28-105 mm doesn't have Autofocus?!

<p>So I just opened up the lens I got in the mail. Nothing else looks questionable, but the autofocus does not work. I'm starting to think I made a very elementary mistake or did something otherwise stupid.</p>

<p>I'm pressing the shutter button halfway down with the AF switched on, but there's no noise or any kind of movement. There are two focusing "barrels" I guess, both of which need to be turned manually to get the image into focus. When switched into manual mode, I have to do the same thing to get a sharp image, so I see no change.<br>

The electrical contacts look clean, and I'm pretty sure I'm snapping the lens into place correctly. I really don't know what's going on, so some help would be appreciated.</p>

<p>Thanks a lot!</p>

Okay, I'm updating this. Somehow I guess I didn't put it in right. I reinstalled the lens several more times later on (got frustrated) and it now magically works.

<p>You may need to clean the contacts on both the lens and the camera.</p>

<p>Could your camera have been set to back button focus some how? </p>

<p>John</p>

<p>A question that I've often thought about, does taking lenses on and off self clean the contacts? I can understand that if the contact surfaces are not wiped for sometime a thin oxide film may form preventing good electrical contact.<br>

Neill</p>

<p>Probably not oxide, given that the contacts are gold plated for that very reason. However, it is perfectly feasible that <em>something</em>, probably grease and dust etc, gets on the contacts, and they do indeed "self clean" as you say. </p>