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  1. #1
    Join Date
    Jun 2008
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    Back focus on Lenses - Tamron 17-50mm and Sony 50mm SAL50-F14

    Ok, so I've tested all my lenses, some many times, and found that the Tamron 17-50mm has back focus (focusing behind the AF point) at 17mm and the Sony SAL-50F14 also has back focus (but more noticeable because it's F/1.4 and at 50mm versus a WA 17mm).

    The Tamron I suppose isn't too affected because at 17mm you wouldn't be using this at its MFD anyway. But it's there nonetheless.

    The Sony is disappointing. It's a brand new lens and would expect Sony to have better quality control.

    Test Setup:

    My very aggressive setup of batteries close together. 1" apart wasn't showing much. Spot Focus, AF on center battery. 2 sec timer on tripod.

    TAMRON 17-50mm F/2.8 (shots at F/2.8)

    above is 17mm showing back focus

    above is 35mm, focus is OK

    above is 50mm, focus is OK



    SONY SAL-50F14


    above is 50mm @f/1.4 showing back focus

    above is 50mm @ F/2 showing back focus


    Question is, what should I do about these two lenses?

    Regarding the Tamron, I think I can live with it unless anyone here things I should send it back to Tamron. It has ~5.5 years of warranty left.

    Regarding the Sony, it's unacceptable and I need to either send it back to JR.com or send it to Sony for servicing (I don't believe I should have to pay for the shipping for their failure, it's brand new).

    I definitely would not send both back at the same time, otherwise I wouldn't have anything to shoot with.

    Could anyone also comment about Tamron and Sony's servicing and turnaround time.
    Last edited by dr4gon; 01-16-2009 at 09:40 AM.
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    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

  2. #2
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    dr4gon, this may be a stupid question, but could it be the camera's AF system.
    Have you tried the test in manual focus?

  3. #3
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    Quote Originally Posted by Peekayoh View Post
    dr4gon, this may be a stupid question, but could it be the camera's AF system.
    Have you tried the test in manual focus?
    No I have not, what would MF tell me?

    Tamron 70-200mm F/2.8 focus just fine and as you can see the Tamron 17-50mm @ 35 and 50mm check out ok.
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    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

  4. #4
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
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    Cool How wide is your vision?

    When I do focus checks with the zoom lenses ... I rarely do the extremes. I base most of the focus in the center of the range. If that is off, I figure the rest has to be. The primes ... well, that's just easy. I would suggest that unless the 17mm focal length was really whacked out ... I would just keep it in mind that you may have to move back up a skosh ... after focus, then take the shot. Normally, wide angle shots like this are done at a distance that would pretty well obscure such findings with a DOF, which is pretty deep.

    DOF: 17mm @ f/2.8 w/ subject at 10-feet from camera

    Name:  DOF-17mm-@-f2.8.jpg
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    When your subject is only 2-feet away ... the DOF is this ...

    Name:  DOF-17mm-@-f2.8-2-feet.jpg
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Size:  37.4 KB

    Okay then ... knowing that, let's say you do pop for a 10-20mm f/4-5.6 UWA

    Subject is only three-feet away ... what's your DOF @ 10mm f/4?
    Name:  DOF-10mm-@-f4-3-feet.jpg
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    Any further away or with a tighter aperture ... the DOF heads towards infinity at a very rapid rate. Unless you just manhandle the camera or shake like a leaf, it's pretty hard to screw up a UWA shot ... as practically everything is in focus!

    I would even go so far as to suggest that if you were planning on shooting considerable ultra wide-angle stuff ... a real UWA lens (10-20, 11-18, 10-24) maybe the correct solution, not this all inclusive 17-50mm lens.
    Last edited by DonSchap; 01-16-2009 at 01:49 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.

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  5. #5
    Join Date
    Mar 2007
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    2,132
    My 17-50 is spot on. I haven't run it through any organized test, but in my own use it nails it every time. Manual focusing never improves on the initial AF.

    If its off by a large margin, I'd send it in and have them tweak it. Keep in mind though it may very well be the camera as well, the two have to interact.
    Nikon D300 | Tamron 17-50mm f/2.8 | Nikkor AF-S 70-300mm VR | Nikkor AF 35mm f/2 D | SB-600 | Lowepro Voyager C | Lowepro Slingshot 300 AW

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  6. #6
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    If you do the test focussing manually, you've taken the AF system out of the equation and you'll know where the fault lies.

  7. #7
    Join Date
    Mar 2007
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    Erm, you can't do an AF test without AF...?
    Nikon D300 | Tamron 17-50mm f/2.8 | Nikkor AF-S 70-300mm VR | Nikkor AF 35mm f/2 D | SB-600 | Lowepro Voyager C | Lowepro Slingshot 300 AW

    For Sale:
    Nikkor AF 35mm f/2 D - Like New (FX compatible)

    Wish List
    Nikkor AF-S 17-55 f/2.8
    Nikkor AF-S 70-200 f/4 VRII
    Tokina AF 11-16 f/2.8
    SB-900 (2)
    Umbrellas
    New Tripod

  8. #8
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    No, but you can test the lens for F/B focus.

  9. #9
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    how ? the only way to do the test using MF is to accurately measure the distance which is not easy considering the very slight margins of the focal plane. a few millimetres out of your distance measurement means you havent accurately tested the B/F focussing issue.

    well...thats the only way i'd know how to check using MF anyway.
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  10. #10
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    Jun 2008
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    Yeah, I can't really tell with MF. It's just too hard to distinguish using the viewfinder and at such minute turns to get a perfect focus.

    I just did some more tests. Farther away from the MFD (so further back) and it resulted in the same back focus.

    So I've decided that it's not working as it should and will either send it for repair or send it back to J&R (jr.com). I know don would advise sending it back to Sony for repair ... but I feel uneasy about it since he's had (having) quite an experience.

    Well I guess I will ponder it over the weekend....
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    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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