Canon EF 16-35mm f/4L IS USM vs Canon EF 17-40mm F4 L Lens in Alamy Quality Control and technical talk Posted December 15, 2014 I have done what you are thinking about, i am very happy with the 16-35 f/4 certainly a better image quality lens than the 17-40, the 16-35 is better balanced when hand held on my 1Ds III, it also has a very professional feel and finish. It is not perfect as all lenses are not, however if i were to make the choice again knowing what i know now i would change without hesitation to the 16-35 f/4. In regard to the 400mm, i don't have one however the reports seem to be very good on this telephoto, in deciding on what lens to buy really depends on what lens would be used the most, if you feel you would use the 17-40 most of the time there is no question, just go get the 16-35. On Ebay the used 17-40's are selling for about 400 euro, so you will loose less than half on purchasing the 16-35. Good luck. Paul.