Camera Lens Depth of Field

I recently learned from Michael Willems that lens Depth of Field (DoF) is a law of physics. It depends on the aperture of the lens only. He taught me that it does not depend on the manufacturer, lens design or quality of the lens components or coatings.

A Canon 50mm F 1.8 will have the same DoF character as its Nikon sibling. A 50mm F 1.8 manufactured 50 years ago will have the same character as one today.

Other aspects of lens quality have advanced, for example aberrations, but DoF cannot.

I would like a choice – a fast lens that gathers lots of light so I can photograph the proverbial “black cat in a coal hole” but also one with good DoF so that its tail is in focus as well as its nose when the lens is fully open. But it can’t be done!

Unfortunately I do not have matched pairs of lenses to check this out for myself. I did have some fun taking some photos with my Canon EF 50mm F  2.5 “Compact Macro” at different apertures.

The F 2.5 is my fastest lens. I set up the Canon EOS-30D on a tripod, locked the mirror up and used a cable release. You can definitely see the increase in DoF as the aperture decreases. Beyond F 8 it does not get much better in practice as “diffraction effects” start to apply. I’m not sure if the diffraction effects are laws of physics too. It seems like we should be able to make improvements in that area using different formulae for optical glass that alter its diffraction capabilities.

By the way, there are some good tutorials on photography concepts at http://www.cambridgeincolour.com/tutorials.htm including one on DoF at http://www.cambridgeincolour.com/tutorials/depth-of-field.htm.

From the DoF tutorial the student can infer that the DoF is indeed a law of physics and not lens design because design is never mentioned.  There’s a theoretical DoF calculator where the brand or design of the lens isn’t a factor.

I ran a calculation using this tool with the following parameters:

  • Camera type: DSLR with a crop factor of 1.6X.
  • Actual lens focal length: 50mm.
  • Focus distance to subject: 0.5 metres.

Then I varied the aperture and got the following results.

Aperture Closest distance of acceptable sharpness (m) Furthest distance of acceptable sharpness (m) Total Depth of Field (m)
F 1.2 0.498 0.502 0.004
F 1.4 0.497 0.503 0.005
F 1.8 0.497 0.503 0.006
F 2 0.496 0.504 0.007
F 2.8 0.495 0.505 0.010
F 4 0.493 0.507 0.014
F 5.6 0.490 0.510 0.020
F 8 0.486 0.515 0.029
F 11 0.481 0.521 0.040
F 16 0.473 0.531 0.058
F 22 0.463 0.543 0.080

Note that if you use a full-frame 35mm sensor camera then the DoF is much greater. It varies from 0.007m at F 1.2 to 0.129m at F 22. Thus accurate focusing is more important on a crop camera.

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