DoF Charts and f/stops

[Pere Casals]

Thanks for the clarification, really a very good explanation.

So can we guess that usual dof scales and calculators are using the engraved aperture as input data?

...as N is asked, or delivered...

[Mark Sawyer]

I just spent a while reading the DoF articles on the LF Home Page, and they confirmed that I'm not as fluent in math as I once was. The article at http://www.largeformatphotography.info/dofknob/#how to use it contained the following passage, (the underlining is mine):

"Good things to know
The min. f-numbers calibrated are correct at magnification M = 0, infinity focus. However, at any magnification, the min. f-number required for a given delta will be less than that at infinity focus. Therefore you're always guaranteed to get min. DOF for a given delta at any magnification.
At magnification M = 1 (life-size, 1:1), the min. f-numbers N on scale are half. At any magnification in between 0 and 1, the min. f-number required will fall in between the f-number scales and their corresponding halves. For an arbitrary N, use N(M) = 1/(1+M)*N(M=0) to figure out exact min. f-number required at M. For example, at M = 0.5, N(M=0.5) = 2/3*N(M=0). So multiply the f-numbers on scale by 2/3."

From this, I'm presuming that one uses the effective, not marked, f/stop. But from Emmanuel's post above, and from looking at varying results given by on-line calculators, I'm guessing some calculators/charts may figure this in, and some don't.

BTW, here's a deeper look at some of the math...

http://www.largeformatphotography.in...DoFinDepth.pdf

[Emmanuel BIGLER]

So can we guess that usual dof scales and calculators are using the engraved aperture as input data?

Hi, Pere! In short, yes, exactly, at least this is how I feel things.

Well, it is always uncomfortable when DoF calculators do not precisely define their inputs, or worse, do not tell which formulae are actually in use, and the limits of validity or relevance (for example, the good old model for DoF is based on geometrical optics assumes that diffraction is negligible).

However, it is reasonable to think that authors of those calculators aim at a general public who never heard about anything else than the engraved f-number, which by nowadays standards, denotes a very deep knowledge in photographic optics: look at modern digital cameras, most have no F-number scale engraved at all!

More seriously, general formulae valid at any distance are not much complicated, even when insisting on using exotic formulae --that actually nobody uses-- with lenses exhibitig non-unit pupillar magnification ratio.
Not very complicated, I mean: taking into account that anybody who carries a smartphone with him 24/7 has access to a computer more powerful that the ones in use for the Manhattan Project

General formulae do not tell us immediately that in macro, DoF no longer depends directly on the focal length, which is of course somewhat counter-intuitive but very interesting in practice, immediately applicable to LF photography in the macro range.

The interest of deriving approximate formulae valid in the macro range is that the maths eventually explain why the focal length magically dissappears from the simplified version of the formulae, and yields a very simple expression, that you can compute as you wish, no software needed.

[Pere Casals]

Well, it is always uncomfortable when DoF calculators do not precisely define their inputs, or worse, do not tell which formulae are actually in use, and the limits of validity or relevance (for example, the good old model for DoF is based on geometrical optics assumes that diffraction is negligible).

A calculator also doesn't ask if the lens is unit focus or internal focus, I'd ask... may this be a concern?

[Dan Fromm]

A calculator also doesn't ask if the lens is unit focus or internal focus, I'd ask... may this be a concern?

Focal length is focal length.

Some lenses that are internal focusing -- my infernal 200/4 MicroNikkor AIS, for example, and not a lens for LF -- focus closer by reducing focal length. With them knowing the focal length when the lens is focused closer than infinity is a little difficult. Focal length engraved on, e.g., the trim ring, can be quite different from actual focal length.

Are there any internal focusing lenses for LF?

[Bob Salomon]

A calculator also doesn't ask if the lens is unit focus or internal focus, I'd ask... may this be a concern?

Maybe you should read the instructions for the Rodenstock/Linhof/Sinar calculator.

It doesn’t care what the focal length is. It does ask what the difference is between the near focus point on the lens or the rail and the far point, also the angle of the camera to the subject and the magnification. It then tells you the required marked f stop and where to focus the lens.

[Mark Sawyer]

From: http://www.largeformatphotography.in...DoFinDepth.pdf

"Lenses are assumed unit focusing, and except for the section Depth of Field for an Asymmetrical Lens, are assumed symmetrical. Large-format lenses are unit focusing, and except for telephotos, are nearly symmetrical. Because the effects of asymmetry are minor unless the asymmetry is substantial and the magnification approaching unity or greater, this simplified treatment is justified in most cases for large-format lenses.

Many, if not most, small-format lenses of other than normal focal length are asymmetrical, and many are not unit focusing. However, for other than closeup lenses, the effect of asymmetry is minimal, and close focus usually is approximately 10× focal length; consequently, the change in focal length from the non-unit focusing is minimal, and the simplifying assumptions give reasonable results. However, lens asymmetry must be considered when determining the DoF of an internal-focusing macro lens, as discussed in the section Effect of Lens Asymmetry."

and

"Most treatments of depth of field assume a symmetrical lens for which the entrance and exit pupils are the same size, and for which the pupils coincide with the object and image principal planes. Although this assumption is reasonable for most large-format lenses of short and medium focal length, it may not be appropriate for telephoto designs."

[Jac@stafford.net]

Of course, CoC presumes a lens can actually produce a point of focus. CoC is a variable human-determined abstraction.

[Dan Fromm]

Of course, CoC presumes a lens can actually produce a point of focus. CoC is a variable human-determined abstraction.

Jac, I made this point earlier in this discussion and Pere Casals corrected me. Correctly, as it turns out.

It seems that circle of confusion has two meanings. The one that you and I like and use, especially when calculating depth of field, is the largest blur circle that will allow the desired enlargement. The other, that some in this discussion have used, is the blur circle the lens produces.

[Pere Casals]

From: http://www.largeformatphotography.in...DoFinDepth.pdf
Although this assumption is reasonable for most large-format lenses of short and medium focal length, it may not be appropriate for telephoto designs."

It's reasonable for most plasmats, these are a lot of cases and most may be quasi-symmetric... but beyond all teles... a verito is not symetrical, nor raptars, then we have heliars and tessars(Nikkor M), Ektars ? and all modern wide "view" angles derived from biogons sporting tilting pupils... there are many LF glasses that are not near to symmetry, including new MF Digarons etc for digital backs that are mounted in optical benches replacing Pro film LF.

Sadly Through Focus MTF charts are scarce for LF products, IMHO that feature is a key factor to realize the presonality of a glass.