17-55 Aperture Advantage 2.8 New to DSLR

Lemming51 wrote:

If you're shooting at f/5.6 or smaller (larger f-number), then both lenses will give very similar result. Both the 17-55/2.8 and 15-85/3.5-5.6 have image stabilization to help hand-holding in low light with long shutter speeds.

The viewfinder AF works with aperture wide open, and many EOS cameras will AF with greater precision with f/2.8 and wider maximum apertures. And f/2.8 zooms often have more lens elements with better correction of optical aberrations than do f/3.5-5.6 zooms. Despite the shorter zoom range, the 17-55 has 19 lens elements (2 are Ultra-low Dispersion) compared to the 15-85's 17 lens elements using 1 Ultra-low Dispersion element.

Shooting the image with f/2.8 and larger aperture allows you to

• give shallower depth of field, separating your subject from the background
• use a higher shutter speed at the same ISO to freeze motion - both the subject's and yours if shooting hand-held

No right answer, both are very fine lenses. I chose the 15-85. YMMV.

Thanks very much for the detailed reply, Nice to get some solid facts behind the decision. Cant believe it's been so difficult to choose between the two lenses! I actually went out this evening (SA time) and shot with the 50 1.8 but set to 2.8 to kind of see how it handled the light and I must say in some dark (ish) places not too well. That made me realise that maybe 2.8 is not even fast enough for very low light and perhaps the choice should be 15-85 with a tripod for evenings and night. The only thing that slightly bugs me is the DOF and perhaps not being able to get as creative images out of the 15-85 as I would with the 17-55. I do have a point and shoot for general pictures I need a lens that will allow me to get creative looking images with high IQ, although maybe i need to study more techniques and learn how to shoot at all the different lengths the 15-85 allows.