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What do those red marking do (just to the right to the focus distances?

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It looks like a Chinon 40-150mm f/3.5 zoom, correct?

In either case, those are focus marks for infrared photography.

Infrared light will focus at a different point than visible light, so if you want to make infrared pictures you'll use one of the red markers instead of the white mark for the focus scale.

There are multiple red marks because the IR focus varies with the zoom setting, so you'll use the red 105 mark when the zoom is at 105 mm, etc.

Many older lenses have some sort of focus marking for IR, here are a few common variants.

If you have a digital camera, it will most likely have a built-in UV/IR-block filter, so the sensitivity to IR wavelengths is very low. With the help of an IR-pass filter (which blocks visible light), a tripod, and long exposures, IR photography is still possible.

The IR sensitivity of digital cameras can be improved by removing the built-in filter. To get an approximate idea of how much difference it makes, see this page on DSLR modding, which compares a modded and an unmodded DSLR on a 940nm IR remote. Briefly, the test indicated about 10 stops increase in sensitivity after modding. For that wavelength, those cameras, and those test conditions.

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  • \$\begingroup\$ Thanks you two for the answers. So is it like this? First we have to select the focal length (zoom ring), then we rotate the focus ring so that needed focus distance aligns with the red marking that shows the focal length which we selected on the zoom ring.......? \$\endgroup\$
    – user152435
    Commented Jun 18, 2016 at 12:22
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    \$\begingroup\$ @user152435 Yes. But only for IR photography; for normal pictures you ignore the red marks and use the white one. \$\endgroup\$
    – j-g-faustus
    Commented Jun 18, 2016 at 12:31
  • \$\begingroup\$ but hey, although almost everything is similar of the lens you have linked and my one, there's no a 'CHINON' printed left to 'ZOOM', instead my one has 'expert' printed left to the 'ZOOM'..... \$\endgroup\$
    – user152435
    Commented Jun 18, 2016 at 12:32
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    \$\begingroup\$ @user152435 Looks like I misspoke: Digital cameras do indeed have some sensitivity to IR even without modification, the mod is to increase IR sensitivity. The spectrum of an IR remote typically peaks at 940nm. The sensor sensitivity at that wavelength might be something like 1/5th of the peak - this graph is for a dedicated astronomy camera, but it's a CMOS with Bayer filter, so the sensor tech is similar to webcams and DSLRs. \$\endgroup\$
    – j-g-faustus
    Commented Jun 19, 2016 at 12:15
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    \$\begingroup\$ With an IR-block filter on top of that, the net sensitivity to IR without modding will be very low. But it appears to be doable \$\endgroup\$
    – j-g-faustus
    Commented Jun 19, 2016 at 12:17
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Those markings are for focusing infrared. Infrared light focuses at a different point to visible light. It will also vary with zoom length. To focus at infrared on that lens you would select the mark witch matches the zoom you are using. Then you would match this mark with the correct distance from the focus ring above.

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    \$\begingroup\$ Hmm does this suggest that focusing on a predominantly red or predominantly blue scene will slightly alter the accuracy of the visible-light focusing marks? If wavelength affects things that much? \$\endgroup\$
    – Lightness Races in Orbit
    Commented Jun 18, 2016 at 15:04
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    \$\begingroup\$ @LightnessRacesinOrbit In a limited extent--that is the cause of chromatic aberration. However, IR itself also spans more orders of magnitude in wavelength (700*nm to one *mm) than visible light. \$\endgroup\$
    – nanofarad
    Commented Jun 18, 2016 at 16:50
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    \$\begingroup\$ @LightnessRacesinOrbit Also, standard camera lenses are to some extent corrected to focus different visible wavelengths to the same point. Higher-end (more expensive) lenses are typically better corrected, but even the cheapest lens will have some sort of attempted correction. But lenses designed for visual light won't be corrected for wavelengths outside the visible range. See this graph; most camera lenses are some variant of achromat. \$\endgroup\$
    – j-g-faustus
    Commented Jun 18, 2016 at 21:12

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