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*Subject*: Re: [Rollei] OT DOF indices, circle = 1/1750*From*: bigler@ens2m.fr*Date*: Tue, 13 Nov 2001 14:54:47 +0100 (CET)*References*:

From Richard Knoppow: > FWIW, Kodak states that the depth of field charts published for > their large format lenses is based on a circle of confusion equal to > 2 min of arc or approximately 1/1750 of the focal length. I like the simplicity and (apparent) universality of this definition for the acceptable circle of confusion for DOF scales very much, since it is exactly twice the angular resolution limit of the average naked eye (1 minute of arc), and thus *potentially* as universal and less questionable as was the definition of the metre (equal to 1/40,000,000 of the Earth circumference, blue, white, red flag again: personally instead of 1/1750 I would have chosen 1/1789 ;-);) More seriously, it means that if the final image is observed at the "standard" viewing distance (i.e a viewing distance equal to the focal length times the enlarging factor) we'll get exactly 4 "eye-pixels" per "image-pixel" in the zones of the image where the acceptable limits of sharpness are reached, hopefully a small fraction of the whole image only, since we usually want the main subject to be the sharpest. Probably the "1/1750-th" rule should apply only to "standard" lenses covering a "standard" angle of about 54 degrees; wide angle or telephoto lenses requiring probably a more detailed analysis. So let's put this definition to work: here is what we get focal length circle c=f/1750 | focal length circle c=f/1750 f(mm) (in microns) | f(mm) (in microns) 35 20 | 75 43 40 23 | 100 57 50 29 | 150 86 The values of c=24 microns for the Rollei 35 (f=40) and c=30 microns for the Bessamatic (f=50, and many others 35 mm cameras) fit very well within this "1/1750-th" rule. The value of c=50 microns for the R-TLR is slightly larger than a "suggested" c=43, but clearly a value of c=75 microns as chosen on some Z**-H** CF lenses, surprisingly, does not match the rule at all. - -- Emmanuel BIGLER <bigler ------------------------------

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