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Re: [Rollei] OT DOF indices, circle = 1/1750



  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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