Sat Nov 21, 2009 8:35 am
Take the lens off and check the mount ring on the camera. There is a small pop-up pin (the lens lock pin) on the face of the ring right next to the lens release button. If it's worn or broke the lens will not lock into position. When you put the lens on the camera, as you turn the lens there should be a "click" sound as the pin seats in the hole on the lens. If you use a lens with a plastic lens mount, check to see if that hole (in the 9 o'clock position as you look at the rear of the lens with the contacts in the 6 o'clock position) is worn or broke. If it is, chances are the the lens will lock in position but then slowly turn as you zoom the lens in or out to the point where the contacts misalign. If the pin is worn or the spring under the pin is broke the same thing will occur. I've been using my 300D for a number of years now are have not had that problem but have noticed that my lenses with plastic lens mounts are getting a wear groove where they contact the mount ring on the camera. If the same thing happens with a lens with a metal lens mount, it's probably the lock pin. Hope this helps.
Paul
The trouble with being retired...you never get a day off.