What you are missing there is aperture size. As your focal length goes up so does the diameter of the aperature for a given f-stop. The larger aperature (not f-stop) means a larger circle of confusion. If you make an 8x10 print from a 35mm and from a 4x5 the magnificaton stays the same (with reguard to the subject to image size), but while a 50mm has a aperture of 25mm at f2.0, a 200m has an aperture of 25mm at f8.0 so to get the same DOF you would need to set your 200mm 4x5 lens at f.8.0 and your 35mm 50mm lens at f2.0.
The problem most folk seem to have is from us using f-stop and aperture interchangablely. The strictly speanking are not the same thing at all. Note in the forgoing example both lenses have a 25mm aperture while the f-stops are far different. --graywolf ----- Original Message ----- From: William Robb <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, October 26, 2001 1:31 AM Subject: Re: DOF and format size > I am not quite sure about this. Presuming that DOF is (as I have > been taught) is a function on reproduction ratio (both on the > negative and on the print), it would logically follow from your > arguement that medium format would have an edge over 35mm, and > large format would be better still. > Here is why. The larger negative, because it needs to be > magnified less than the smaller one for any given print size, > will show a smaller COC on the print. > This would go against the generally accepted norm that smaller > formats give better DOF. > Or am I missing something???? - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
