Archived posting to the Leica Users Group, 2000/11/10
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]on 10/11/00 8:19 am, Brougham at brougham3@yahoo.com wrote: > Mark Rabiner <mark@rabiner.cncoffice.com> wrote: > >> If you fill the top to bottom of the chair with a 28mm it should give >> you just as much front to back dof as when you pull way back across >> the street and fill the chair top to bottom with a 90mm lens. >> >> But it doesn't'. > > Why should it be the same? DOF is based on aperture and distance from > the lens. No, that's wrong, sorry. Go and look at Ansel or any textbook and work through the equations. They're not that difficult if you did geometry in High School. DOF is based on: 1. Size of the circle of confusion. This itself is a function of average magnification of the image when viewed, which is why different formats use different circles of confusion. So the DOF is format dependent. 2. Aperture. 3. Not distance but *magnification* It is NOT directly a function of lens focal length, which is what we've been trying to say. For the same aperture and magnification of the subject, DOF is the SAME for different focal lengths. So Mark's example above is correct. If you don't believe me, you can use the formulas to confirm that a 25mm lens focused at 1m, a 50 at 2m, a 100 at 4m and a 200mm lens focused at 8m, all of which will produce and image of the same magnification, all have the same depth of field (55 centimetres in case you're interested). By extension, if you crop an image from a wider lens so that the subject is magnified to the same degree as it is in a picture taken of the same thing from the same distance using a longer lens... you get the same DOF. It's the photographic equivalent of the constancy of the speed of light. Or the laws of thermodynamics. Or, no matter how hard you shake it the last drop runs down your leg. As to Mark's point about why you might not see the same DOF in practice, who knows? Differences in lenses, differences in aperture accuracies, differences in the way depth of field is perceived in the different perspectives of a wide and a long lens? - -- Johnny Deadman http://www.pinkheadedbug.com