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| p.1 #18 · Question: Keep my 17-40 f/4L or trade for 16-35 f/2.8L II + boot |
Two, is the FF scenario. Here the benefits of the 16-35L II are far greater than the benefits of the 17-40L (provided you have an average/good copy of each). The 16-35 has better overall IQ, less vignetting, less edge softness, faster aperture -- and that important 1mm, which is very noticebable in head-to-head comparisons. Still, the 17-40 is a good lens, but the 16-35 is in a differernt realm.
It isn't that simple at all.
Neither lens (17-40 or 16-35) is intrinsically better than the other in an overall way. Unless you subscribe to the "if it is bigger and more expensive, it must be better" school of lens acquisition, the choice is about matching the lens to your photography, no more and no less.
The 16-35 f/2.8L II is an excellent lens. It provides one additional stop compared to the 17-40 and it is less prone (though not immune) to corner softness wide open. It is a larger, heavier, and more expensive lens. Optically, the primary improvement of the II version was at f/2.8 - from all reports it isn't significantly different at other apertures. The 16-35 uses a slightly larger and non-standard 82mm diameter filter thread, so you'll need to update your filter collection in most cases. If you need a large aperture ultra-wide lens for hand held, low light shooting on full frame, this is great choice.
The 17-40mm f/4L is an excellent lens. It provides a slightly larger focal length range. Stopped down a bit it is an excellent performer - as good as or (insignificantly) better than the 16-35 at smaller apertures. It is also regarded as being a bit more resistant to flare. It is a bit smaller/lighter - not significant on its own, but when carrying a pack full of lenses it makes a contribution. The 17-40 uses the same 77mm filter threads that are found on a good number of the other popular L zooms. If your primary need for an ultra wide angle lens for shooting subjects like landscape, architecture, and similar from a tripod, then this is a great choice.
There are those among us who could choose to own the 16-35 and who would if we believed it offered advantages in our photography. For us cost is not the issue. Performance for the kinds of photography we do is. One photographer might be better served by the 16-35, while another equally good photographer doing a different sort of work might be better served by the 17-40.
Neither is in a "different realm" and you cannot differentiate among photographs made with the two lenses by looking at them.