DoF Charts and f/stops

[Pere Casals]

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

Dan, I'm honored by correcting you in nomenclature this time, but I've to say that you are correcting me x10 more times in concepts, so learning a lot.

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

I'll read it, but I think you explained it to me perfectly, I find the included tilt feature pretty powerful...

[Emmanuel BIGLER]

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

Well, providing that you have access to the true F-number N, and that you can estimate your magnification ratio M, focal length does not play a direct role in traditional DoF for magnification M greater than about 0.1
The real question, for a lens with internal focus, is to have access to the true f-number, as discussed below.

And Dan says:
Focal length is focal length.
Yes, of course for a LF lens with fixed elements.
But "modern" macro lenses with internal focusing in use un 35 mm, medium format and DSLRs are actually a kind of a zoom. The "commercial" focal length, to the best of my knowledge, corresponds to the use in infinity-focus setting.
When the lens is used down to 1:1, the focal length may shorten significantly, but in this position, nobody has access to the focal point which is recessed somewhere inside the lens mount, and plain users cannot easily estimate the actual focal length of a macro lens with floating elements outside the infinity-focus position.
And since DoF in macro does not depend on the focal length, users of those lenses, in principle, should not have to care for the actual focal length.
But the real question is: when my lens with floating elements is set for 1:1 ratio, what is my actual f-number N or or effective f-number N(1+M) (i.e. effective = 2N at 1:1 for M=1) ???

I do use such a macro lens from time to time [100 OFF TOPIC !!], but I realize that I did not pay attention to the meaning of f-numbers that show up in the view finder on the liquid crystal display!

Regarding the use of general formulae valid for asymmetric lenses with non-unit pupillar magnification ratio, I tried to play with them and compare with traditional formulae valid in principle only for quasi-symmetrical lenses.
I never found any practical differences at large distances, between general formulae and simplified formulae valid for symmetrical lenses.
One of the reasons is that the hyperfocal distance, i.e. the setting distance for which DoF extends to infinity, is the same for assymetric lenses, i.e. at large distances both formulae yield almost the same result for DoF limits.

And since most applications in macro work rely on quasi-symmetrical lenses, at least with non-floating element lenses, is is likely that nobody has ever experienced a huge discrepancy between actual, real-world DoF and simulated DoF based on simplified formulae valid for quasi-symmetical lenses.
Either you use any lens type including a telephoto at large distances, and in this situation the hyperfocal distance is the same regardless of the lens formula, or you work in macro with a symmetrical lens.

[Mark Sawyer]

I think I withdraw my original question. This is making my head hurt...

[Pere Casals]

and that you can estimate your magnification ratio M, focal length does not play a direct role in traditional DoF for magnification M greater than about 0.1

In a calculator, when focal does not play a direct role it still serves to calculate magification M, operating focal and subject distance... for those calculators asking focal and distance.

The real question, for a lens with internal focus, is to have access to the true f-number, as discussed below.

In general IF lenses (prime) are designed to avoid focus breathing, so image circle size can be considered constant and true aperture should be mostly invariant (vs focus range) and equal to the engraved one, I guess...

My understanding is that what is engraved is the true optical aperture and not the light transmision.

________________

I think I withdraw my original question. This is making my head hurt...

But returning to the original question, if calculation can be done with either N or Neff it would we weird asking Neff to users, under the KISS design principle , ( noted by the U.S. Navy in 1960, coined by Kelly Johnson at Skunk Works) https://en.wikipedia.org/wiki/KISS_principle

The other challenge is that with N input (or output) the calculator has to deal with IF and UF lenses at the same time, but IMHO Emmanuel illustrated that this is possible with posted formulas, if accepting that IF glass holds aperture across focus setting.


So IMHO I'd say that there is no reason for a DOF calculator to torturate a user with a Neff values... is it there?

[Dan Fromm]

I think I withdraw my original question. This is making my head hurt...

Mark, your question was answered early in this discussion. Depth of field at 1:1? There is none, don't bother calculating it.

