Need help selecting a new lens.

[neuroanatomist]

i know the 17-55 is faster and better for low light but the 17-40 is an L what do you think?

The build quality is better and presumably the autofocus; thats about it for advantages between the two.

The build quality of the 17-40mm f/4L is better, yes. Not sure that the AF is better. Both have ring USM with FTM. An f/2.8 lens like the 17-55mm enables the high-precision center AF point (the 400D does have that feature). Also, twice the incoming light with an f/2.8 lens vs. f/4 means better focusing in low-light situations (important indoors). So, given that, I would thin AF on the 17-55mm would perform better.


The 17-55mm has less barrel distortion at 17mm than the 17-40mm (and that's despite cropping with a lens compatible with FF; on FF it suffers pretty badly from distortion). The 17-55mm has more vignetting wide open (here the crop factor helps the 17-40mm more). The 17-55mm has slightly better resolution even at f/2.8, especially at the edges, than the 17-40mm.


Overall, I think the optical quality of the 17-55mm f/2.8 is slightly better than the 17-40mm f/4L. The build quality of the 17-40mm f/4L is better, and it's less expensive (and includes the hood, which must be purchased separately for the 17-55mm). The 17-40mm is compatible with FF if you plan to go there soon.



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Pros 17-40 are weathersealed (with frontfilter) so it would mean a good outdoors weathersealed combination with your 7D.


I chose between them half a year ago, but not considering the weathersealing as important. I chose the 17-55, but nowadays I don't know becasue I also like to wander around when weather ain't so great.

<div>Weather-sealing is the main reason I got the 24-105mm f/4<span style="color: red;"]L IS. I do like that lens, but I still find the 17-55mm f/2.8 IS generally more useful. The f/4 lens hunts for focus more often indoors unless I have the 430EX II mounted (I hate the strobe assist from a pop-up flash).</div>
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