Aperture to depth of field normalisation across various formats

[Emmanuel BIGLER]

.. despite the method having been mentioned twice ..

Hi, Helen ! Remember the old Roman sentence : "testis unus, testis nullus"; so stating exactly the same thing 3 times, by independant witnesses, who never met before, exactly complies with the ancient Roman wisdom

BTW I checked the rule of thumb against my home-made DOF calculator, with the following entries into the program :
- a fixed subject-to-lens distance used by Richard Avedon in his famous series of protraits in the West, the distance was about 6 feet, say 1,8 meter.
- for the 35 mm camera, a focal length of 90 mm and reasonable f-numbers: 2 - 2.8 - 4 - 5.6 - 8
- in 6x9 cm with a "simulated" focal length of 192 mm delivering exactly the same "filling factor" in "portrait' mode
- in 4x5" with a f.l. of 270 mm and in 8x103 with twice as long 540 mm

I found by a side-by-side comparison of DOF curves that scaling the f-number in proportion of the ratio of f.l. works so well that I doubt that any DOF calculator could be really useful to me in LFphotography in the future, I just have to keep handy an old 35 mm zoom lens with its series of engraved DOF Scales ;-)

[Bob Salomon]

"BTW I checked the rule of thumb against my home-made DOF calculator, with the following entries into the program :
- a fixed subject-to-lens distance used by Richard Avedon in his famous series of protraits in the West, the distance was about 6 feet, say 1,8 meter.
- for the 35 mm camera, a focal length of 90 mm and reasonable f-numbers: 2 - 2.8 - 4 - 5.6 - 8
- in 6x9 cm with a "simulated" focal length of 192 mm delivering exactly the same "filling factor" in "portrait' mode
- in 4x5" with a f.l. of 270 mm and in 8x103 with twice as long 540 mm"

But if you are talking about those portrait shots taken in extreme high key he also used 6x6, a Rollei 3.5F with the 75mm Planar and a Mamiya TLR with the 80mm. How do I know? I was at his studio after he had his assitant, Gabriel, call me in because they were having a flare problem on the 3.5F but not on the Mamiya TLR. Reason? The 75mm lens was picking up some stray light that the 80mm on the Mamiya did not see due to its longer length. Cure? Rotate the offending light a couple of degrees.

[Jeff Conrad]

The “ratio of focal lengths” rule is long established—Stroebel discusses it in View Camera Technique (I have the 1976 3rd ed.). So perhaps we have a fourth independent witness ... The derivation is straightforward—see this article, under, of all things, “Depth of Field and Camera Format.” Do note that the approximation is valid only for a limited range of distances—greater than macro but significantly less than hyperfocal, for all formats compared. This of course assumes the “same picture” criterion that we’ve discussed here several times: among other things, it assumes the same subject distance and angle of view and the same-size final image viewed from the same distance, with the same final-image sharpness criterion for all formats.