24-105 Focus

<p>I have two 24-105's; the one used by my wife (on a 5D) seems a little odd in that, even with the focus point set to one point in the center, the foot/meter scale on the lens can give wildly different readings with successive auto focus on the same point. E.g., focus on a point about 10' feet away three times and the lens gives distances anywhere from 5' to near infinity, especially with a short focal length. <br>

My copy (on a 5DII) does not do that. I have not tested her lens on the 5DII but plan to.<br>

Is the lens due for a trip to Canon?<br>

Thanks.</p>

<p>It seems very obvious that before you do anything else, you should swap the lenses over, not only testing your wife's lens on your 5DII but also testing your lens on your wife's 5D. That should tell you whether the problem (for there does appear to be one) lies with the 5D body or with your wife's lens. The behaviour you describe is certainly not how things should be. However, it is worth remembering that the focusing scale of the 24~105 is a joke. Use Live View on your 5DII to focus your known-good lens accurately on a distant ("at infinity") object at various focal lengths and look at what the focus scale says.</p>

<p>I don't think you should be seeing different behaviours between these two lenses on these two bodies, but as the first response says, standard troubleshooting procedure is to swap then and see whether this is a lens thing or a body thing.</p>

 

<p>You should definitely not expect exactly the same reading on the distance scale every time, particularly at wide angles. Remember that AF is done with the lens wide open, and in most cases* (including this lens on either of these bodies, using any focus point) the specs say that it should get within "depth of focus" (i.e. depth of field with the lens wide open). On a full-frame body with a 24mm lens at f/4, a subject that's 3m away will be within DOF as long as the lens is focused anywhere between about 1.85m and 7.89m. You're seeing a somewhat wider range than this, which doesn't sound good, but you shouldn't expect it to lock on exactly 3m every time. Zoom out to 105 and things should be much better, with a range of approximately ±0.1m, which would hardly be noticeable on this lens' focus distance scale.</p>

 

<p>If you repeat the test and take pictures each time regardless of what distance the AF system found, is the subject adequately sharp in the pictures?</p>

 

<p>*: the central AF point on both of these bodies (along with a couple of the invisible assist points that are not user-selectable and only used when tracking a moving subject) goes into high-precision mode if your lens is f/2.8 or faster, and this mode is supposed to focus within 1/3 of DOF. But with an f/4 lens, this feature is not available.</p>