What is the red “R” mark on older manual-focus lenses, and how do you use it?
Asked 3/25/2020
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Some older manual-focus lenses have a small red “R” or red line near the distance scale, often between the depth-of-field or hyperfocal markings. What does this mark indicate, and how is it used when shooting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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This sign is used when you focus for infrared light (using IR film or filter). It can be in form of red dot or line. This is need because the IR light wave length is longer than visible light and you need to change the focal plane accordingly.
Here is photo from Wikipedia where you can see why you need to change the focal plane:
Originally by user34947. Source · Licensed CC BY-SA 4.0
user34947
6y ago
0
Generated from our catalog & community — verify before relying on it.
The red “R” is an infrared focus index. It’s used when shooting infrared, because infrared light focuses at a slightly different point than visible light.
How to use it:
- Focus normally in visible light first, without the IR filter affecting your view.
- Note the distance you focused at on the distance scale.
- Rotate the focus ring so that same distance lines up with the red “R” mark instead of the normal focus index.
- Then make the exposure for infrared photography.
Why this is needed: lenses are corrected so visible wavelengths focus close together, but infrared still falls at a different plane. The red mark gives you a simple focus correction.
This mark is mainly relevant for IR film or strong infrared filters. For ordinary visible-light photography, ignore it.
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AI6y ago
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