4odonates wrote: ↑Sat Feb 11, 2023 10:14 pm
What's your opinion of the Macro Depth of Field Calculator by PhotoPills?
It has some aspects that bother me a lot. See below...
One thing about that calculator puzzles me.
Focusing distance: [example = 225 mm]
Distance from the subject to the camera sensor. It must be at least 4 times greater than the focal length. <— Why? Why doesn't the calculator work when the minimum focusing distance is used?
It's because in the thin lens model, 1/f = 1/o + 1/i, so the distance between object and image can never be less than 4 times the focal length. This occurs at 1:1 magnification when o = i = 2f.
Unfortunately, the thin lens model is notoriously inaccurate for modern lenses used at close focus distances, partly because in modern lenses the focal length is not fixed, and partly because there is often significant distance between the two principal points from which focus distances have to be measured.
The fact that the calculator imposes what appears to be a thin lens restriction is puzzling, since it's not difficult to get around simply by allowing magnification as an input.
But indeed, that seems to be what it's doing. I cannot see any way to specify magnification directly. Instead, it seems that the only control is to specify "focusing distance", which the calculator then uses to infer magnification using 1/f=1/o+1/i. In addition to the issues of variable focal length and separation between the principal planes, the calculation seems to allow only magnifications less than or equal to 1. If it did allow for magnifications greater than 1, then there would be an ambiguity because the object-to-image distance is the same for magnifications m and 1/m.
Ah, I see that a deep dive is possible. The calculator can be embedded in other web pages, which means that its inner workings can be observed. I see a script at //photopills.com/widgets/ppdofmacro.min.js, which with some reformatting can be read. And yep, it looks like the calculator computes m based on thin lens assumption, then either uses the specified effective aperture or computes effective aperture using the usual correction for pupil magnification factor, then computes DOF using the usual 2*C*Feff/m^2. That last part is fine; the problem is the thin lens stuff before it.
3. Clearly the step size calculator in Zerene Stacker is better than any of the ones I have discovered (although the one from PhotoPills looks promising). Would you consider making your calculator available online (for free)?
I'll take a look at that. In the meantime, if you can handle an Excel spreadsheet file, see
viewtopic.php?p=126606#p126606 (in "A new way of thinking and calculating about DOF"). That provides an explanation and a link to the spreadsheet. If you want to actually use the spreadsheet then you'll have to download it, possibly overriding anti-malware protections, and enable editing. But the posting alone shows the formulas that are used, and the surrounding thread explains why they work. The formulas are not quite up to date, but the differences are insignificant for anything you'll be doing. In the spreadsheet, effective f-number is only computed. If you want to enter effective f-number directly, then you can just replace the formula with a number, but in that case don't save the spreadsheet or you'll wipe out the formula until you do a fresh download.
What happens to the step size calculation when the macro rig is complicated by the addition of extenders, extension tubes, diopters, etc.?
The whole calculation is driven by magnification, effective aperture, and COC if provided. See the
"FAQ: How can I calculate effective aperture?" for a plethora of ways to do that, depending on setup.
--Rik
Edit Feb 13, 2023, to fix erroneous statement about pupil factor in thin lens model.