Goldenspark
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I've never been happy with the lack of sharpness on my 100-400, so today I decided to bite the bullet and really open the lens up to see what was going on.
(mine is an old, slightly tatty 100-400).
As I removed the main guts of the lens from within the fixed barrel, a small screw fell out!
It turns out there is a three way plstic "spider" that supports the front elements, and each leg has two small screws holding it. The screw that fell out was from one of these, but one of the other legs was missing BOTH screws!
That meant only 3 screws of 6 were in place,but more importantly, one leg was unsupported (a bit like a tripod with only 2 legs!). This woul have the effect of alowing the front lens groups to be tilted.
Luckily I had a couple of screws from another lens I dismantled so I could replace the 2 lost ones.
All back together, and the transformation is amazing, Now at 400/5.6 shots are much sharper - like I'm sure it should be with this lens. Also, the AF adjust problem I had seems to have gone away as well (even at -20 on my 50D it was not quite correct).
I appreciate DIY repair is not for everyone, but it shows the sorts of things that wear and tear can do over time. The screws are meant to have thread-lock on them, but that has not stopped them coming loose. I'm a little worried about where the other 2 screws have gone. They do not appear to be inside the lens as far as I can see.
Just need to get out and use it now!
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