So what are the distance markings on a lens for?

Discussion in 'Photography Beginners' Forum' started by Pixel Guy, Jul 10, 2006.

  1. Pixel Guy

    Pixel Guy TPF Noob!

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    Hi all,

    I've got a basic question. Even though I've been using an SLR for a number of months now, I've never learned about the distance (feet and meter) markings on a lens. What are these markings for and when would I use them (I assume they're for focal distance, but I can't figure out how they would be applied).

    Also, I hear the term "focus at infinity" a lot -- can anyone explain exactly what this means and when one should focus at infinity?

    Thanks.
     
  2. fightheheathens

    fightheheathens TPF Noob!

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    The distance markings indicate the distance from the lens to the object in focus.

    well, i use the distance markings for a number of things,
    first off i have a manual focus lens (i assume you do to). If say you are focusing on flowers 1 or 2 feet away then switch to a landscape, rather then focus while looking through the camera i just turn my focus ring to infinity and then do some fine tuning. I use the markings in this sence as a guide. rather then akwardly looking through my camera while turning the focus from 1 foot to infinity or anytime there is a big jump in the focus distance, i just sorta make a guess.

    secondly, if you have a good lens, you should also have (in addition to the focusing distances) numbers coresponding to the f-stops on your camera arranges in this pattern

    22-16-11-8-5.6-4-2.8-1.7-2.8-4-5.6-8-11-16-22

    this gives you the approximate accaptable infocus limits.
    what this means is if you focus your camera to 4 feet, the 4 foot mark on the focusing ring would be right above the 1.7. If your f-stop is 1.7, only the object at 4 feet will be in focus, however, if you are at f-22 focused at 4 feet, (4 still above 1.7) now what you do is you look to the numbers on your focusing ring corresponding to distance from lens above the 22 on either end. What this now tells you is that these distances will be in focus as well.

    so what you can do with this is quite cool. if say you want to focus on something 12 feet away and something 1 foot away, you look at your f-stop scale and find what f-stop numbers both 12 and 1 fall between. if say both 12 and 1 fall between the two 16's, then by setting your camera to f-16 and focusing so that the distance markings on your lens fall between the two 16's both objecs will be in focus. i have used this technique on my wide angle lens to have objecs appearing in focus from 2 feet away to infinity.


    Infinity means objects at "infinate" distance from the lens will be in focus (logrhythmic scales etc)
    anywho, basically what happens is as you focus father and farther away, more and more becomes in focus. after a certain point, (about 15 meters away from a 50mm lens) everything past this distance will appear to be infocus. this is focusing at infinity because everything past 15 meters, including the moon, stars or other far away objects will appear in focus.

    i hope that helps...
     
  3. Soocom1

    Soocom1 TPF Noob!

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    If you already know this, I apologize. For those reading this and hearing about this for the first time I hope this helps:

    On most lenses (though not all) there are two line typically. One white or green, and one red. There are sometimes red numbers as well. These numbers and lines represent the focusing of the lens for infrared.

    Infrared is at a different wavelength than visible light. It focuses at a different distance. So, lets say you are shooting at a distance of 4 feet. You dial in the lens to focus as normal where the number 4 lines up with the white line. Then you dial the focus ring to where the number 4 meets up with the red line and you are set to shoot in infrared.

    Many of the newer DSLRs are designed NOT to shoot in infrared for some unknown reason, and the manufacturers are under the false impression that people no longer shoot in infrared. This is why some of the newer lenses do not have an adjustment for infrared. Shame, with the new technology, who knows what we could not do with photography that we could do with film.
     
  4. Pixel Guy

    Pixel Guy TPF Noob!

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    Thanks very much for the replies -- very helpful, especially since you both have taught me things I wasn't aware of. You know, one thing I forgot to mention in my original post is that I use a digital SLR. I have three lenses, all of which have manual focusing rings and distance markings. Only one of the lenses (a 50mm prime) has both the distance markings and the aperture markings.

    Given that I'm using a digital SLR and that my two main lenses are newer style auto-focus lenses (with no aperture rings), I tend to use auto-focus most of the time (also, since I'm a novice I haven't become proficient at manual focus yet). But then I started looking at those distance markings and began wondering what benefit they might bring to photographing various subjects. Also, I've been reading Bryan Peterson's book Understanding Exposure, and he talks about setting the distance on the lens, but doesn't fully explain what the markings are for and when and why someone would use them.

    So the replies help a great deal.

