Why is my lens only somewhat sharp near the center and very blurry on the edges?

Question

I bought a canon 50-250mm IS lens (for my T1i), maybe two years ago and haven't used it much, but whenever I do, I always regret using it. Pictures are blurry, the depth of field too shallow and so on. Here's an example:

I shot this one in auto mode, and it is awful. I can definitely see that some of the blur is from handshake, but the surrounding is extremely blurry. Is my lens broken or is this normal?

I am still learning about photography, and did get a much better shot, but I had to hop on a boat and get much closer.

Answer 1

There is definitely something very wrong here. That's not a focus or DOF issue, but looks like the lens suffers from extremely strong field curvature.

It's normal for a lens to be sharper in the center than around the edges, and this effect gets stronger with wide open aperture and at the extreme ends of the zoom range (your last sentence indicates that you get sharper images when you zoom in less). But it should never be as extreme as what is visible on your picture, at least not in a lens produced later than 1900.

I can't think of any way that user error could produce this result. It definitely looks to me as if that lens is broken, i.e. has a strongly decentered lens element. You should have sent it back right after you bought it, but you should contact Canon's service department in any case; hopefully they will still fix it - maybe not for free, but decentered elements are usually not the "broken beyond repair" kind of problem.

Answer 2

The softness at the edges is definitely caused by a decentered lens. This just means one of the internal glass elements is not properly aligned with the others. Like so;

(It could be any of the elements...this is just an example of the far left being off.) I'd recommend that you immediately send it in for repair to Canon. You can start here:

http://www.usa.canon.com/cusa/consumer?pageKeyCode=onlineRepairLanding

You can actually find out exactly what's wrong with your lens by using a homemade star chart. Place some circular binding reinforcement stickers in an x on a black sheet of paper. You can find these at some office supply stores.

Take a picture of the x arrangement and zoom in on the circles. You should notice the black "bleeding" on to the edges of some (if not all in your case) the white circles. Note which direction the bleeding is occurring as this will give the direction of the offset of the misalignment.

You might mention these results when you send it in to Canon for repair. Hope this helps.

Answer 3

One of the lens elements is badly decentred, send it to your nearest Canon service centre for repair.

Answer 4

This problem has been existent even on some macro lenses. The lens shows a "linear" (instead of circular) area as sharp and rest of the area as blur. This problem, to my way of thinking, is existent due to the lens being poorly manufactured. The second reason might be due to extreme sensitivity of the lens towards the light rays that refract all around the edges.

Answer 5

This effect can be observed on some lenses after repair when the lens is reassembled in a wrong way. Particularly, I have experienced this when the rear element on 135mm lens was turned backwards (with convex surface turned to the camera body, while the right position in that case was with convex side directed inside the lens barrel). So it is better to bring this to the decent service.