How is DoF calculated in Photo Tools?

I often receive letters form Photo Tools users asking «How can DoF be different for different cameras having the same sensor size?»

Photo Tools app calculates DoF based on the pixel size, not the sensor diagonal as it is usually assumed. This idea was proposed and proved by the photographer Vladimir Medvedev.

The circle of confusion diameter nowadays is usually taken as ~30 microns, and this size is standard for the full-frame matrices or 35-mm films. In the era of digital technologies these standards introduced as long as in 1940s have become obsolete.

See Vladimir’s illustration for better understanding:

Schematic circle of confusion
Comment by Vladimir Medvedev: «The pixel of the digital camera’s sensor such as Canon EOS 5D Mark II is shown in orange, the pixel of Canon EOS 7D is blue. The circle of confusion of 30 microns diameter is green. Red ring is the diameter of the circle of confusion equal to 1/1500 of the 35-mm camera diagonal (28 microns)».

Practically, a focusing scale of objective lenses fits the circle of confusion of 1/1500 of the sensor diagonal. The scheme shown above helps us understand how many pixels would be lost during such calculations.

Let’s make a simple experiment to prove this point. I’ve taken Canon EOS 7D camera with Canon EF 24–105L lens, set the aperture to f/9 and made two shots:

  1. Focus is set to the hyperfocal distance calculated by traditional means (circle of confusion is equal to 1/1500).
  2. Focus is set to the remote object.

Then let’s compare the full-size photographs (clickable):

First method
The shot made with the help of standard DoF calculators
Second method
The shot made with the focus set to the remote object.

As we can see, the calculator based on the circle of confusion of 30 microns wrongly considers the image as sharp while it is really not.

All previous standards being not actual any more, Vladimir suggests to base the calculations on your camera’s sensor pixel size.

Photo Tools uses the circle of confusion diameter approximately equal to 3.2 pixels, and this dimension has been picked experimentally.

Let’s make the third shot with the focus set to the hyperfocal distance calculated by Photo Tools.

Photo Tools calculation
The shot based on Photo Tools calculations.

As we see, the second and the third images are almost identical, therefore the DoF calculator of Photo Tools app allows to predict the depth of field more precisely.