    In response I have a few questions (they might seem a bit basic, but I thought I'd ask to get an even fuller understanding of focal distance):

    First, is there a particular reason why it's good to know the focal distance? I know you mentioned that it allows you to refocus without having to look through the camera, but is it important to know the focal distance every time I take a shot?

    Second, are the distance markings mainly useful only in manual focus, or do photographers use them when autofocusing as well?

    Third, if a lens doesn't have aperture markings to align with distance markings, is there really any advantage to having distance markings at all?

    Finally, I looked at my 50mm prime. It has a slightly different arrangement. It has the distance markings: 20 (feet), 7, 4, 3, 2.5, 2, and 1.75 (though it turns a ways past 1.75). Then, just below the focus ring it has a solid white line (which I assume is for aligning the distance); on the left of the line are numbers 22 and 11, and on the right of the line are numbers 11 and 22. Finally, there are two rows of aperture markings, but they look like this:

    22 16 11 8 5.6 4 2.8 1.8
    22 16 11 8 5.6 4 2.8 1.8

    And the only difference is that the second row of numbers is smaller.

    Any hints on how to use all of these markings? Any thoughts would be appreciated. Thanks.
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    The focal distance markers on lenses are absolute godsends when the autofocus fails which it often can in some situations. With my old Nikon FE if the light is too low I can not see the focusing prism through the eyepiece. So when I am at a party or out in the dark I simply turn on the flash, lock the lens at the correct aperature for the flash, eyeball the distance the subject is from the camera and just click. More often then not the result is perfectly in focus despite not being able to set the focus the "normal" way.

    For autofocusing I do not really see how it can be much of a benefit. The camer will focus for you, but I assume you could use it as some crude measurment tool by focusing on something then reading the value off the lens :)

    Yep the distance markings are still useful for when you know the subject distance but standard focusing techniques fail. Infact I am not sure if they currently still do it but my Nikon FE even had a film plane indicator which would alow someone to measure the exact distance of the subject from the film plane.

    I'm having trouble visualising what these markings look like. Can you possibly post a picture of the lens?
     
  6. Pixel Guy

    Pixel Guy TPF Noob!

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    Garbz, thanks for the reply. Apologies for this late reply (I've been away the last few days). I appreciate you answering my questions, and I think I now understand when/where one might rely on the distance markings. Regarding my confusion about all the lens markings, I think I've figured some of it out. After reading fightheheathens' post, I was looking for the exact same markings, and I think I misinterpreted the aperture ring on my lens with the depth-of-field scale that he described (I had confused the depth-of-field scale with the aperture ring). My lens has depth-of-field markings, but it only has 22 and 11 on one side and 11 and 22 on the other. These are between the distance markings and the aperture ring. Here's a photo:

    [​IMG]

    I'm not sure why the depth-of-field scale on it has 22 and 11 on each side, as opposed to the kind of complete scale fighttheheathens described. I'm also not quite sure how to use these markings in light of the fact that it only has 22 and 11 -- wouldn't one need the other depth-of-field measurements?

    Any thoughts?

    Thanks.
     
  7. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    When your lens is set to f/11, the distance shown between the 2 11's is your depth of field (what is in "acceptably" sharp focus). For instance, it looks like if you focused your lens so 20' was lined up with one of the 11's, then it looks like the other 11 would be close to 7'. Where you are actually focused would be somewhere around 12', but at f/11 everything between 7' and 20' is in focus.

    If you focus the lens so that infinity is at one 11, then everything from the distance at the other 11 to infinity is in focus. This is one of the main uses for a DOF scale. If you actually focus on infinity (put it at the main white line), at f/11 it looks like you get a DOF of approx 30' to infinity. But if you rotate the focus ring so that infinity lines up with one 11, then it looks like you might be able to get a DOF of approx 18' to infinity.

    A more detailed DOF scale would be nice, but they would have to make it larger, and also make it so the focus ring was slower (meaning you'd have to turn it more to get from say 4' to infinity than you do now) in order to squeeze in the apertures with shorter DOF. You can google "DOF calculator" and find lots of sites where you can plug in your lens data, and it will show you a DOF chart for all the apertures.
     
  8. Pixel Guy

    Pixel Guy TPF Noob!

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    ksmattfish -- thanks very much; that's helpful. I think I finally understand it now, especially in terms of the particular distance markings and DOF scale on my particular lens. I can see how there's a bit of guessing/approximating, since the DOF scale on the lens isn't complete. But knowing how it works is the main thing, and I can get more precise through practice.

    Thanks also for explaining why the DOF scale isn't as detailed as some, as well as for the tips about a DOF calculator. I think I'll Google for one and experiment with it as I use my lens.

    Thanks again.
     

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