Infinity, and beyond

Discussion in 'Beyond the Basics' started by Josh66, May 6, 2012.

  1. Josh66

    Josh66 Been spending a lot of time on here!

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    OK, so I decided not to derail this thread any further...


    I was going to reply to this post, but thought a new thread would be more appropriate.
    Now, on all of my lenses, the red dot for IR focus reaches infinity before the 'normal' focus mark. That means that you would never have to go past infinity to focus IR at infinity.

    pgriz says that it's to allow for temperature variances (and cites the manual to a lens). I said that it's to allow the AF mechanism to overshoot it and come back to prevent damage (just seems right - I don't have any references).

    So, we have 3 theories - (1) IR focus; (2) Temperature; (3) Built in 'slop' to prevent damage.



    Since IR reaches focus before visible light, there is no reason to go past infinity in IR. The temperature differences seem kind of small to me (relatively) for it to affect a lens to that degree. The 'slop' theory sounds good to me, but I can't back it up with anything.



    So, the question is - why will some lenses allow you to focus past infinity? Also, is there any situation (other than those mentioned) where it would be desirable to focus past infinity?
     
  2. Garbz

    Garbz No longer a newbie, moving up!

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    How about correct for variance in manufacturing? It's much easier to manufacture a lens with tolerances that allow it to focus past infinity then use calibration tools to adjust the focusing system.
     
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  3. EDL

    EDL No longer a newbie, moving up!

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    The auto-focus theory seems to be one of the most prevalent if you google it and I guess that makes sense, allowing the mechanism to roll past infinity.

    Another point I saw was that zoom lenses might have a slightly different infinity focus at different focal lengths.
     
  4. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    I can't say definitively, but: Physics tells us that it is definitely NOT #1, and as for #2, all of my old glass (the good stuff anyway) stopped dead on infinity, and I'm sure that with the all-metal construction, thermal expansion would have been even more of a factor (and by that token, wouldn't there need to be slop in the actual element mounts so that when the lens go very cold they weren't compressed to the point of damage?). Therefore, it seems reasonable and logical based on the 'overshoot and return' design of most AF systems that choice #3 is the correct one.
     
  5. Buckster

    Buckster Been spending a lot of time on here!

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    All I can say for sure is that the title of this thread makes me think of Buzz Lightyear.
     
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  6. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Different wavelenghts of light focus to different distances - that's what chromatic aberration (CA) is all about.

    IR has a lower frequency than visible light, so in a simple lens IR will focus the furthest, distance wise, from the lens.
    [​IMG]

    However, CA can be corrected by making a compund lens:
    [​IMG]

    Consequently, the issue of 'Infinity, and beyond' usually boils down to who made the lens, and how much design time and manufacturing effort is put into making a lens.
     
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  7. Josh66

    Josh66 Been spending a lot of time on here!

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    The kids have been watching Toy Story 3 a lot lately, lol.
     
  8. Dao

    Dao No longer a newbie, moving up!

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    Thanks to you ... now the words "Buzz Lightyear" stuck in my mind.
     
  9. Josh66

    Josh66 Been spending a lot of time on here!

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    Nobody really touched on this:
    "Those mentioned" being the possible reasons that it goes past infinity in the first place.

    I can't think of any time where you would want to focus past infinity... UV photography maybe. That's the only thing I can think of. In order to focus UV light at infinity, you would have to 'go past' infinity in visible light, correct?

    Edit
    UV is sort of in a special place though. When you start talking about quartz elements, I would just sort of expect them to account for the different focus point in some way. A lens like that might not even have focusing marks on it for visible light...
     
    Last edited: May 7, 2012

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