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DOF in digital pictures - Another Test

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DOF in digital pictures
Another Test
Diffraction & sharpening
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Another test
In this test I didn't try to keep CoC constant but just moved the camera 30, 60 and 90 mm, still based at 3 meters. This test was done using an Sony Alpha 55 camera with a sensor pitch of 4.77 microns. The camera was focused using live view and images were made at apertures between f:4 and f:16.
I have added the diameter of the first Airy ring, which is due to diffraction, and the CoC for the amount of defocus chosen. Interpretation:
  • Left column, sharpness is increasing while stopping down because lens aberrations are subdued until diffraction starts to dominate.
  • Left to right, sharpness does not drop perceivably until the CoC is larger than the Airy ring.
The grid corresponds to the sensor pitch. Red disk is diffraction and green disk is CoC. For each row sharpness should not change until green disc is larger than red disk.
Remark: In my opinion the figures below overestimate the effect of diffraction. The figure shown is the diameter of the Airy disc for green light of 0.420 microns. The intensity distribution within the Airy ring center dominated, like a bell curve, so using the disk diameter ignores the fact that most of the photons will be well inside the disk.


offset = 0 mm offset = 30 mm offset = 60 mm offset = 90 mm
f/4

coc = 0.0
dif = 4.1

coc = 8.8
dif = 4.1

coc = 17.5
dif = 4.1

coc = 25.9
dif = 4.1
f/5.6

coc = 0.0
dif = 5.7

coc = 6.3
dif = 5.7

coc = 12.5
dif = 5.7

coc = 18.5
dif = 5.7
f/8

coc = 0.0
dif = 8.2

coc = 4.4
dif = 8.2

coc = 8.7
dif = 8.2

coc = 13.0
dif = 8.2
f/11

coc = 0.0
dif = 11.3

coc = 3.2
dif = 11.3

coc = 6.4
dif = 11.3

coc = 9.4
dif = 11.3
f/16

coc = 0.0
dif = 16.4

coc = 2.2
dif = 16.4

coc = 4.4
dif = 16.4

coc = 6.5
dif = 16.4

 



Last Updated on Saturday, 12 March 2011 21:05  

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