DoF may not exist...

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Erik Kaffehr
Erik Kaffehr Veteran Member • Posts: 6,120
DoF may not exist...
1

If we presume have a perfect optical system, the zone of optimal sharpness is near zero in depth.

Jim Kasson has some excellent graphs that illustrate this perfectly:

This shows as a function of focus distance at different apertures. Assume that the X-axis goes from 2.00 to 2.16 m and the lens is nominally focused at 207 cm.

Note that peak performance is 3500 cy/PH at f/2.8 and focused around 207 cm:

We can see that sharpness drops very fast around accurate focus.

Now, if we look at an image, we may perceive it as sharp, even if it is quite a bit blurred.
Back around 1930 German scientists arrived at acceptable blur being 1/1500-th of the image diagonal.

For the GFX cameras the image diagonal is 55 mm, so the criteria translates into 55/1500 mm which is 0.037 mm or 37 microns. What would that be in our figure?

Image height / blur circle / 2 that is 33 / 0.037 /2 -> 446 cy/PH.

Looking at f/2.8, which is the sharpest aperture, that would yield good focus from 2.03 to 2.15 cm.

Now, let's assume that we make a 16"x23" print, which is about the largest sheet paper size and look at 20"/50 cm distance. Conventional wisdom says that human vision can resolve 180 PPI at 20". So we need 16 * 180 pixels which is 16 * 180 /2 cycles. That number is 1440 cy/PH. For cimplicity, let's say 1500:

Now, we can see that f/2.8 yields acceptable sharpness from 206 to 208 cm, while f/11 yields acceptable sharpness from 2.035 to 2.08 m.But, maximum sharpness is reduced from 3500 cy/PH to around 1800 cy/PH when stopping down. That is due to diffraction. So, we gained depth of field while giving up on maximal sharpness.

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Erik Kaffehr
Website: http://echophoto.dnsalias.net
Magic uses to disappear in controlled experiments…
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