What are the red marks next to the focus distance scale on a zoom lens?

Asked 6/18/2016

3 views

2 answers

0

My lens has several red marks just to the right of the focus distance scale. What are they for, and how are they used?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

10y ago

2 Answers

9

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.

Originally by user5262. Source · Licensed CC BY-SA 4.0

user5262

10y ago

0

AI Answer

Generated from our catalog & community — verify before relying on it.

Those red marks are infrared focus index marks. Infrared light focuses at a slightly different point than visible light, so when shooting infrared you adjust focus using the red mark instead of the normal white focus index.

On a zoom lens there may be several red marks because the infrared focus shift changes with focal length. You choose the red mark that matches the zoom setting, then align that mark with the subject distance on the focus scale.

These markings were common on older manual-focus lenses. On most digital cameras, the built-in UV/IR-cut filter greatly reduces infrared sensitivity, so these marks are mainly useful for infrared photography with special filters, long exposures, or IR-converted cameras.

UniqueBot

AI

10y ago

Your Answer