You know... it's still an interesting question... Whilst the 17-50 is 2 stops quicker, a lot of night time street photography appears to be about contrast and capturing movement (often blurred) - not about tack sharp images of people/things moving. The IS system on the 15-85 is excellent (4 stops) so is the equivalent of carrying around a non-IS 1.4 prime (or better at the shorter focal lengths) for
static subjects. You will also have depth of field advantage of f5.6 but with the ability to handhold at ridiculously slow shutter speeds. If that suits your style of night time walk around, then the answer isn't so clear cut
It isn't
always about wide apertures for night time - particularly when you consider the 15-85 is sharper to the corners wide open than the 17-50 OS when stopped down to the same (or smaller) aperture as the 15-85. Take a look here if you don't believe me:
http://www.the-digital-picture.com/...LensComp=729&CameraComp=0&FLIComp=0&APIComp=0
+1 and in addition......
People seem to get very hung up on being able to shoot at f2.8 and whilst fast glass has it's place, it isn't always necessary.
A mid-range f2.8 zoom would be useful to a wedding photographer where using a flash in a church/registry office isn't an option but for someone doing a bit of street photography at night, even at f2.8 you would probably still need a tripod to stop lens shake and the DoF might not be that useful to you. You can still get a nice blurry background using f5.6 but you may need to be a little more creative
I'm only an amature photographer but I have added around 6K-8K images in my Lightroom catalogue since March/April this year.
I can't think of many instances where I've thought "damn..... if only my 15-85 was faster"
My brother-in-law's favourite rule of thumb is 80-20.... as in 80% of the time you won't need a fast lens but 20% of the time you might do.
But then you could apply the same logic/thought to the focal length of the 17-50.
For me being able to shoot at 15mm all the way up to 85mm is fantastic and it's very rare I need to go longer (I have my 100-400mm for that
)
Before I chose the 15-85, going on advice I was given on here, I downloaded a program that analyses the focal lengths my pictures were shot at (at that time I was using a 400D & Sigma 18-200). It turns out that 40% were around 80mm and an equal amount at 18mm and 200mm with a smattering of other lengths in between.
Given the inferior IQ of the 18-200mm lens and the 400D's 10MP to the 7D's 20MP, my thoughts were that I would be able to crop any shots without too much loss of IQ so wouldn't miss the reach of the old lens.
From what I can tell, there isn't too much difference in IQ between the lenses, they tend to have their own weak/strong lengths/f-stops, it really comes down to what you actually think will work for you the best and of course budget
