On Thursday, January 10, 2002 at 22:01, Bernd Moeller
<olympus@xxxxxxxxxxxxxxx>
wrote re "Re: [OM] [OT] two OM items for sale -> browsers -" saying:
> Wow. But do you actually work with the aperture wide open? Usually I
> step down my Rodenstock 3.5/50 to f11 or f16 in order to achieve good
> DOF and better consistency of exposure time.
>From http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=001Kur
"You may be surprised that I suggest using these lenses at full aperture.
However, diffraction effects swamp improvements in optical distortion even
closed down a single stop. Howard Bond has done extensive testing in this
area (as readers of Photo Techniques will recall), and I concur with his
conclusions. I should also mention that most LF lenses (with the possible
exception of certain fast -- i.e. faster than f5.6 -- press lenses) are
sharpest wide open (assuming you have adequate coverage for your chosen
format at that aperture). If you were comparing prints made with your
lenses stopped down, you were not seeing either at its best. Try them both
wide open, compare them against each other and against prints made at
smaller apertures. Then decide which you prefer. "
There is a lengthy discussion there. The smaller apertures are useful to
lengthen exposure times for easier manipulations.
The diffraction limit in lines per mm is equal to the f/stop times the
wavelength for a high-contrast target:
The diffraction limits in lines per mm for blue and red light are:
F/stop Blue(400 nm) Red(700nm)
1.0 2500 1429
1.4 1768 1010
2.0 1250 714
2.8 884 505
4.0 625 357
5.7 442 253
8.0 313 179
11.3 221 126
16.0 156 89
22.6 110 63
32.0 78 45
45.3 55 32
and so on.
DoF doesn't matter for enlarging if the film carrier, lens, and easel are
properly aligned (using levels) and the lens' curvature of field is
minimal, which is normal for an enlarging lens.
Stopping down will usually reduce any disparity in exposure between centre
and edge.
tOM
------------------- http://groups.yahoo.com/group/ottawa-
photo-clubs
tOM A. Trottier, ICQ:57647974 http://abacurial.com
758 Albert St, Ottawa ON Canada K1R 7V8
+1 613 860-6633 fax:231-6115 N45.412
W75.714
"The moment one gives close attention to anything,
even a blade of grass, it becomes a mysterious,
awesome, indescribably magnificent world in itself --
Henry Miller, 1891-1980
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