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  1. #1
    Join Date
    Nov 2009
    Location
    Houston, Texas
    Posts
    8

    Focus on 70-200G Lens

    I have an A700 Sony and among other lenses that I have is the 70-200G Lens. It is a great lens.The problem I have is in low light conditions and I have to switch to manual focus. When I rotate the focus to infinity(because the subject is quite a distance away), it's out of focus. I have to rotate back to try to get the subject in focus. It is a nuisance especially when it may be an action shot. Sony support tells me it may be the dioper adjustment, and that I have to adjust the dioper for each lens. I don't have a problem with any other lenses, I simply rotate the focus ring to infinity and get after it. Any suggestions????

  2. #2
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,560

    Lightbulb Some high end lenses ...

    Yeah ... as I recall from my use of that very lens,

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    the "infinity" focal point is NOT at the end of the focus range, as it is on MOST zoom lenses. This particular lens is of an advanced DESIGN so that it works well with a teleconverter ... and as such, there is slop built into the end of the focus travel to allow it "autofocus" properly with the T/C added to the length of the lens.

    When you "manually" focus, you have to be cognizant of this "extra" area at the end of the focus limit, backing off to the properly marked "infinity" setting on the focus distance window (just under the upper "focus stop button").

    Look closely at it, as you adjust the lens ... you will see what I am referring to. You should see that the focal index mark on the lens goes just past the actual "infinity" setting.

    It has been a couple years, but I am pretty sure this is right.

    Let me know what you find.

    BTW: I wish my TAMRON SP AF 200-500mm f/5-6.3 Di LD lens had this capability, because when you put a T/C on it ... infinity focus is shot to hell. Since you cannot, you are stuck with 200-500mm anywhere beyond 27-feet.
    Last edited by DonSchap; 11-06-2009 at 08:27 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

  3. #3
    Join Date
    Dec 2007
    Posts
    1,760
    Your diopter adjustment has nothing to do with the lens..it adjusts the focus of your eye to the focus screen
    Last edited by SONYNUT; 11-06-2009 at 12:47 PM.

  4. #4
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,560
    Steve,

    ain't it hell getting decent advice from the SONY Support Line? My god, next thing you know they'll be saying that the diopter is the source of issues with "AF Micro-Adjust"

    Last edited by DonSchap; 11-06-2009 at 12:52 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

  5. #5
    Join Date
    Dec 2007
    Posts
    1,760
    I love companys that can't even tell you how their product works..

  6. #6
    Join Date
    Nov 2008
    Location
    Monmouthshire, UK
    Posts
    2,152
    It's my understanding that low dispersion glass is pretty sensitive to temperature change and as a consequence focus point varies. So an "infinity" stop would only be correct for one temperature, hence the lens is designed to focus beyond infinity (and that's a conundrum).

  7. #7
    Join Date
    Dec 2007
    Posts
    1,760
    I think for the most part it gives the af some breathing room rather slaming up into the stop...

  8. #8
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,560
    As I recalled, the Canon EF 70-200mm f/2.8 IS USM operated the same way. The lenses are quite similar.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

  9. #9
    Join Date
    Nov 2008
    Location
    Monmouthshire, UK
    Posts
    2,152
    It's only an issue with the heavyweights where the size of the elements mean that expansion/contraction is a significant factor.
    Anyway this doesn't help Travel58 with his problem. If the AF can't lock on at f2.8 then manual focus will be impossible so the only answer is to prefocus.
    Don is correct, use the ∞ mark on the lens, you could even pull back a bit because at that distance the DOF will be substantial even at f2.8.

  10. #10
    Join Date
    Jun 2008
    Posts
    2,204
    All of my lenses that I've seen focus beyond infinity. They're built this way as everyone has discussed.
    flickr

    Canon 7D - 5D | 550EX - 430EX II - (2) PW FlexTT5 | 24-105 f4L | 70-200 f2.8L IS | 100 f2.8L IS | 50 f1.8 II

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