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Calculating step distance for medium format sensor

 
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Davids



Joined: 31 Jan 2016
Posts: 157

PostPosted: Wed Apr 18, 2018 7:34 am    Post subject: Calculating step distance for medium format sensor Reply with quote

I normally use Johan's excellent step distance calculator for my stacks, however his largest sensor is FF. While the GFX sensor (43.8 mm x 32.9 mm) isn't that much bigger than FF, I'd still like to calculate appropriate step distances. Any help?

David
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mawyatt



Joined: 22 Aug 2013
Posts: 1674
Location: Clearwater

PostPosted: Wed Apr 18, 2018 10:59 am    Post subject: Reply with quote

David,

There's a formula that includes pixel size I recall. Off the top of my head it's:

DoF = Wavelength (Green ~ 0.55um)/((NA)^2) + Pixel width/(M*NA)

Think this is correct, but check to be sure.

Best,
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ChrisR
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PostPosted: Wed Apr 18, 2018 12:43 pm    Post subject: Reply with quote

My reference is still this: DOF Two ways
http://www.photomacrography.net/forum/viewtopic.php?p=126606#126606

You can of course alter it, put fixed numbers in, etc.
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rjlittlefield
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PostPosted: Wed Apr 18, 2018 3:48 pm    Post subject: Reply with quote

If you are pixel-peeping to see all the detail captured by your sensor, then a good starting point is to use the same values as for FF. In this model the larger sensor covers a larger FOV but presumably with more pixels so that the pixel density is the same.

If you are doing anything else, for example making prints to be viewed at standard viewing distance, then a good starting point is to enlarge the step size by the ratio of the sensor sizes, so about 1.25X in your case using the diagonals. In this case the model is that the larger sensor permits a larger COC, and DOF is directly proportional to COC in the formulation that I think Johan's calculator is using.

The reason I've written "good starting point" is that if the step size is critical, then there's no substitute for determining the right value by experiment. All of the calculators -- including my own which ChrisR links to -- incorporate assumptions that may or may not be appropriate for your subject, setup, and application.

Another good reference is the simplified tables (also mine) at https://zerenesystems.com/cms/stacker/docs/tables/macromicrodof . The numbers there are conservative in the sense that they assume a perfect lens, limited only by diffraction, plus an arbitrarily good sensor that can capture however much detail is present in the optical system. Still, some photographers use even smaller steps because they get better results that way for their subjects and setups. There are good physical reasons for that, but I won't take space here to mention any of them. On the other hand, other photographers routinely use larger steps because they find that the results remain adequate for their purposes and larger steps make the shooting and processing go faster. Again, if the step size is critical, you have to determine by experiment what size best meets your own needs.

mawyatt wrote:
DoF = Wavelength (Green ~ 0.55um)/((NA)^2) + Pixel width/(M*NA)

Think this is correct, but check to be sure.

I don't recall seeing that particular form before, but it looks like it has all the right parameters raised to the right powers. The first term is due to diffraction and the second is geometric blur. Unfortunately the model is fundamentally wrong because the two components don't really combine that way. But fortunately that fundamental error doesn't matter much because the resulting number is close enough to be plausible and ultimately a human ends up adjusting it anyway. There is much more discussion of these points at http://www.photomacrography.net/forum/viewtopic.php?p=148498#148498 and in the surrounding thread, if you're a glutton for punishment.

Personally I've become convinced that the whole question is squishy enough that fretting over the nuances of formulas is basically a waste of time. But I confess, it was interesting when I did it, and I don't begrudge the time spent.

--Rik
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