Introduction. Having used the Sigma 15-30 F3.5-4.5 on a Canon D60 for several months, it was becoming quite apparent to me that the lower edges of all my images, particularly those shot at 30mm, were never really sharp. The centers of my images were extremely sharp...even wide open. But not the corners.
So, I decided to revisit my earlier comparison of this lens with the much cheaper Tokina 19-35 F3.5-4.5 lens. Just for fun, I also decided to compare them both (at 30mm) with an old Canon 28-70 F3.5-4.5 zoom lens I have. (Note: The Canon lens is not the popular, highly regarded and expensive "L" lens, but an old consumer level zoom lens.)
All images were taken using a tripod, mirror lockup, self-timer outdoors in overcast daylight. I used a Perl Script to crop the corners and center of each image and create the table below. Based on my experience using these lenses for several months, I feel that these test images are quite representative of how these lenses perform on "real-world" subjects.
Results. My findings are consistent with my earlier results, but underscore just how weak the Sigma is at the bottom corners.
Conclusions. Same as last time. Unless you really need the extra width, save some money and go for the Tokina!
About the Image Crops. The image below was automatically generated using a Perl script that I wrote for this purpose. I decided that this script might be useful to others and have decided to make it publicly available here. I wrote the code quickly for my own use...so there are no guarantees, warranties, and you use it at your own risk!