if you shoot a standard subject with a fixed manual exposure using a canon 300mm f/2.8 prime and the sigma 120-300mm zoomed out all the way to 300mm.. the sigma lens usually has the image underexposed relative to the canon lens on the body by about 1/3 a stop. its rather obvious, just by doing histogram comparisons.
the other way to see it.. is to take the 300 and divide it by 2.8 and realise that it is actually larger than that largest filter size used by the 120-300.
go search around on the net for the issue. Its quite acknowledged actually.. however.. if you don't use the zoom all the way.. say.. stick to around 270-285mm in focal length.. yup.. your max aperture will be an effective 2.8.
