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
The fact that your lens is a variable aperture model and you are having difficultly focusing on the long end points to the likelihood that your camera's is not acquiring focus because it is getting insufficient light to use the center auto focus point as a cross-type (reading both horizontal and vertical in phase mode, through the viewfinder) and is defaulting to only a horizontal type which needs a vertical image point to acquire focus.
So that would still be an issue when I used manual focusing to fix the problem. I tried manual and the problem persisted
Is the remedy to change the focus area setting to the broadest possible or the narrowest possible (i.e. spot)?
Also wouldn't the issue be addressed by opening the aperatrure to f22 to give the lens the most leyway in focusing with increased depth of field?. I did that too and the focus problem persisted
Thoughts per the above?
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.
Of course, the best thing is to try it on a different camera and see if the issue presists. And/or try your camera with some other lenses and see if they focus properly.
One thing to remember is a long lens does not only "magnify" the subject but any and all problems connected with it.
Couple thoughts on the suggestions provided which are appeciated:
The problem has persisted since I have owned it and exisited before it was dropped. Also the long lens was not on the cmaera when dropped. The blue shift problem seems resolved now that the setting has been changed to Auto from Fluorencent
I live in a hot but dry area (Califirnia near SF) and it has never been in a hot humid area for any length of time even on phot trips. So mold is not an issue. I also use it maybe once a week, so it has been used enough to not have oil issues
The problem may be gradual as suggested but when I blow up photos at 135mm or above it is pronounced. I can not tell if there is a shift above 135mm or just an acceleration. Photos below 100mm are acceptably sharp
Becasue I have tried to manually focus, any issue with spot metering or auto focus should be moot. I have been tried to focus slightly above or below sharp to see it that makes a difference and it did not. Shooting wide open with the greatest depth of field also did not work
Of all the items mentioned, the variable aperture issue seems to maybe be relevant because I think I have controlled for everything else
Any thoughts. Maybe it is just a bad lens.