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up vote 10 down vote favorite

Take this lens I have for example, it has red values on the distance scale (not sure what do you call it exactly) next to the focus ring. What does these values mean?

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3 Answers

up vote 12 down vote accepted

EDITED FOR 2 CASES

If the markings show aperture values, those are hyperfocal-distance markings. It lets you know where the hyperfocal distance is at each full F-stop.

For example, one of the lines has a 16 in red next to it. That will let you set the focus to the hyperfocal distance at F/16.

If you do not know what the Hyperfocal-distance is you can read this short article. It even has a calculator to calculate the distance for you.

If the markings show focal-lengths, those are infrared focusing lines. They are used to adjust your focus for infrared photography.

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Hmmm where exactly did you find the 16? I'm only looking at 28, 35, 50, 70, 135. – jon2512chua Jan 11 '11 at 15:55
Sorry, I could not read the numbers (too small) off the lens you linked to, so I looked at one of my lenses which is obviously not the same model. Looking another picture of the lens you sent, I realize this one is marked by focal-length on aperture. I will edit my answer to explain both cases. – Itai Jan 11 '11 at 16:10
Ok got it now. Thanks for explaining both cases, and also about hyperfocal-distance. – jon2512chua Jan 11 '11 at 16:22
up vote 1 down vote

That seems to be a DOF-marker for the different focal lengths, like there used to be markers for the aperture. The longer the focal length, the shallower the DOF. Edit: (make that "the shallower the relative DOF becomes" (due to magnification, thx ysap)).

Take http://www.kenrockwell.com/canon/lenses/28-135mm.htm, there you can see that it is "28,", "35", "50", "70" and "135".

Edit: Jukka Suomela actually read the article (and did not only look for a clearer picture, like me) and pointed out that Ken mentions them as infrared focus markers. That clearly makes a bit more sense.

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are you shure about "The longer the focal length, the shallower the DOF"? I think the DoF gets larger the farther away your subject is (think Macro photography). Maybe you mean that the "relative/apparent" DoF is shallower? – ysap Jan 11 '11 at 15:37
@ysap- You are confusing focal-length and focus distance. DOF gets smaller at shorter focus distances AND it gets smaller at longer focal-lengths (and vice-versa). So Leonidas is correct. – Itai Jan 11 '11 at 16:12
The article in your link actually mentions that they are infrared focus indices (see also, e.g., thomasmayphotography.com/photography-techniques/…), not DOF markers. – Jukka Suomela Jan 11 '11 at 16:17
@Itai - then it seems like I really have to refresh my memory regarding DoF relationships. I still find it hard to accept that it gets shallower with increasing FL, but for now I'll take your word for that ;-) – ysap Jan 11 '11 at 16:34
@ysap: Looking over the equations, I think you are right pointing out my mistake. Total DOF stays the same at fixed magnification of subject, but relative DOF (relative to distance) shrinks. I mostly ignore magnification, making relative DOF the important factor. – Leonidas Jan 12 '11 at 3:05
up vote 4 down vote

IIRC, these are the focal length marks for IR (infrared) photography, but I can be terribly wrong...

EDIT: to be more precise, I think these lines mark the infinity focus for the different focal length (28-135) for IR photography.

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