... It is worth considering, if you are concerned when looking at the chart, that photographic paper can only resolve about 5 l/mm. ...
Photographic paper resolves well above this figure. I don't have targets fine enough to measure it's capability, but I know from contact printing negatives that Ilford MGIV for example, resolves at least 50 lp/mm. And why shouldn't it? Paper emulsions have a very fine grain, much finer than film.
... Light passing through a 2mm aperture will have the same diffraction regardless of what camera it's mounted on-----however as you move up in format size, the film plane moves farther away from the iris, increasing the effects from diffraction. ...
I believe this is not the case. You can combine the absolute aperture measurement and the film distance to the lens aperture and, in fact, show that the diffraction limit does not depend on a fixed aperture value.
The diffraction limit is dictated by the Airy disk. If the lens is focused at infinity, the calculations for the radius of the Airy disc simplify to:
r = 1.22 * l * N
and the maximum lens resolution (diffraction limit) is then given by:
R = 1/r = 1 / (1.22 * l * N)
where ‘l’ is the wavelength of light, and ‘N’ is the lens aperture in f/stops.
As you can see, the diffraction limit depends on the wavelength of light and the f/stop and is not limited by an absolute aperture value.
Not the same diameter but the same f/stop (regardless of film format) creates the same amount of diffraction.
Newbie here. I've been following these threads and they may go a long way to explaining some of my less than desired results on some shots as far as sharpness goes. I'm a bit of sucker for flowing water shot at 1 sec. or so. Using ISO 100 film I can sometimes get this by stopping way down. Now I see it would be better to use a more optimal f-stop and ND filters. Now to my question, if, for example f22 is optimal for 4x5, why are there additional f-stops way beyond f22?? Is it simply a matter of making fuzziness available for those who like it?
Some people contact print 4x5 (in which case Airy disk size can be acceptably large). The lens manufacturer doesn't dictate how you will use the negatives so you have to know what you are doing when you move that aperture lever (unless you use a Minox, in which case Walter Zapp specifies the aperture for you )
A practical interpretation of what Ralph posted can be summarized as follows: diffraction is proportional to aperture diameter (in millimeters) across formats, whereas it is proportional to aperture numberwithin a format.
So, one way to understand it is that within a format a long lens magnifies the Airy disk, so you need a larger aperture diameter (but it works out to the same aperture number) and a wide lens shrinks the Airy disks, so you can use a smaller aperture diameter (but it works out to the same aperture number).
The equation for the diffraction limit, posted in #22, is totally independent of film format. The f/stop and the wavelength alone control the diffraction limit.
Diffraction is an optical phenomenon not limited to photography. Original research was done for telescopes and astronomy. The diffraction limit is a 'perfect' lens characteristic and has nothing to do with film formats.
Last edited by RalphLambrecht; 09-15-2010 at 07:20 PM. Click to view previous post history.
Diffraction is good to understand, but remember that it is just one thing to consider when crafting your pictures. Most good lenses are of sufficient technical quality to be used at any marked aperture while producing prints that would be acceptable to most people. In many cases, depth of field or exposure time is a larger concern than absolute sharpness.
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