Maximum DOF without stacking

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rjlittlefield
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Maximum DOF without stacking

Post by rjlittlefield »

The following comment was posted in response to one of Doug's recent posts:
Interesting that you used stacking, I thought that was for photomicrography, and this view is about 2 inches across, must have had a shallow DOF!
This issue comes up often enough that I thought I'd post a quick summary here.

DOF drops in proportion to 1/(Mtot*Mtot), where Mtot is total magnification = lens magnification * subsequent enlargement.

It's a huge problem at high magnifications, but a significant problem at lower magnifications too.

To take the numbers for Doug's image, suppose you make a print that's 20 cm wide, of a subject 5 cm wide. Then you have Mtot=4.

Using 6 lp/mm as the criterion for "sharp", the maximum DOF is only 70/(4*4) = 4.4 mm, no matter what camera or lens you use.

That 4.4 mm is achieved at an optimum aperture setting that does depend on the camera and lens (more precisely, on the lens magnification & sensor size).

Stopping down less than optimum gives you less DOF because of geometric blurring; stopping down more than optimum gives you less DOF because of diffraction. Stopping down more than about 1 stop from optimum gives you no DOF at all because the whole image becomes so blurred from diffraction that 6 lp/mm is not reached anywhere.

These formulas come from the current best reference on DOF limits, which is Ted Clarke's article "Applying an Abbe Criterion to Photomacrography".

It's enlightening to realize that 6 lp/mm = 12 pixels/mm = 2400 pixels in 20cm. Web resolution is more like 800 pixels in 20 cm, a factor of 3 less. This goes a long way toward explaining why macro shots that are optimized for the web or for video can have large depth of field, relative to traditional prints. Quite simply, they are shot under conditions such that "What you see is all you get!" Any attempt to enlarge the image or to print or display it at higher resolution would just reveal more blur.

You've probably gathered that I'm a fan of stacking. See this article for an introduction to its use in photomacrography.

--Rik

DaveW
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Post by DaveW »

Is that not the reason why all the natural history close-up pictures on TV or computer monitors seem to have more depth of field than you can get on a print?

Poor image reproduction disguises the difference between the critically sharp parts and the less sharp parts, so all detail beyond the resolution of the TV screen or computer monitor appear equally sharp.

I also remember reading about photographic competitions where poor on screen projection has disguised the difference between super sharp in focus slides and poorer ones.

DaveW

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