Stacking: am I getting this technically right?

[austrokiwi1]

Over the last few weeks I have gone back to basics. My interest is coin photography and from what I have reviewed on this site, so far, coin photography is very straight forward in comparison to what many of you are doing. So I decided to look at the, for me, technical points. I have been going through Enrico Savazzi's book over the last few weeks( most of what I read was previewed on line as I only received the hard copy on friday)

What I have successfully digested so far, from the book and this forum, seems to have reaped dividends: my copy stand is 15 kg heavier and is ,now, perched on Sorbothane feet. My use of lighting has become a lot more directed than it used to be. I have a lot to unlearn and a lot of new things to learn. In the area of stacking I believe( as far as my knowledge allows) I think there is heaps of unlearning and learning to do. I down loaded From Enrico Savazzis site the DOF tables and went over them. I then took two stacks of which the second doesn't seem to be as good as it could be( the first one though good wasn't positioned particularly well):


Coin is 13mm in diameter in real life

I think I was wrong in my use of the DOF table( and even entered the wrong circle of confusion value): My camera is a Sony A7r. I was using a SK Makro Iris 50mm 2.8 @ F4. the steps I took Using "cambridgeincolor" advanced diffraction calculator I worked out the circle of confusion for my camera as 12.25 microns. This was the only figure I entered into the depth of field table and I entered it as 0.01225mm. Don't laugh( much) at these questions.....did I actually enter the amount I had calculated? And did I actually calculate the correct figure in the first place?

Then using the tables I worked out( and I am absolutely sure I read the table wrong and then entered it into the stack shot incorrectly) That for the magnification I was using ( approx 1.6-1) the DOF was 0.41mm. I programmed that into the stack shot as 0.410um I then set the start and finish points and let the stack shot calculate the number of pictures( 10 in all). Since then I have been over analysing what I did...and doubting my self.....now that I have utterly confused myself could some person tell me what I did right and what I did wrong. Any constructive comments greatly appreciated( the stack was processed by Helicon soft using the depth map method. the picture is completely unedited( after stacking)

[rjlittlefield]

I worked out the circle of confusion for my camera as 12.25 microns. This was the only figure I entered into the depth of field table and I entered it as 0.01225mm. Don't laugh( much) at these questions.....did I actually enter the amount I had calculated?

No laughter from here -- DOF usually makes my head ache!

Yes, 12.25 microns is equal to 0.01225 mm. The Sony A7R sensor has pixels that are 0.00488 mm per side, so your circle of confusion is 2.5 pixels wide. That's a good value.

Then using the tables I worked out( and I am absolutely sure I read the table wrong and then entered it into the stack shot incorrectly) That for the magnification I was using (approx 1.6-1) the DOF was 0.41mm.

Here things get a little uncertain, and in any case I think it's very likely you made a mistake.

The uncertainty is because while f/4 means the same thing for all lenses focused at infinity, by the time they're refocused, put on extension, and maybe reversed for macro use, one lens's "f/4" may be the same as another lens's "f/5.6" or even "f/8".

But I assume you're talking about the lens shown HERE, and that particular lens behaves a lot like the assumptions that I think are bundled into Enrico's tables. Let's assume that, anyway.

This brings us to magnification. You've shown an image of a coin that is "13mm in diameter in real life". The image was shot on a full frame sensor, 24 mm x 36 mm, and I assume you're showing the whole frame (not cropped). In that case, I calculate that the coin on sensor is about 22.6 mm wide and 22.3 mm high. Taking the average of those and calculating magnification I get that you were running at a magnification around 1.73X, pretty close to what I assume you mean by "1.6-1".

But now things fall apart. Using my own calculator from HERE, and plugging in 1.73X, f/4.0 with pupil factor 1, sensor width 36 mm and 7360 pixels, COC of 2.5 pixels, I get that the classic DOF is only 0.089 mm, very far away from your 0.41 mm.

That number 0.089 mm cross-checks OK with the values of 0.11 mm and 0.084 mm that I get from Enrico's "DoF Calc" for f/4 at 1.5X and 1.8X and f/4, and it crosschecks OK with the values of 0.10 mm and 0.079 mm that I get for 1.4X and 2.0X from Zerene Stacker's table 2-A HERE.

I can reproduce your number by plugging in a magnification of 0.625X (=1/1.6).

