A typical lens have white focus indicators on their lenses [5m 50m and infinity] but some have red points for 85m and farther and some lenses have a red curve instead of numbers.

What are these red focus numbers and curves?

  • \$\begingroup\$ Could you include an image of what you're talking about? \$\endgroup\$
    – osullic
    Commented Feb 19, 2019 at 13:01
  • \$\begingroup\$ @osullic encrypted-tbn0.gstatic.com/… \$\endgroup\$ Commented Feb 19, 2019 at 13:10
  • \$\begingroup\$ @osullic phone screen is broken and I am in middle of an lesson. \$\endgroup\$ Commented Feb 19, 2019 at 13:12

1 Answer 1


The red dot conventionally marks the hyperfocal distance where the long end of the depth of focus will just extend to infinity. It's the setting for the maximum amount of in-focus content in landscape photography. This sounds like your description (putting the red dot among the distances close to infinity). It doesn't match your images (putting the red mark (which I cannot actually see) opposite to the distances, offset from the usual mark indicating which distance is selected).

The images (where it is totally unclear what your arrows are supposed to point at) would rather indicate where to read off distances for infrared. Infrared light has considerably longer wavelengths than visible light. While this would in theory be offset by the measures correcting chromatic aberration, they cannot really extend all that much. The glass of the lenses slows down infrared like other wavelengths but results in quite less of a phase difference, so infrared is refracted by smaller angles. For the same distance, you need to exercise more of the collecting power of the lens, so whatever distance is opposite to the "infrared" mark is larger than the distance opposite to the "visual light" mark.

  • 5
    \$\begingroup\$ That is what hyperfocal is, but a red dot seems not applicable for it, since hyperfocal for a given focal length greatly depends on aperture and focus distance. Where would you put the dot? I didn't understand the red dot question, but lenses often have a dot (I've only seen white) next to the focus index mark which indicates where infrared would focus. \$\endgroup\$
    – WayneF
    Commented Feb 19, 2019 at 13:52
  • \$\begingroup\$ encrypted-tbn0.gstatic.com/… \$\endgroup\$ Commented Feb 19, 2019 at 14:14
  • \$\begingroup\$ @WayneF encrypted-tbn0.gstatic.com/… \$\endgroup\$ Commented Feb 19, 2019 at 14:14
  • 1
    \$\begingroup\$ That can't be hyperfocal, because hyperfocal doesn't work that that way (it varies greatly with aperture and focused distance). The Canon lens manual page 11 says those markings on the 70-200 lens is "Infrared index", which is found on many lenses (more so on primes). I don't see anything suggesting hyperfocal on the other lens (or any lens for that matter. because it doesn't work that way). Depth of field calculators usually show hyperfocal for the settings entered. \$\endgroup\$
    – WayneF
    Commented Feb 19, 2019 at 17:59

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