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On my Canon EF35mm F2 IS USM (my first prime lens ever which I just bought used) there are the numbers 22, 11, [line with red dot], 11, 22 written below the distance scale.

From the manual of the lense I learned that they are seemingly called "distance index", but the manual doesn't mention what they are. Even googling this word, and also several other related search terms like "lens numbers below distance scale", doesn't find me anything.

What do the numbers and the red dot mean, and what are they used for?

enter image description here

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The 22 and 11 indicate the depth of focus when stopped down to that particular aperture. E.g. in the image, with aperture f/22 everything from about 0.6 to 2 meter away is in focus (as a rough guide).

The red dot, if I'm not mistaken, indicates the focus distance for infrared imaging instead of visible light.

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  • \$\begingroup\$ Thanks, that makes sense. I already suspected it might have something to do with aperture, but couldn't think of a reason for the numbers being symmetric. \$\endgroup\$
    – MaxD
    Jun 2, 2021 at 14:45
  • \$\begingroup\$ Do note that these numbers will only be remotely valid when the resulting image is viewed at assumed "standard" conditions: an 8x10 inch or 8x12 inch display size viewed from a distance of 12 inches by a person with 20/20 vision. Change the display size or viewing distance and DoF changes. \$\endgroup\$
    – Michael C
    Jun 2, 2021 at 19:19

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