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Question about Tamron 90 2.8

Discussion in 'Lens Lust' started by ChrisC, Jul 27, 2008.

  1. ChrisC

    ChrisC

    130
    Jan 7, 2008
    Maryland
    I bought this lens a few months ago and have recently noticed that it won't always open to 2.8. I just put the lens on and was able to set it to 2.8, took a shot and then it just changed itself to 3.0 Sometimes the widest I can get it to go is 3.8 and it doesn't matter if I am on manual or aperture priority.
    I am using it on a D200 and have never had a problem with any of my other lenses opening all the way before. Is there something I don't understand about the lens or do I have a faulty one?
     
  2. Leif

    Leif

    Feb 12, 2006
    England
    Switch to manual, focus to infinity, then check the aperture. It should be F2.8. The camera will display the effective aperture (not the real one), which increases as you focus closer. I suspect you were focussed on something not too far away. Move the focus to the other extreme (1:1) and you should lose several stops.

    I am sure a Google will find a good explanation of effective aperture, but basically as you focus close, so the image is magnified, and hence the image gets dimmer. The camera takes that into account when it tells you the effective aperture. Personally I wish I could set the camera to display the true aperture, but there you go. It doesn't!
     
  3. ChrisC

    ChrisC

    130
    Jan 7, 2008
    Maryland
    Thank you Leif for the fast reply. I have much to learn!
     
  4. CraigH

    CraigH

    691
    Mar 21, 2008
    Orlando, Florida
    Macros and Variable Aperture

    Chris, you might want to read the review of the Nikon 105 VR at Thom Hogan's website. Starting at the second paragraph in the performance section he discusses variable aperture in macro lenses. At the end of the review he does a very flattering mini-review on your Tamron 90.

    http://www.bythom.com/105AFSlens.htm
     
  5. ChrisC

    ChrisC

    130
    Jan 7, 2008
    Maryland
    Thank you Craig, I will check that out!
     
  6. Jonathan

    Jonathan

    676
    Jun 11, 2005
    Southern Maine
    It is very normal. I thought I had a defective lens when I first bought mine.

    Thom's review is great.

    Great lens isn't it?:smile:
     
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