Introduction: The Matchup
Canon's EF 16-35mm f/2.8L II USM ($1699 MSRP) and EF 17-40mm f/4L USM ($839.99 MSRP) are two of the most popular “Ultra-Wide Zoom” lenses available for its mid- and upper-level DSLRs. The 16-35mm f/2.8 II is an update of the previous 16-35mm f/2.8 lens, and is said by Canon to offer a complete optical redesign for increased resolution and contrast. The 17-40mm f/4 is billed as a high-quality, budget-priced "L" lens by Canon, at a stop slower than the 16-35mm f/2.8 II and just a bit more than half the price.
The lenses feel similar in use, with the faster 16-35 f/2.8 slightly larger and heavier. The 82mm filter size of the faster lens is going to be only a bit more difficult to find and more expensive than the still-large 77mm filter of the f/4 lens.
The sealed construction, handling and even the number of lens elements/groups favors the more expensive zoom, but not as significantly as the price tag would lead you to believe. The bottom line is, how do these lenses perform optically?
Our optical tests are principally divided into four areas: Chromatic Aberration (the tendency for different wavelengths to be focused at different spots on the image sensor, resulting in color fringing in sharp borders), Resolution or MTF50 (Modulation Transfer Function - a rating of how efficiently the lens transmits both resolution and contrast), “SQF” or Subjective Quality Factor (which is an incorporation of measurable test results put into the context of the limits of human vision and perception). and Light Falloff or Vignetting (how much the light values transmitted by the lens vary from the center to the edges of the frame).
We ran two settings on each lens test: at full-open aperture (bringing out some key vulnerabilities - corner falloff, color fringing, resolution issues), and at two stops down from full-open, often considered the optimal setting for almost any lens, using the “sweet spot” in the center of the glass, without causing the diffraction problems you generally get when you stop the lens down further.