So, I'm thinking that what you might have done was to enter the magnification incorrectly, essentially treating 1.6:1 as if it were 1:1.6.

Is this helping?

--Rik

[austrokiwi1]

Thank you thats a great help.
You picked the right lens. I attended a course on Coin Photography two years back. I was taught that the best lenses, for coins, were around 100mm. At that time I had a horrible sony 30mm macro so changing to a 105mm sigma EX was a huge improvement. That little SK was, for me, counter intuitive; but I have come to appreciate it more and more. I believe it is the best resolving lens I have, its shape allows close in lighting and it produces a 43mm image circle which is more than adequate for a FF sensor. The biggest pain is setting the aperture ( have to undo grub screws). However to my eye F4 is the best setting so it is locked at that setting.

Taking your calculations( well spotted on my magnification error) I redid the stack adjusting the DOF to 89 um however For DOF overlap I now realise I should have dropped it to something like 80um( correct?) There is a marked improvement, at least to my eye:

[Chris S.]

Austrokiwi1,

I quite like your coin shot, which looks clear and well-detailed to me. Would that this coin could talk--of the places it's been, the hands that have clutched it, the times it's seen. . . .

But I am not a coin person. One of these days, I'd be grateful if the experts in coin photography (we're graced with a few, in our community), will teach the rest of us the finer goals of coin photography. Beyond the obvious--sharpness, detail, proper focus, correct-color, etc.--I'm certain there are additional elements that only coin enthusiasts know to look for, in what they view as top-quality images. And I suspect that some of these elements would surprise folks who can easily make outstanding images of bugs, plants, protozoa, and other subjects.

Rik obliquely referenced his excellent Web resource on calculating step sizes, but I think he was too modest in not recommending it more emphatically. Enrico's solid and encyclopedic Website is a treasure trove, and when he published his approach to DOF calculation, it was state of the art. I developed my own, unpublished spreadsheet a bit before Enrico published his. I continue to use it, and having worked with it for years, familiarity makes it function well for me.

But Rik later published his approach, in which he simplified the art of calculating focus increments for stacking. Though simplified, nothing important was omitted. Rik factored out some confusing elements that end up not mattering. So if you're starting out now, you can take a shorter path with a far simpler learning curve, and just use Rik's approach, which he shares freely at the link above.

Once upon a time, forum members would routinely PM me for a copy of my spreadsheet. During this period, I happily complied. But since Rik created his approach, I've advised folks to instead look at Rik's Webpage. It's just as good, and much easier.

Of course, anyone's approach--Rik's, Enrico's, mine, etc.--just gives you a starting point for determining focus increments. Such starting points usually work quite well. But for truly optimal results, you need to do some empirical testing above and below the specified increments, and carefully determine what works best for you. My preferred increments tend to be a bit smaller than the ones on Rik's page. But that's with my perception, my equipment, and my sense of balance between slightly improved results (which come from smaller increments) and increased time/trouble (my workflow is highly automated, so finer increments don't much bother me). So your choices may vary. And it's also true that if one needs to use a larger increment--perhaps because the subject will quickly wilt, melt, or otherwise degrade--this larger increment can produce very good results. In doing this, one may sacrifice a bit of detail contrast, but capture a story-telling image. And the story, within an image, is usually the most important element, superseding all others.

On the other hand, if you prefer to continue using spreadsheets, I'd suggest considering this: Most such approaches base their calculations on a middling wavelength of visible light. This has long struck me as a bad idea, especially for those of us who use apochromatic objectives, which include correction for shorter, more persnickety wavelengths such as violet, which benefit from smaller increments. If you proceed with a spreadsheet calculator, consider plugging in 380 nanometers--a wavelength for violet light--as your light wavelength. This will give you a smaller increment than wavelengths for green or red--and in my opinion, get you closer to optimal results.

All this said, I'll repeat my recommendation to start with Rik's approach, and test a bit on either side of these starting points. Much simpler!

Cheers,

--Chris

[austrokiwi1]

Thanks Chris: As has been my habit lately I will now go back to heaps of reading and digesting. Once I think I have the right understanding I will test it out again. I have already downloaded Rik's table... as you say its a lot simpler.

The coin is pre christian Axumite ( Ethiopia) Axumite coins are the only way archeologists have been able to reconstruct some of the history of Axum. This is from memory, so could be wrong; Axum was the first empire to adopt Christianity as its official religion. As you say if these coins could talk