Folks, > DOF is a function of the lens focal length and aperture selected, > not the design of the lens. The shape and number of aperture > blades effect the appearnce of out of focus areas. The design of the lens does have a minor impact on the DOF. Aberrations tend to decrease the DOF at larger apertures faster then "expected" (those that are influenced by aperture anyway), and of course diffraction tends to decrease DOF at very small apertures (this is also influenced very slightly by the iris design and manufacturing quality). Having said that, modern lenses should behave (diffraction effects and soft focus lenses excluded) pretty much as expected, whilst older lenses generally will have a small problem with residual aberrations. > Different lenses have different DOF markings, and I suspect > different assumptions about DOF since only one plane of the image > is actually in focus. The size of the enlargment or projected > image effects how much DOF the human eye sees. I am sure an > optical expert can provide a better explaination. I don't have access to the lenses that you are commenting on but ... The most likely reason for differences in marking, is as you say, differences in assumptions. If you have access to the lenses then you could check this out by calculating the size of the circle of confusion that applies for the given depth of field at the given apertures for each of the lenses. The other possible reason that comes to mind for giving few marked points is mechanical. Depending upon the lens construction and the design of the cams, the actual travel of the focus ring may make the markings too close together without the results having changed. Have a look and let us know what you find. Andy Shaw
