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DOF in digital pictures

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DOF in digital pictures
Another Test
Diffraction & sharpening
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Updates:


2011-02-08

Added some new images showing the effects of both defocus and diffraction. I also started looking at how much fine detail contrast can be regained with convolution type sharpening.

 

2011-02-10

Some formatting changes, added some words about overestimating the effect of diffraction in the figures.

 

 

 

We are accustomed to having significant Depth Of Field in photography. Now that we have digital sensor that are very high resolving and essentially flat the old figures are less valid.

 

The main reason I started this investigation is that there was a discussion on Luminous Landscape regarding focus on distant objects. The author was focusing at infinity and saw that infinity was in focus but objects say 300-400 feet away were not in focus. My immediate reaction was that with high resolution digital DOF is virtually nil, especially if we are pixel peeping at actual pixels. I do, you do, we do! Of course, we shall not pixel peep but we do it anyway.

 

Infinity is a bit less than practical but I sat up an experiment. The set up was as follows:

  • Camera on tripod 3 meter from test object
  • Test object consisted of a "Lens Align device", a dollar bill and test target for Imatest
Focusing distance was calculated for defocus corresponding to 6 micron and 12 micron circle of confusion (CoC). It was assumed that focusing was exactly at 3000 mm and an offset was calculated for each aperture and CoC. Something like this:
F-stop CoC Offset lp/ph "Megapixels" Corresponding near focus
in meters when lens
is focused at infinity
f/8 0 0 2836 24.6
f/8 6 17 2749 23.1 467
f/8 12 35 2454 18.4 235
f/11 6 24 2644 21.4 341
f/11 12 48 2424 18 170

 

The "lp/ph" figure measures at how many line pairs at picture height the lens can transfer 50% contrast. The "Megapixels" figure is normalized to 24.6 MP at "optimum" focus. Optimum focus was very difficult to find.

 

f/8 coc=0 CoC = 6 CoC = 12
f/8
f/11

 

Preliminary findings

To me it is pretty much obvious that significant sharpness is lost at a circle of confusion of 12 microns, which is approximately twice pixel pitch of the Sony Alpha 900 used for the test.
With a 150 mm lens at 3.0 meter 12 micron corresponds to moving the camera backwards with 3.5 cm if f/8 is used or 48 mm at f/11. Would the camera be focused at infinity a CoC of 12 microns would be achieved at about 235m at f/8 or at 170 m at f/11.
Using a CoC of 6 microns (corresponding to the pixel pitch) I can see some image degradation, but this is quite small. A CoC of 6 microns would be corresponding to about 470 m on a 150 mm lens focused at infinity using f/8. At F/11 CoC 12 microns would be achieved at around 340 m.


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

 


Diffraction

Stopping down affect the image by diffraction. The two images below show the zero offset image at f:5.6 and f:16 respectively. Some of he sharpness lost can be regained using advanced sharpening. The amount of sharpening in the samples below may be a bit excessive. Alternatively the original images could have more sharpening.

Diffraction only

f:5.6 f:16 f:16 Smart Sharpen
LensBlur
1.3px 165%
f:16 Topaz InFocus

Defocus and diffraction

f:5.6 zero offset f:16 defocused 90 mm f:16 defocus
Smart Sharpen
LensBlur
1.3px 165%
f:16 defocus
Topaz InFocus
EstimateBlur
Last Updated on Saturday, 12 March 2011 21:05  

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