Among my DSLR lenses, I own a Tokina 124 ATX PRO. It's a 12-24 mm which give very good results. Unfortunately, for 1 year, it started to do Err 99 on the Canon EOS 10D body. Initially I thought the problem could be related to the contacts between the lens and the body. Recently while reading the forums, I found finally that the problem was on the lens, and more particularly the diaphragm.
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To qualify the problem, do the following test:
Mode aperture, f=4.0 --> OK
Mode aperture, f=4.5 --> OK
Mode aperture, f=5.6 --> Err 99
Mode aperture, f=4.5 --> OK
Mode aperture, f=5.6 --> Err 99
For Europe the repair center is in Netherland.
If you have a Tokina Lens with such problem, you are aware :) !!!!