Prof Dr Bigler and Ms Bach aren't wrong, as far as they go.
I'll guess that genotypewriter was hinting at the problem that DoF and hyperfocal distance calculations necessarily make assumptions about CoC. Those assumptions may not conform to your picture-viewing requirements. I always appreciate it when the author of a DoF calculator allows for the CoC criterion to be set freely.
There's also a subtler point, something Harold Merklinger explained nicely - placing the focus point so that the calculated DoF reflects your CoC criterion doesn't necessarily result in a picture that looks "sharp".
Since we're talking about it, there's another point that's subtler still: the optical formulas on which DoF calculations are based assume idealized behavior in the defocus transition. In fact, real lenses vary in their bokeh. Perceived DoF varies as a result.
The upshot is that it's not possible to calculate your way to perfect focus.
To be clear: used sensibly, together with other information, DoF calculations can help you make pictures that look more or less the way you want them to look. Just don't kid yourself that you can turn the crank on one of these calculators and use the result to make a picture that's sharp from 15.2 to 47.3 feet.
Oren, thanks for the reply. I'd still like to hear from genotyprewriter. To my literally little mind, "highly variable" and "subject to assumptions" aren't quite the same.
I thought everyone knew that. My macro DoF calculator insists on being given a CoC, also points out the diffraction limited resolution ...I'll guess that genotypewriter was hinting at the problem that DoF and hyperfocal distance calculations necessarily make assumptions about CoC. Those assumptions may not conform to your picture-viewing requirements. I always appreciate it when the author of a DoF calculator allows for the CoC criterion to be set freely.
Agreed. But you can find out what probably isn't possible.The upshot is that it's not possible to calculate your way to perfect focus.
Cheers,
Dan "limits are often worth the trouble of learning" Fromm
Last edited by Dan Fromm; 29-Mar-2012 at 17:23. Reason: typo
The issue of diffraction effects is a subtle point.
Since we are often dealing here with the best lenses ever designed for LF photography, used at their best f-stop, we can consider that residual aberrations and diffraction roughly contribute to the same extent and actually limit the ultimate sharpness, at least for objects that we focus on at best.
For far out of focus objects, we can simplify the model by taking only into account geometrical optics en neglecting diffraction and aberrations. Hence from a very crude, oversized, value of the circle of least confusion, and for objects far out of focus, geometrical optics and classical Dof equations do apply. However if you insert in your DoF calculator very small values for the circle of least confusion, the model fails because small "c" values can be smaller than the minimum diffraction spot size, roughky equal to "N" microns where "N" is the f-number. For example, following the Holy Example of Saint Ansel, with your left hand, you stop down your lens to f/64 ; and at the same time, with your right hand on your palmtop calculator, you enter "32 microns" for the CoC. Doing so your are plain wrong. And a good calculator should warn you against this dangerous behaviour (using a palmtop with your right-hand only, while your left hand manipulates the controls of your lens at the same time is the ultimate challenge for any LF photographer)
However (and not kidding) we all know that the transition between sharp and unsharp play a very important aesthetic role. Hence let's experiment and see what happens ;-) I want my swirling bokeh ! Like a natural product, I do not want it to be modelled by a palmtop.
An additional (marginal) remark for those you love precision in equations (they'd better go out and take pictures, but this is another story )
If you use the general DoF formulae valid in general for asymmetric lenses, you'll find that the hyperfocal distance, namely the distance where you expect DoF to extend to infinity at the back of the object on which you focus "spot-on", is the same as for a plain, simple, symmetrical lens (or a single thin lens element).
Hence people should not care for general DoF equations with a telephoto, mosty used for landscape at long distances : for all lens designs used for far-distant objects, the hyperfocal distance f^2/(Nc) (of f + f^2/(Nc) if you prefer) is the same regardless of the pupillar magnification ratio.
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Reminder : Jeff Conrad's DoF articles explain everything (for those who prefer to read maths prior to take pictures ;-)
part I, Introduction to Depth of Field
http://www.largeformatphotography.in...IntroToDoF.pdf
Part II, Depth of Field in Depth
http://www.largeformatphotography.in...DoFinDepth.pdf
When you are focused on the hyperfocal distance in the subject space, the distances from the focus point to the near point and far point (infinity) on the rail will be just about equal. Whenever you use the near and far points on the rail to focus, theoretically, you should focus so the distance from the rear standard to the rear principal point of the lens is the harmonic mean of the corresponding distances for the near and far point. But it is almost always true that this point is so close to the midpoint between the near and far points that you might as well use the midpoint.
If you favor the near point, as suggested above, you are biasing the focus towards the foreground, which may be a good idea in some circumstances, but it is not generally so.
In any case, the rule will not set the focus on the hyperfocal distance.
It is possible that the suggestion is referring not to distances along the rail but to subject distances. But in that case, it makes even less sense. The distance in subject space from the hyperfocal point to infinity is infinite.
There is another rule of thumb which suggests that one should focus so that the near depth of field is one half the far depth of field: i.e., focus one third of the way into the scene. This rule works when you focus at about one third the hyperfocal distance and for no other distance.
No it's not "wrong". In scientific writing, "variable" doesn't imply that something is incorrect or it is bad.
I said the values are "highly variable", as in reliant on other factors. And making assumptions on those other factors mean there will be limitations to the meaningfulness of of the HFD values. For example, you might assume a CoC for a 150dpi 8x10" print which will give a HFD of say 10ft. That HFD value is not valid anymore if you were to print the same image at 300dpi.
Oren read it right. And "variable" means it is "subject to assumptions", although they're not the same... just like not everything that's "edible when cooked" is "raw chicken".
Thanks for that. I also was wondering what you meant. You had written "Why hasn't anyone mentioned that HFD is a highly variable value..." and yet I had mentioned that it was an inexact calculation in the previous post to yours, including a mention of the variation in the value of the maximum acceptable circle of confusion.
G, vagueness is a sin.
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