Canon 10-18mm IS STM
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Since the Rebel is only 10 Megapixels, I set my 5D Mk III to MEDIUM resolution, which is also 10 MP. This puts the Rebel at an even greater disadvantage, because its anti-alias filter blurs its image, while when shooting the 5D MK III at lower resolution the effects of Bayer interpolation go away and thus the 5D Mk III's 10MP images are far sharper than a native APS-C 10 MP image. (For the same-sized image, it's always better to shoot a higher resolution camera at a lower resolution than to shoot a camera at that lower native resolution.)
Here we go. Click either image for the camera-original file:
As the EXIF will tell you, both were shot at f/5.6 at their widest zoom settings, which give the same angle of view, an equivalent of 16mm on full-frame.
The center of the 5D Mk III image is sharper; that's because both lenses are very sharp in the center and the 5D Mk III has a huge advantage of starting with much higher resolution in the first place.
If I had shot the 10-18mm on a modern camera like the Canon SL1, this difference would have gone away since the 10-18mm shot would have been much sharper. In this case, the 10-18mm lens is much sharper than the 10 MP camera can show.
The interesting part is the sides and corners. Look carefully at the original files (you can't see anything directly with the small images on this page) and you'll see that the 10-18mm is sharper on the sides and corners. The 16-35mm II has never been that sharp when you split pixels, which is why people adapt the Nikon 14-24mm and Nikon 16-35mm VR to their Canons for serious landscape work.
Most people don't realize that ultrawide lenses are the toughest lenses to design, and that zooms are also nearly impossible to design well. Combine the two into an ultrawide zoom, and they are so nearly impossible to design well that the world's first ultrawide zoom wasn't invented until 1993, and the world's first really pixel-to-pixel sharp ultrawide zoom wasn't invented until 2007.
Canon's first pixel-to-pixel sharp ultrawide zoom is the 10-18mm, so their older full-frame ultrawides haven't caught up — yet.
I expect that the Canon 16-35mm f/4 L IS should be the first really sharp Canon full-frame ultrawide, but today, a $100 used Rebel with the new $300 10-18mm turns out images about as sharp as the $1,700 16-35mm f/2.8 L II and $3,200 5D Mark III. I just saved you $4,500!
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