DA 50-200 Problems

Right, I'm having a fair few problems when using my DA 50-200. Firstly the focus seems to be off every time with it focusing behind what I want to shoot. Secondly it won't focus to infinity (or lock focus on distant objects) when at 200mm. Anyone got any suggestions? I'm getting quite tired of many of my shots being out of focus and needing to be scrapped. Tends to happen when focusing close (less than 2m) and also very far at 200mm focal length.

Do you get these problems only with that lens?

Are you able to reproduce the problem under controlled conditions?

I've only really noticed it on this one lens because of the very narrow DoF when close but also because it won't lock focus on distant objects at 200mm. My A50f1.7 seems to be back focused as well when I get focus confirmation so it could be the camera (noticed this shooting wide open in Saltaire Church).

How would I go about testing it?

Especially for close objects, are you sure it is back focusing, or is it possible that the focus sensor is simply off from where you think it should be?

If you assume the focus is on center, but is actually 5%-10% or further down the frame, this can cause a huge problem when you're not aware of it.

If you haven't verified exactly where your focusing sensors are, do that and see if that explains some of your issues.

Well first, it seems to me, you have to establish beyond doubt whether it's the camera or the lens, or neither, of course, and you'd be a bit unlucky for it to be both.

To check the camera, use the focus test chart we've been talking about recently, with several different lenses, wide open to ensure minimum DOF, spot focus mode, and all the good stuff about chart flat, straight, tripod, SR off, precise aim.

This should tell you where the camera thinks it's in focus.

If you find the camera to be consistent (not necessarily correct) with at least some lenses, then you can measure any difference between those and the one you're worried about.

One step at a time, I guess.