Hi guys, just wanted to tell the tale of my experience with my 70-200mm over the last 4/5 months, in the hope that it may help someone if a similar problem ever arises for them.
I first noticed that something wasn't right about 6 months ago, of course during my first paid job covering a Christening with my girlfriend. I had been shooting the boy wide open at f/2.8 initially, using aperture priority. I then would have stopped down to something like f/5.6 when getting some shots of the boy with his parents, and then with most of the other guests. The shots that followed were over exposed, so I think I added some negative ev compensation. Of course when I opened back to 2.8 for subjects on their own, I was underexposing, so I switched to manual for the rest of the day. At the end of the day I was quite dejected, as I thought I had been at fault for not checking the meter properly before shooting, and put it down to the pressure of my first 'job'.
I should have checked things out properly back then, and I might have found the real problem, but I guess life gets busy and some things get pushed to the background. I also didn't use the 70-200 a lot, and when I did use it, I'd usually be wide open, so the metering would be fine.
A couple weeks back, I did a photosession of a little boy, and again I noticed a lot of overexposed shots, again I thought I'd faltered under the pressure. The following weekend I went to Victor Harbor with a 24mm, 50mm, and the 70-200mm. I had been using the 50mm to shoot a breakwater leading into a boat, I think at f/5.6, then decided to try a close up angle, so switched to the 70-200mm. I was in manual, so was expecting the exposure to be fine, but to my surprise it was way overexposed, about 2 stops I worked out by the time I'd adjusted the shutterspeed to get a good exposure.
I stupidly thought I'd discovered something new, that adjusting focal length adjusted the amount of light coming through the lens or something. Of course when I looked into this I found that it was a ridiculous notion. So I started googling about problems with overexposure, and came across a couple of people who had found the problem was due to a stuck 'aperture lever'. I'd never heard of such a thing before, but sure enough there was a little lever on the back of the lens, (and every other lens - at least with Nikon mounts). I gave the lever a little nudge, and the saw the aperture blades close down. A gentle push the other direction and they opened smoothly... Eureka!
I think I'm lucky, and maybe I'd gently knocked it when mounting it one day causing it to bind. It seems to move freely now, and the problem didn't occur in my tests since. However, others have reported it being completely stuck, in which case it is important to stress that it only be gently moved - forcing it could lead to further damage if it's more than just a little stuck.
Maybe this is all common knowledge to some, but I wish I had read something about this before hand, as I'm sure the problem would have been resolved much earlier. I always thought f/5.6 was pretty soft, I'll be very interested to see what results I get now that my aperture is working as it should.