stopping down, how much is too much?

[Leonard Evens]

You should look at the discussion of how to choose your f-stop at this same website. In particular, you might be interested in Paul Hansma's method described therein. He has produced a formula which for a given focus spread on the rail---discussed there also---gives you the f-stop which gives you the best balance between diffraction and defocus (related to depth of field). The formula involves a square root and most people would not want to try using it in the field, but it is easy enough to carry a small table with you. The website has the table. I find the f-stop recommendations too conservative in many siutations, but it at least can give you an indication of the smallest f-stop you might want to consider for a given focus spread.

[Emmanuel BIGLER]

I like this discussion very much because we try here to criss-cross
the physicist's point of view with an aesthetic point of view.
Difficult to argue against the Laws of Physics. But it is more fun to
ignore them and do whatever suits our hands and eyes in the whole LF
photographic process.



A closed-form physicist's and engineer's point of view could be :
Brian. There exist and optimum f-stop; say : f/22 for the 4"x5"
format, scale the optimum f-number like the diagonal of the format,
this yields f/45 in 8"x10", etc.... in macro at 1:1 ratio : take into
account the effective f-stop which is one stop further w/respect to the
engraved f-stop. You are "allowed" to close down 1 or 2 f-stops beyond
the optimum engraved f-stop of your preferred LF format. If you dare to close
down even more... you are entering the forbidden zone (forbidden only by theoretical considerations, of course) where so many
bad things can happen to your image that you should not do it, no,
never ;-);-)



The aesthetic point of view says : Brian; as far as LF photography
is concerned no, never listen to theoretical engineer's
considerations. LF photography means : absolute freeedom. You
can "mechanically" stop down to f/256 ? please, do it and enjoy ;-)



Not kidding now, there is an interesting article (sorry, in French) by
Jean-Marie Solichon with some real images taken with, or without
entering the "forbidden diffraction f-stop zone".

http://www.galerie-photo.com/diffraction.html

In a discussion on image "sharpness", the choice of cactus plants as a test object is probably meaningful ;-);-)
Jean-Marie prefers to stop beyond f/22 "only in case of absolute
necessity".



Une impression sur papier photo brillant des fichiers originaux en
TIFF confirme cette impression sans ambiguïté et me conforte dans mes
habitudes tendant à diaphragmer plus que f :22 uniquement en cas de
nécessité absolue.

Stopping down so much that you enter the "diffraction zone" has the
advantage of an homogeneous un-sharpness across the whole image field ;
this may, or may not be acceptable. Pinhole aficionados like very much
the absolute homogeneity of image (un)-sharpness from one inch to
infinity in the image field. My usual objection against alleged infinite depth of field in pinhole photography is : none of any image point will satisfy my preferred sharpness criterium, e.g. 100 microns of defocused spot in 4"x5" ; so to me pinhole depth of field is : zero because none of the object points are actually imaged with sufficient sharpness ; so do not transfom my beloved LF camera fitted with a beloved sharp optics into a pinhole camera ;-);-);-)



Although I accept the idea of homogeneus unsharpness from an aesthetic point, although I respect the quest for absolute freedom in pinhole photography (down with glass and optics ! down with Scheimpflug ! down with focusing !down with expensvie gera, we demand a cardboard shoe baox a s acamera ! down with flat field and flat image detectors, we demand curved image detectors ! ), this point of view this does
not actually suits my taste on the resultiong images ; I prefer sharpness and I agree with the
article by Jean-Marie Solichon, to me large format photography means
excellent sharpness, not homogenous un-sharpness ;-);-);-)



But I was trained as physicist, not an artist !

[Struan Gray]

Even physics argues for an aesthetic approach. The Airy function is quite sharply peaked, especially compared to the top hat of defocus, so 'best' is indeed in the eye of the beholder. For my money, you can get away with quite a bit more diffraction blur than the 'width' of the Airy function would suggest.

[Kirk Gittings]

"I think that insuffcient depth of field will almost always be more objectionable than whatever minimal loss of sharpness results from diffraction so I use whatever aperture is needed for the desired depth of field without being concerned about diffraction." Brian

This is the god's truth. Well said.