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handling the DOF trap

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handling the DOF trap
What options do we have?
A practical comparison
Tilt lens on a shoestring
Using Helicon Focus in the field
Schallachburg example
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Handling the DOF trap

Depth Of Field (DOF) is an illusion. We pretend that there is depth of field within which things are in focus.

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The bad news is that DOF does not really exist. The good news is that we can do something about it. (But, read also this!)

 

What is Depth Of Field and why does it not exist?

Depth OF Field (DOF) is mostly an illusion. Sharpness essentially exist just for a plane that is in focus. There is an area around this plane we perceive to be in focus. The perceived zone of sharpness depends on much, expectations, print size, viewing distance and eyesight. This figure from Wikipedia illustrates the issue:

Once we defocus an image any point will be shown as a small disk. Called "circle of confusion" often called "CoC". If the "CoC" is small enough we will perceive the image to be sharp.

A good dicussion is given in this paper.

 

When we look at pictures on screen at actual pixels the CoC is in fact the size of the pixel, something like 5-10 microns, perhaps a third or sixth of the values used in 135 film days. For that reason "DOF" scales on lenses are totally useless in the digital age.

I have seen this is in part when I bought my first MF camera. DOF scale on the Pentax 67 were calculated on 0.06 mm CoC which is grossly equivalent to the 0.03 mm CoC normally used for 135 mm. Using the DOF scales on the Pentax would give the same apparent sharpness as the DOF scales on the 135 equipment. BUT, I did not buy my Pentax 67 to make pictures that were as sharp as 135, I wanted to have sharper pictures! A solution is to use the DOF scales, but if you use f/16, focus according to marks for f/8 and so on. This would double the resolution at the limits of depth field.

 

The DOF scales we used to have on our lenses were based on a circle of confusion of are based on small print sizes observed at 25 cm. They are clearly not adequate for demanding viewing, like modern "pixel peeping".

Try to test "DOF calculator" with the following values, and try out some variations.

  • Maximum print dimension: 60 cm (about A2)
  • Eyesight: 20/20
  • Viewing distance: 50 cm
  • Camera type:  35mm
  • Focal length: 50
  • Aperture: f/8
  • Focusing distance: 1 m



Last Updated on Tuesday, 09 March 2010 07:10  

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