As stated it should be.
EF-S / EF is compatible with APS-C style cameras (ie they have both the white and red dots on the screw mount)
EF is only compatible with Full Frame Cameras (those with only the red dot on the screw mount)
Just have a look on Canons own site: http://www.canon.co.uk/for_home/product_finder/cameras/digital_slr/EOS_50D/
EOS 5D MKIII + EF 24-105mm f/4L IS USM, Sigma DG 150-500 5-6.3 APO HSM, , EF 55-200mm 4.5-5.6 USM, Speedlite 430EXII
EOS 350D, EFS 18-55mm
