.. 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 ;-)