Bob Blakely <Bob@Blakely.com> writes: > So that all can see that DOF is dependent on the focal length of the lens as > I said, I will provide my example from: There's an "engineering" proof. Here's a "visual maths" proof. Take a camera of a certain format focussed at the hyperfocal distance D. Draw a detailed and precise optical diagram, showing the paths of light rays, producing circles of confusion of just the acceptable values for objects at infinity and D/2. (We don't need to actually draw it, just know it can be done.) Now enlarge this diagram by a linear factor of 2. Obviously it's still optically valid, and the image size has been (linearly) doubled, but so have the circles of confusion. Therefore the image quality is exactly the same. At this point, 37% of candidates jump in the air, saying: "See, the DOF is constant!" but fortunately most of them realise very shortly afterwards that they've forgotten to scale up the **hyperfocal distance** as well. Therefore a shorter focal length lens does indeed provide greater depth of field. (Remember that the distance to infinity is exactly the same as twice the distance to infinity.) You can check that the near focus is D/2 using my calculator: http://imaginatorium.org/stuff/focus.htm Brian Chandler ---------------- geo://Sano.Japan.Planet_3 http://imaginatorium.org/
