Wrote Charile Loeven:
>These calculations on the focus distance were made when film had a
>resolution of around 30 lines per millimeter.
>Today film has a resolution of 100 to 160 LPM.
>The suggestion was to use about half the DOF range or close 2 additional
>stops than indicated for the desired DOF.
The resolution of film never figured in these calculations. The resolution
of the eye did. This is 5--7 lines per mm at a viewing distance of about
30 cm (1 foot). Any print looks sharp if it resolves at least 5 lp/mm. So
when only contact prints were made from large format negs, no neg ever
needed more resolution than that. This is why people got good results from
pretty primitive lenses such as two element achromats, three element
Cooke triplets or four element aplanats.
When enlargement began, the eye resolution had of course to be multiplied
with the enlargement ratio. Here are some numbers:
If a 35 mm neg is enlarged to 13 x 18 cm, the enlargement is 5 x so we
need 5 x 5 = 25 lp/mm in the neg. If we go up to 18 x 24 the enlargement
is approximately 6.7 x and resolution must be 33--34 lines. 24 x 30 means
8.3 x which is 41 - 42 lines -- in theory, because we do not view a foot wide
print from a distance of one foot! We cannot take in the whole image at
that distance, and we insist on *smelling* the print, the worse for us. So
in practice, 18 x 24 is the critical size and what goes at this size, goes.
I think that the calculations of the maximum acceptable circle of confusion
used nowadays in the industry are based on a circle of 1/30 mm, i.e. 30
lines. Formerly, you could find medium format cameras with d.o.f. scales
based on 1/10 mm, and 35 mm cameras at 1/20. --Of course sharpness is
at a maximum in a plane at a right angle to the optical axis. Please note that
a plane has NO thickness! So if we demand absolute max sharpness, we
have no d.o.f. at all. This obviously would be silly. And who goes at a
negative
with a microscope? Not me.
Find a 35 mm camera of the 'thirties, and if it has a d.o.f. scale, you
will find
that it lists much more generous d.o.f. than a modern 50 mm lens does.
This is of course because the maximum permissible circle in the neg has
shrunk with increasing enlargement ratios, so as to keep the circle in the
print blow 1/5th of a mm.
Hälsningar/Regards
Lars Bergquist
Timberwolf Type, the independent
specialist in text typefaces - visit me at
<http://www.timberwolf.a.se/>
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