Leica User Forum - View Single Post - Accuracy of depth-of-field scale of lenses on M8

lars_bergquist · 05/10/07, 09:52 AM

This is an embarrassing subject. Leitz should have changed their tables and scales more than seventy years ago.

The rationale behind the 1/30th of a millimeter circle of confusion (c.o.c) was the following, back in the 1920's: A print, to appear sharp at normal viewing/reading distance (abt. 30 cm or 12 inches) must have a maximum c.o.c. of 1/10th of a mm. The competition was a contact print from a 6x9 cm roll film negative. In order to beat that, the 35 mm neg must be enlarged 3x. So, 1/10 ÷ 3 = 1/30 maximum c.o.c in the negative. And we have lived with that since then; Leitz obviously felt that if they changed to a more realistic value, customers would complain that the re-scaled lenses had deteriorated, because depth of field had shrunk! (Early Retina cameras used a c.o.c. of 1/20, actually).

Today a c.o.c of 1/60th would be realistic, in the M8. That would give us a reasonably good 18x24 cm or 8x10" or A4 print. You get that c.o.c. simply by reading d.o.f at 4 for an actual aperture of 8, and so forth, doubling the numbers. This of course means that you can forget about calculated d.o.f. when using anything longer than 35 mm. From 50 mm on, you just point-focus. Of course d.o.f. exists, but it is too shallow to be used in a premeditated way.

I agree with Sean of course that circumstances and subjects and intentions are important. I don't agree however that anything but aperture and reproduction ratio (a combined product of focal length and subject distance) decides d.o.f. For the same c.o.c. criteria, any 35 mm lens whatever at, say ten meters, will produce the same d.o.f. I could go even further: Any lens, irrespective of focal length, and at a given aperture and ratio of reproduction on the sensor, will produce the same d.o.f.

The old man from the Age of Tape Measure Focusing

05/10/07, 09:52 AM	#14 (permalink)
lars_bergquist Erfahrener Benutzer Join Date: 11/15/05 Location: Greater Stockholm Posts: 1,295	Re: Accuracy of depth-of-field scale of lenses on M8 This is an embarrassing subject. Leitz should have changed their tables and scales more than seventy years ago. The rationale behind the 1/30th of a millimeter circle of confusion (c.o.c) was the following, back in the 1920's: A print, to appear sharp at normal viewing/reading distance (abt. 30 cm or 12 inches) must have a maximum c.o.c. of 1/10th of a mm. The competition was a contact print from a 6x9 cm roll film negative. In order to beat that, the 35 mm neg must be enlarged 3x. So, 1/10 ÷ 3 = 1/30 maximum c.o.c in the negative. And we have lived with that since then; Leitz obviously felt that if they changed to a more realistic value, customers would complain that the re-scaled lenses had deteriorated, because depth of field had shrunk! (Early Retina cameras used a c.o.c. of 1/20, actually). Today a c.o.c of 1/60th would be realistic, in the M8. That would give us a reasonably good 18x24 cm or 8x10" or A4 print. You get that c.o.c. simply by reading d.o.f at 4 for an actual aperture of 8, and so forth, doubling the numbers. This of course means that you can forget about calculated d.o.f. when using anything longer than 35 mm. From 50 mm on, you just point-focus. Of course d.o.f. exists, but it is too shallow to be used in a premeditated way. I agree with Sean of course that circumstances and subjects and intentions are important. I don't agree however that anything but aperture and reproduction ratio (a combined product of focal length and subject distance) decides d.o.f. For the same c.o.c. criteria, any 35 mm lens whatever at, say ten meters, will produce the same d.o.f. I could go even further: Any lens, irrespective of focal length, and at a given aperture and ratio of reproduction on the sensor, will produce the same d.o.f. The old man from the Age of Tape Measure Focusing Last edited by lars_bergquist : 05/10/07 at 09:54 AM.