I wonder if the person who tested the 50-500 tested the OS version, it should be sharper then the non-os version , and Before i purchased the 50-500 OS 2 years ago I had read many reviews which said it was sharper then the 150-500.
Also he states that the canon 100-400 is less sharp then the sigma 120-400, which surprises mehttp://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=453&Sample=0&FLI=7&API=0&LensComp=683&CameraComp=453&SampleComp=0&FLIComp=4&APIComp=0
Thats the 100-400 and 150-500 at 400mm, i doubt the 120-400 that sigma has is that much sharper then the 150-500http://forums.dpreview.com/forums/post/39554709
, here people discuss the 150-500 and 50-500http://www.lenstip.com/244.4-Lens_review-Sigma_50-500_mm_f_4.5-6.3_APO_DG_OS_HSM_Image_resolution.html
'''Quote'' The Sigma surpasses noticeably the 150-500 mm OS model here, which, on going from 400 mm to 500 mm, had a lot of problems with reaching similar results. What’s more, even after significant stopping down to f/11 the 150-500 mm model at the maximum focal length barely brushed against the usefulness level. The Sigma 50-500 mm OS doesn’t have any problems with exceeding it and, after all, its focal length range is a lot wider.
When you compare the tested lens to the Sigma 120-400 mm OS we see a very similar situation. The Sigma 50-500 mm OS at 400 mm fares better than the 120-400 mm OS model and additionally we get a useful 500 mm at our disposal.
How does the Sigma compare with its most dangerous rival, the Canon 100-400L? With the Sigma we get a wider range and a bit better stabilization but this device is a little bit more expensive. As far as the image quality is concerned, the Sigma loses…the Canon at 400 mm is not only faster but also fully useful up to the maximum aperture. Near f/8 it has results at the level of 35 lpmm and it should be emphasized that it was tested on the sensor of the EOS 20D. The Sigma reaches 36 lpmm but only on the 50D’s sensor with 15 million pixels. If we tested the 100-400L on the 50D we think for the combination of f/8.0 and 400 mm it would exceed easily the level of 40 lpmm. The 400 mm focal length is definitely better in the case of the Canon. The Sigma, however, boasts 50 mm and 500 mm focal lengths for a change which the Canon lacks…
I hope you can make your decision !, I would personally go with the 100-400 that canon has,
My sigma 50-500 OS had so much dust inside the lens after only 2 weeks, the autofocus was not 'that' fast,
But being able to zoom from 50-500 was great...