This is, of course, a frivolous answer but it gets at the big problem with conventional closeup photography. There's no way to get enough depth of field. This is why focus stacking, which is practical only with digital and static subjects, is so widely used.

[Mark Sawyer]

I frivolously disagree with your frivolous answer, Dan. I do mostly close-up work these days, enough to know there's usually something in focus, but yes, it may be a bit of a scavenger hunt finding it...

In playing with plugging the same 1:1 values into various internet DoF calculators, I received differing answers, confirming 1.) different calculators use different formulae (and I saw a few variations in my readings), and 2.) yup, at 1:1 with longer focal lengths and wide apertures, DoF is scary shallow no matter how you calculate it. Fortunately, my calculator is a loupe, an eyeball, and a ground glass, so for me, the original question just curiosity.

But I still haven't been able to pick out any authoritative documentation on whether one should use the engraved or actual f/stop, and I'm still curious...

[Mark Sawyer]

In a calculator, when focal does not play a direct role it still serves to calculate magification M, operating focal and subject distance... for those calculators asking focal and distance.

But does inputting the magnification or subject distance in the calculations compensate for the Neff value being substantially different than the N value? Or is it simply allowing for the inherently shallower DoF at close focusing distances?

So IMHO I'd say that there is no reason for a DOF calculator to torture a user with a Neff values... is it there?

The reason would be that the Neff is the actual f/stop you're working at. And to me it makes sense that as such, this is the number you should use. But I'm also stupid and rather ignorant, and like to make things complicated...

[Pere Casals]

But does inputting the magnification or subject distance in the calculations compensate for the Neff value being substantially different than the N value? Or is it simply allowing for the inherently shallower DoF at close focusing distances?

Mark, my guess is that it is possible to calculate DOF of a "unit focus" lens (LF lenses...) by operating N, or calculating N, and never using Neff for it. At the end we can find a law that links N to Neff from magnification, so we can make a sustitution of any Neff variable by an N variable (https://en.wikipedia.org/wiki/Change_of_variables).

If we compare what happens with an Internal Focus lens, the Unit Focus at 1:1 has one half of the aperture but it also has twice Nodal to film distance, so enlarging by x2 the confusion circles, and one thing may excatly compensate the other... just a possibility I guess.

So I bet that the Rodenstock pocket DOF calculator uses the the engraved aperture to predict well DOF also for LF, if not they would be saying it in the instructions, would Rodenstock missled people by one stop in a calculator for a Sironar S? I don't think that about Rodenstock...

Other calculators... who knows ??? but it can be checked !

The reason would be that the Neff is the actual f/stop you're working at. And to me it makes sense that as such, this is the number you should use.

Let me suggest a test, attach a DSLR in the back of the view camera without the lens, take a USAF 1951 glass slide or a target sporting Group 6, we need that at 1:1.

Then displace the target (back and forward) from perfect focus at 1:1, perhaps a micrometric rail for macro would be useful.

Then plot the through focus lp/mm graphs, measuring from the DSLR images in the back, this is the same graph we plot to get the optimal holder height in a V750 scanner, you will get a graph similar (but using lp/mm rather than _um) to this:



This would be first hand trustful information, and you would find if that matches the DOF calculated by introducing N or Neff in a particular calculator... IMHO this is the way, testing !

But I'm also stupid and rather ignorant, and like to make things complicated...

Mark, the Kelly Johnson's KISS calls "stupid" to designers that complicate things for end users, not calling "stupid" to end users, but defending them from "stupid" engineers.

"The principle is best exemplified by the story of Johnson handing a team of design engineers a handful of tools, with the challenge that the jet aircraft they were designing must be repairable by an average mechanic in the field under combat conditions with only these tools. Hence, the "stupid" refers to the relationship between the way things break and the sophistication available to repair them."

Since then KISS is a principle applied to many manufactured goods. Exception are cars, that are designed to invoice customers with spare ECU boards