Canon T3i won't focus with a

Please provide more information about the conditions under which this occurs.

Try another EF lens. There is a circuit in the lens too.

I've not come across this situation before. Any FD lens will only work manually for aperture and focus with an FD to EF adapter. I'm not sure how you are mounting the FD lens. An EF lens should work auto on focus and aperture.

Most likely your camera is set to M manual focus. There's a little switch on the side of the camera that has S/C/M. The S is for single servo focus so it focusses on one subject and does not re-focus. The C is for continuous for action and movement and the camera continuously re-focusses, and the M is manual focus.

The other possibility is that both lenses are switched to Manual. There is a switch on the lens that says M/MA. Be sure it's on MA

You could have set your camera in MF and on the lens is a switch, you could have set (accidentally) to MF.
If both are correct and you camera still does not focus automatically let me know.

Quantaray makes lenses with a variety of lens mounts. Assuming the lens has a Canon EF mount, it will fit onto the T3i just like any Canon lens. If it has a different mount, you'll need an appropriate lens mount adapter.

Assuming your lens has a Canon mount, yes, it will work. Since the lens is completely manual and nonelectronic, you won't get any autofocus or autoexposure.

The Raynox 250 is an adaptor lens that makes a "macro lens" from a standard lens. Normal you only can focus a standard (zoom) lens say about 2 feet in front of the camera lens and anything beyond that. Small things, would be very small in your frame. With a macro lens, you can come very close to an object, because they can focus from a few inch in front of the lens. With the 250 you "transform" a standard lens to a macro lens, by just clicking it on the front of your lens. Read the manual, how to do that.

You than can make macro pictures, but great chance you have to focus in the MF position. When you have focused, you should be able to take a picture in automatic (green camera symbol) when you want to change things, you only look what shutter time and what aperture the camera choose and from that you can go into manual mode to make the aperture smaller (greater f number) and an other shutter time.
For macro photos you always should use a tripod, because shutter times can be a bit long and every movement of the camera will be greater than the object you try to get sharp in the picture.

Have a look at this page for discussion about this adapter. Depending on the lens you're using it with - the DOF can be extremely shallow and make focusing difficult. If you're in Auto Focus mode - try switching to manual and see if you can get better results.

Good luck!

Wideangle: 0.6m to infinity (from lens front) / Telephoto: 2.0m to infinity (from lens front) Wideangle: 0.685m to infinity (from CCD) / Telephoto: 2.085m to infinity (from CCD)