>> > 3) Depth of Field is controlled by two things and two things only. >> > Subject magnification and aperture. It doesn't matter whether you >> > increase your subject magnification by moving closer or by using >> > a lens with a longer focal length your depth of field will be >> > reduced. > >This isn't entirely true. DOF depends on focal length whenever >you are not in macro range, IOW, if you start to approach >hyperfocal distance. The DOF of a 20mm and a 200mm, both at f/8 >and at a magnification of 1:100 is extremely different. Looking >through the viewfinder or your average DOF calculator will >show this without a shadow of doubt. True. Out of macro regions (Distance >> focal) the range of DOF is roughly: D_near = ( h * D ) / ( h + D ) D_far = ( h * D ) / ( h - D ) Where D is focusing distance, D_near is the close limit of DOF, D_far is the far limit of DOF, and h is hiperfocal distance: h = f^2 / (N*c) Where f is focal length, N is aperture (f/ number) and c is circle of confusion. Magnification will be roughly: M = f/D Using the formulas you can see that h escales with f^2 while M only scales with f. That's why using shorter focal values (at the same magnification) giver bigger DOF (out of macro). >> > In addition, if you use a camera with a larger format >> > you are magnifying the image more so you will have less >> > depth of field. > >If this is meant to be done with "equivalent" focal lengths, ok. Yes. But if you consider a equal final paper size, the global magnification (negative plus copy) will be the same, but the bigger format will get les DOF. Just talk to big format folks or test it on the field. >> > Now most people think that if you switch to a wide-angle lens you >> > will get more depth of field, that's wrong. > >Hmmm. See above. Yes. A wide angle lens gives more DOF out of macro. >> > If you switch to a wide angle lens and do not move closer to your >> > subject, you are decreasing subject magnification and your depth >> > of field will increase. > >Yes. > > >> > However, if you move closer to have the same subject fill the frame, >> > you depth of field will be the same. > >True only for close ups. (= big magnifications) Yes. Only for close ups. At close-up DOF is roughly (from the Lens FAQ): Frontdepth=Reardepth=Ne*c/(M^2) Where Ne is effective f/ value, c is circle of confusion and M is magnification. >....... > >> Assuming this is true, and I strongly believe it is, >> then your conclusion (below) doesn't hold up: >> >> >> > This explains why digital cameras have a lot of >> > depth of field, there CCD's are so small, subject magnification >> > is very low and depth of field is great. >> >> Assuming I enlarge the image on film and from the CCD >> to the same final size, on paper, then total >> magnification is equal, it seems to me. So shouldn't >> DOF be equal, too? > >It should, but it isn't. In fact it shouldn't. You only need to use the formulas o do a field test. Sentences like "DOF only depends on magnification and f/ value" and "DOF is 1/3 in front and 2/3 on the rear" are common errors which only holds for a given situation. The first sentence only holds for macro and the second only for one subject distance. >Actually, even at same print size, the >apparent DOF of the print made from the CCD image is bigger, >although it needs more enlargement in the printing step. Yes. That's because h scales on f^2 and M on f. >Thats also why large format photographers don't get more DOF >on a print, even though their lenses can be stopped down much >more than 35mm lenses. Ok, not really, they can and will use >the movements of their camera to their advantage. > >Thomas Bantel Best regards Vicente * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
