Question about Tamron 90 2.8

Jan 7, 2008
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?
Feb 12, 2006
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!
Mar 21, 2008
Orlando, Florida
Macros and Variable Aperture

Thank you Leif for the fast reply. I have much to learn!
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.
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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