Archived posting to the Leica Users Group, 2003/04/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I just used the spreadsheet that Mr. Deadman mentioned to "run some numbers" and confirm what I had expected: For a given size of final print, angle of view, and lens aperture, the apparent depth of field is identical, regardless of the format of the capturing device. What does this mean? Well, I can shoot an image with a 35mm camera using a 50mm lens, and the same image with a 4x5 camera using a 200mm lens (roughly the same angle of view). Both lenses set at f5.6. Then I print the images at 4x5 inches. The prints will be identical. How can this be? It all has to do with the circle of confusion. Using the calculator, I plugged in some numbers. For the circle of confusion in the 35mm format, I specified a value of 1/100". When I print the 35mm image to 4x5 inches, I will enlarge the CoC by 4x (enlarging it to 1/25"). To get a matching print from the film shot on the 4x5 camera, I can specify a CoC of 1/25". This matches the enlarged CoC from the 35mm print. With these numbers plugged into the calculator, I get the following results: Focus set to 1 metre. 50mm lens on 35mm camera, f5.6: nearest focus .92 metres, farthest 1.09 metres. 200mm lens on 4x5 camera, f5.6: nearest focus .92 metres, farthest 1.09 metres. Of course, when I am using my 4x5 camera, I will not be satisfied with a CoC of 1/25", especially if I want to enlarge the print to 16x20. I will want to stop down my lens to get a CoC more like 1/100". Depth of field is really an "illusion". By choosing a maximum circle of confusion, you are merely saying that you will consider a "fuzzy" point "sharp" until its fuzziness exceeds a given threshold. If you want to enlarge your film to higher degree, the maximum CoC you are willing to tolerate becomes smaller, and you must stop your lens down more. (If you don't believe me, I invite you to download the spreadsheet, and run the numbers for yourself.) BTW: the spreadsheet is very nice, and very useful! - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html