i have a sigma 28-200mm lens which i use on film camera, a canon eos 7e
the lens works fine on my film camera
when attached to my canon rebel xsi the focus is very soft and some out of focus completely
apparently the 'chip set' on sigma lenses 5yrs or older are incompatable with the newer technology and you have to send gthem back to sigma to be 're-chipped'
however sigma does have parts to do this anymore or at least that was the response they sent me
send sigma an email with the serial number of your lens and see what kinda response you get
hope this helps