Sigma Lens focusing problem. *pics included*

Ok, I got this lens used from someone at a SUPER cheap price, but it seems to have problems at longer focal lengths. It is the Sigma 70-300mm f/4-5.6 APO Macro Super.

At longer focal lengths the lens does not want to seem to focus on the object in the AF points. The lens will focus on the object, give the AF beep, and the AF point will flash, verifying a focus, but the object in the viewfinder is still blurry. It's not so much blurry as it looks like a double of the object. Kind of like if you cross your eyes a little.

Example: This was taken at 300mm, f/7.1, 1/1250s, iso200. This is a 100% crop as well.

Most of the other ones were taken at similar settings with at least a 1/1250" shutter, and you can see the propeller in the photos actually stopped, so I don't think it is motion blur.

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What AF mode are you using? The only way to get AI servo focusing on the 300D is in "sports" mode. The fact that you're getting a "beep" means you're in one-shot mode or AI focus (worthless) mode. Neither will get the job done with moving airplanes.

If you get the same scenario with still subjects then I don't think this is your whole problem, but I think it's part of it.

I was in AV mode, but this occurs with still objects as much as moving ones.

Have you done any test with still subject to confirm the problem? Place a beer bottle or something similar 30ft away from you on the ground, and see if you lens can focus accurately.

the sig is soft at 300mm. Look at the photozone's review. I don't remember mine being that soft at 300mm though. Perhaps some tests on still subjects on a tripod is warranted to confirm.

Here is another example from a while back. What is strange, is this only occurs sometimes. I suppose it is possible to be motion blur, because messing with the camera right now, it works fine, but other times, it will not do this. I'm fairly certain it isn't motion blur though because of the fact of it not focusing in the viewfinder.

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I agree. Do some controlled testing to see if you can narrow it down. Shoot a high-contrast target taped to a wall to avoid focus error (focusing on something other than your target). Try different focal lengths and different apertures.

I am pretty sure that this is motion blur and/ or camera shake - it is the cause of a lot of blurry pictures. I'd suggest try a series of test shots with a tripod and see if the problem goes away.
I have read that because the pixels on the sensor are so sensitive, digital cameras are mose senstive to camera shake. also i have read that the crop factor actually increases the risk of camera shake also.
i would ahev thought a speed of 1/1250 second would freeze just about anything but if your camera isn't steady and the subject is fast moving, that could account for your pics.
Just my 2c and quite happy to be wrong...

If those were at 1/1250, you'd have to be on a 30 cup a day Starbucks habit to get that much motion blur.

Set up on a tripod, aim at a good contrasty target and see whether it looks sharp. Then try manual focussing and see what you get. From what you say, I suspect impact damage, and something getting knocked out of line at longer f.l. in which case it'll need to be repaired (may be why it was so cheap).

id imagine with the planes its because by the time u hit the shutter release after the focus beeped the planes had moved a lot. using a static object would be a lot better test. with the bird it looks like possible motion blur.