factor that scales with aperture. Light gathering power and diffraction do too.