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The EOS 50d Mark II is what's known as a "full-frame" camera. Its image sensor is the same size as a frame of 35mm film. The Sigma DC lens is intended for cameras with smaller sensors. The lens is compatible in that it will work with the camera to take pictures, but the pictures are cropped due to the smaller imaging area of the lens.
In order to use the full image sensor area you need to use full-frame lenses. In the case of Sigma, this means lenses without the DC designation.
Sigma makes some lenses for Nikon cameras-- though the Nikon D40 requires a lens with its own internal focusing mechanism, since it does not have an autofocus motor. This entirely depends on what Sigma lens you have, i.e. if it were a Sigma lens made for a Canon camera, yes there would be an adapter for that-- but the likelihood of success is less with the limitation of the D40 needing an internal focus motor on the lens.
You have to make sure you bought the sigma lens with a NIKON mount. Sigma makes a Sigma mount as well as a Canon mount, of course neither of those mounts will work with your camera. Also you must also make sure your lens is the Sigma 70-300 HSM (/Hyper Sonic Motor) or it will not AF on the D40. I just bought this lens from Abes of Maine and I love it on my D40.
there is a defective flexwire inside the lens or unfortunately the opening or fstops motor is defective. flexwire is easy to repair but a replacement motor is not available or hard to find, (the magnet inside the motor is slowly disintegrating causing it to stall)