I purchased the T5i Sunday with the EF-S 18-135 IS lens. In addition, I purchased the EFS 55-250 f/4-5.6 IS II lens.
I only, today, did I put the EFS 55-250 lens on the camera body. When I put it on autofocus, the lens goes thru a bunch of jerking, in and out motions. Even after I have the camera aimed at a subject, it still does this. Is this normal? If this is the case I can't, for the life of me, understand why anyone would want to use anything but manual focus.
I have a T4I (basically the same just a little older) with the 2 kit lenses which includes the 55-250. I have not seen this tendency with mine. Based on your description, sounds like it's happening when you mount the lens or turn the camera on.
I you just bought it and it was local, I'd have them check it out and probably replace it.
It was at night. At first I was outside. When I went inside I had the same problem but then, again, the room I was in didn't have the best of light. I am ready to chalk it up to low light conditions. I went to a concert Friday night and took the camera with the 55-250 lens. I got as close to the stage as I could. I was probably about 8 rows back. The auto focus was slow to respond so I ended up switching to manual focus.
Thanks for everyone's input!
Just to make sure it really was just an issue of inadequate lighting, test your lens outside during the daytime and verify there are no issues focusing.
Focus performance is part of the reason why people are willing to pay a premium for a lens that can maintain a focal ratio of f/4 or f/2.8 all the way through the zoom range. Your lens is f/4 when it's at the "wide" end (55mm) but it drops to f/5.6 at the "long" end (250mm). Unfortunately you can't just go "wide" to collect more light, let it focus, and then "zoom" back in because the lens is not "parfocal" -- meaning that if you change the level of zoom after focus, you have to re-focus. There are a _few_ lenses where you can re-focus after zooming and they remain focused (it's not as important for still images, but people who shoot video like that feature and you can imagine why.)
Canon makes an EF 70-200mm f/4L lens -- VERY high quality and actually not too terribly expensive. There's a version which does _not_ include image stabilization and it lists for about $700 (street price is closer to $630) and a version which _does_ include image stabilization but that's a bit more expensive... a little above $1300 (street price is closer to $1150).
They also make the EF 70-200mm f/2.8L ... the non-IS is about $1300 and the IS version is about $2500. VERY desirable ... just not cheap.
An f/4 lens collects twice as much light as an f/5.6 lens. An f/2.8 lens collects twice as much again... or four times more than an f/5.6 lens. This makes it possible for the lens to focus in situations where the f/5.6 can't do it.