problem focusing with my 50mm f/1.8

[benhasajeep52 50]

Well, it might have. I am not too familiar with the Rebels but one of the other modes should allow focusing tracking. Since things were moving, it may have done a quick refocus when the guy moved forward. It will not correct for slow shutter / motion blur though. Just for moving subjects.

Single is more a focus, lock, fire AF mode.

 

[GeneralBenson]

Shouldn't be camera shake. I can shoot my 50 3-4 stops slower than that (one of the advatnages of pentax's in camera SR). I think 1.8 DOF isn't much, and if you're in single AF mode, then the scene is changing too much between focusing and shooting. Either you're moving, or the subject is. In the example you posted, the focus is definitely best in the front of the scene, and gets more out as it goes back. It looks like you focuses on the closer person's back, then they moved 6 inches away before you shot.

 

[Samanax]

Has this particular lens ever taken a sharp picture while using auto focus?

It takes good pictures if you manual focus, right?

 

[usayit]

one shot AF.

1, you are shooting at f/1.8. DOF is slim and thus focus is critical
2, one Shot AF will focus and give confirmation of focus AT THAT MOMENT. Meaning once it confirms focus it will stop and hold... it will not continue to achieve focus.
3, The subject (I assume the one in front) is moving away. My guess is that he walked outside the field of focus right after focus confirmation and is beyond by the time the shutter trips.

It doesn't look like camera shake.

To rule out equipment failure, place the camera on a tripod and point it downward at an angle. Place on the floor a ruler or something with distance markings. Focus on a marking and take a photo at wide aperture. The marking you focus on should be in focus. Stop down one and shoot again. Repeat. If the markings either nearer or farther away are in focus, then your lens/camera is not focusing properly.


Read up the manual for your camera. Canon has an AF mode specifically for moving subjects and such; AI Focus, One Shot, and AI Servo.