I want tack sharp images, Canon 17-50 or15-85?

kewlguy wrote:

Have you tried the 55-250 STM? It's far from poor. But if you want to invest on a lens (good thing), buy a FF telezoom like 70-200/4 IS or 70-300L.

As for standard zoom, if you can stretch the budget, try the 16-35/4 IS. The 17-55/2.8 is a good lens but due for an update...

most of the time I will only recommend full frame lenses for FF cameras or if you think you may up grade to FF later

The reason is most of the time FF lenses dont do so well on APS-c as they do on FF...this "sweet spot" thing is a bit of a myth most of the time

here you can see the 16-35F4is vs 17-55f2.8 is the 17-55 is slightly sharper on APS-c ...the only "sweet spot" advantage is with vignetting  but the 17-55 is cheaper a stop fast and a bigger FL range..

Regarding 15-85 vs 17-55 they are both very good with the sharpness edge going to the 17-55..it down to you whether you want bigger FL range or the faster constant F2.8 ..remember you will gain IQ if you can keep your ISO lower with the latter  which is 2 stops at the long end

I put the prime in to show how good Canon zooms can be as you can see the zooms are almost as good ..but you ask how good is the canon 24mm stm prime very good you see it match the £800 GBP/$1000 UD  zeiss 24mm f1.8

also of note when looking at the links if you click > measurements >sharpness > field map you can see the sharpness at different FL and F stops  you will see the 17-55 has a week spot at f2.8 and 17mm  but once stopped down is sharper and is sharper at all other FL

My choice would be the 17-55