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?
Open up a stop for each, and it matches my tests.
Originally Posted by Sirius Glass
Originally Posted by bobwysiwyg
But it comes at a price.
A bit more fuzziness in parts that else would be, uhm..., a bit less fuzzy, in exchange for a bit less fuzziness in places that else would be more fuzzy (i.e. increased depth of field).
The depth-of-field requirements may demand it. Selecting film speed, aperture and shutter speed is full of compromises. Ultimate sharpness is not always the guiding factor.
Originally Posted by bobwysiwyg
That makes sense, thanks.
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This question comes up every couple of years.
Leaving out the mathematics,a 2mm iris opening is the sweet spot
between difraction and coma --as a general rule. The f stop will change
according to the focal length of the lens. I imagine there may be modern very well corrected(and expensive) lenses where this rule may not apply.
Lynn (Jones?) is pretty good at explaining how/why this works
give him a google.
forgot to mention--
the iris in your eye will stop down to a minimum of 2mm--
I assume that'a a built in technology.
Originally Posted by Donmck
I don't think that works very well. If that's true my 6x6 normal lens would have a sweet spot at f/45 and my 4x5 normal lens at f/64. You are deep into diffraction with both lenses at that point. See the graph in my post above.
Actually, I don't think it's an absolute measure.
Diffraction is IMO, a much over-rated concern for MF and Large Format unless you are producing exceedingly large prints.
If you look at Ralfs’ excellent graph of performance v aperture, for a MF Lens it gives you a working range of f11 to f16 for critical work and f11 to f22 for standard observation.
If you have a 5x4 the operating range is even greater f16 to f32 for critical work and f16 to f45 for standard work.
If you are at all concerned about the effect diffraction has on image quality you could perhaps try looking at some of John Sextons work.
Fabulous images of the very highest quality, which tend to be shot at very small apertures (typically f22 to f45).
John kindly provides technical data for each of the shots in his books.
It is worth considering, if you are concerned when looking at the chart, that photographic paper can only resolve about 5 l/mm
I believe that is correct, though maybe, somewhat misleading.
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.
At the same time diffraction is increasing,film size is increasing-----at a linear rate ,keeping the signal to noise ratio the same.
So, a print from 135,6x9,and 4x5 should all be "as sharp."
So, I should have said,if your using small f stops, below 2mm is where diffraction becomes a big problem.