My Canon EFS 55-250m 4-5.6 is blured the 135 setting

Question

This is a relatively unused and new lens (2-3 years old) on my Canon Rebel T2i. Never been damaged. It will produce shape photos until about the 100mm to135mm range settings. Not above that and at 250mm it is a disaster.
I have tried to test for this problem by shooting using a tripod and 200 ASR, f8 setting with a 2 second and then 30 second timer. However, it remains fuzy. I have tried various subjects (backlight, full sun, shade, etc) all on a tripod with the same results. I have also tried to change the f stop to the extremes and the shoot time as well, all to no avail. Ditto for 400, 800, 1600 exposure settings
I have also tired the same routine as oultined above with the stablizer off and with manual focusing, but the problem persists.
I have not tried to lock the mirror but the lens should produce acceptable photos on a tripod at 150mm I have been told
So I am wondering if it is a defective lens or if this lens abover 135mm is useless.
Any comments welcome
Cheers

Answer 1

I thought of the manual focus issue but I can't rationalize that with the auto focus issues.
It would be best to select the broadest AF setting. All points active.
Your comment about aperture is confusing. As the numbers get bigger the size of the lens opening gets smaller. But your camera acquires focus in the largest aperture available to it. Your lens varies from an f4 on short end and f5.6 on the long end. The fact that your lens works properly on the short end where the widest aperture is f4 but not on the long end where the widest aperture is f5.6 suggests that either the camera can't acquire focus (set to use the center point and is unable to acquire a horizontal image point onto which to focus or the lens isn't opening to its fullest aperture above 135mm. (If this were the case, the viewfinder might appear unusually dim, shots at f5.6 wouldn't be properly exposed, ... )
The ISO shouldn't make a great deal of difference to whether the lens focuses or not. But in low light situations, your camera may not be able to acquire focus. Cranking up the ISO won't change the camera's ability to capture focus.
What you're photographing also makes a difference for focusing. There has to be sufficient contrast but that usually isn't a problem so I hope you don't take offense that I mentioned it.
Another thought... the comment regarding its age and condition made me think that it may be suffering some sort of dried up lubricant or other issue related to storage and lack of use. Some things just work better if they're used with some frequency, even light frequency, than if they sit unused.
A final thought is that the lens may be suffering from some sort of mold or contaminant. Mold can grow under certain hot and humid conditions and I've read that expensive lenses are often protected in climate controled cases. Certainly a unlikely issue but I don't know your location.