DxO labs has tested and reviewed the Canon EOS 17-40 f/4L.
From their review:
“Due to its more glamorous sibling this modest lens is often overlooked though it’s well known for its high-performance by Canon users. As a small, light, highly portable zoom it would make a great choice for travel, landscapes and general-purpose photography.”
Our own review by Justin echo'd this feeling. Justin has owned and used this lens from his very first day shooting, and it's a completely viable alternative to the Canon 16-35 f/2.8 L if you can spare the extra stops of light. You can read our review here, and check out DxO's full review here their lens comparison tool is an excellent resource if you're into sharpness charts.
Source [DxO]
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Canon 17-40 f/4L at B&H | Canon 17-40 f/4L at Adorama