Loose mount

I picked up a 100-400 not to long ago and have noticed that when it is mounted on the camera it seems loose. I have tried it on the 400D and 30D and it is about the same on both. It locks in securely but there seems to be more play than with my other lenses. It doesn't seem to effect anything but I am curious to see if is a "noraml" thing.

Yeah, mine is the same, quite a bit of play in it even when it was new. It's never been a problem with mine, it's always taken great images so I wouldn't worry about it.

Did you try tightening the screws on the lens? Sometimes they become loose over time.

Strick wrote in post #4013895
I picked up a 100-400 not to long ago and have noticed that when it is mounted on the camera it seems loose. I have tried it on the 400D and 30D and it is about the same on both. It locks in securely but there seems to be more play than with my other lenses. It doesn't seem to effect anything but I am curious to see if is a "noraml" thing.

What direction(s) is the play in?

If the play is felt only when you attempt to rotate the lens in the mount and NOT when you try to push the lens from side to side relative to the body, there is nothing to worry about.

If, on the other hand, the lens is not held tightly enough to the camera body to keep it aligned with the camera then you have some repair work - maybe as simple as snugging a few screws - to deal with.

It is the former, it just as a little play in the rotation. I figured it was not a problem but it can't hurt to ask.

Yes same with mine. Just in the rotation, so nothing that affects the alignment. It's not due to screws loosening over time as mine was like that out of the box on day 1, and has no loose screws anywhere (I check all my lens mounts periodically so that I don't get one stuck on the body).

I have seen other posts about this matter in the past, it doesn't seem to be a problem to anyone and it certainly hasn't affected any of my results from this lens.

I have the same problem (loose mount) with my 100-400 but my 24-105 is ok. I think the cause of the problem is the following: On the mating surface of the lens there is a hole into which the locking pin from the camera body goes. This hole is a little bigger in the rotational direction for the 100-400 than the 24-105. Thus the locking pin has a little play with respect to the hole causing the body to rock. A close look at it seems to indicate this is intentional on Canon's part. I do not know why. This little play can potentially be bad. The lens is fixed onto the tripod which is presumed to be stationary. The body can rotate ever so slightly due to shutter motion. If it happens then the picture sharpness is compromised.