I’m underwhelmed by both these Canon zooms, and with DSLRs likely to hit 30-40 megapixels soon, neither of these lenses can deliver the quality that such a camera will demand, since neither one is up to the task with the 21-megapixel Canon 5D Mark II. While the quality is satisfactory (mostly) by f/8, that’s limiting and not versatile.
Sure, you can stop down to f/8 or f/11 and get decent results, assuming you can find a good sample. But my tests and field experience show the Zeiss ZE 21mm f/2.8 Distagon to be superior wide open at f/2.8 to either of these mediocre zooms— see Guide to Zeiss ZE / Zeiss ZF Lenses.
Then there is the Canon 35mm f/1.4L and TS-E 17mm f/4L, which also show superior results. So it boils down to whether you prefer the convenience and lower cost of one zoom lens, or higher quality and more expense of fixed focal length lenses.
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