Is there a chapter on "Making guesses on the intended end format of the lens" in these books?
Try not making ridiculous, assumptive statements, Sonny.
No it is well documented on books on optical engineering and lens design. See Kingleslake for examples. Also Modern Optical Engineering by Warren Smith. Optics Eugene Hecht. Try doing your homework in advance instead of shooting from the hip sonny.
The only point of any relevance is whether diffraction can be seen in the print at normal viewing sizes - and let's not get into a discussion about what's "normal". One person's kick in, is another's "dominant influence on the blur circle".People who say "diffraction kicks in at f/whatever" are annoying. Diffraction is always there. It doesn't "kick in" at an aperture, at some aperture it becomes the dominant influence on the blur circle. And this has nothing to do with what's capturing the image.
I didn't write that cropping changed things. I wrote that it is impossible to put a predetermined criteria on a lens if you don't know its intended application. Now do you understand why your unsubstantiated statement is wrong?
In terms of LF lenses, how do they know what format the lens is to be used on? For example, a 14" lens will have different (visual) diffraction character on 4x5 compared to 8x10.
I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?
What's the difference between shooting a 14" lens on 8x10 and cropping to 4x5 and shooting it on 4x5?
I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?
I'm certainly not afraid of numbers and calculations but this procedure would seriously discourage me from taking any photographs at all. I just aim for f/8-11 and liberalyrely on the DOFscale on the lens.As others have said, if this gives you satisfactory results, thats all that matters.
Could you do better? Using the largest aperture practical for the situation gives the best resolution due to restricting the percentage of diffracted light forming the image. For critical purposes it is possible to select the optimum focusing distance and aperture for a given format and situation. It involves calculation (which many of us would rather avoid).
The hyperfocal calculations can be used to quickly generate a table of hyperfocal focusing distance and aperture combinations for any lens and format when we require the depth of field to extend from one-half the hyperfocal distance to infinity. We can also calculate the required point of focus and aperture for a finite depth of field.
For a finite depth of field, if you know the following distances from the lens:
A = distance from the lens to the nearest point at which the depth of field must begin
B = distance from the lens to the farthest point to which the depth of field must extend
Then we can calculate the ideal lens-to-subject distance s.
For example, if weve composed a scene for which the near limit of DOF is A = 6 meters and B = 47 meters, then s = 10.6 meters. This is true for any format and any focal length lens.
If we also know the circle of confusion diameter c for the format and the focal length of the lens f, we can compute the maximum aperture (minimum f number) that gives us depth of field from A to B. In some cases the required aperture number is impractical or not even available. The calculation gives us the required information.
In the above example, for the 35mm format and using a circle of confusion diameter of c = 0.029mm and 50mm lens, we get aperture value N = 6.3 (aperture closed about 0.31 stop from f/5.6, or opened about 0.69 stop from f/8).
For the 6 x 7cm format using c = 0.059mm and 110mm lens, N = 15.1 (aperture closed 0.83 stop from f/11, or opened 0.17 stop from f/16).
For the 4 x 5 format with c = 0.10mm and 150mm lens, N = 16.6 (aperture closed 0.11 stop from f/16, or opened 0.89 stop from f/22).
This procedure maximizes resolution for a given scene with finite depth in two ways:
1. It uses the ideal point of focus relative to the near and far limits of the field.
2. It uses the largest aperture (smallest f number) that produces the required depth of field and thereby minimizes the percentage of diffracted light forming the image.
For casual shooting the above procedure is overkill. But in some situations, the extra steps might be justified, such as the case of a costly long-planned trip to a distant, hard-to-get-to location of unusual interest and beauty, or a scene that will soon be obliterated due to development giving us oneand only onechance to capture it on film before it is gone forever.
I'm certainly not afraid of numbers and calculations but this procedure would seriously discourage me from taking any photographs at all. I just aim for f/8-11 and liberalyrely on the DOFscale on the lens.
I calculate (or view using the manual stop down lever) the f stop based on the DOF I need. Then I stop down one additional stop to be sure. Is this a bad practice?
I have known some nice stoats at work too.
Larry, let LJH speak for itself.
Sorry. Just trying to add a little clarity to the discussion, as it seemed a little blurry.
The difference is you will enlarge the 4x5 crop twice as much as the 8x10 to make the same sized print. So diffraction effects will be twice as visible in the print from the 4x5 crop.
Also the 8x10 will show a larger angle of view, so you would normally choose a lens that was half as long if you were to switch to 4x5 to capture the same image (it won't ever be the same).
Sorry. Just trying to add a little clarity to the discussion, as it seemed a little blurry.
Some in the running discussion were and are not what we would call terribly focused to start with.
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