[- Paul -]

Originally Posted by dcstep

Low f numbers don't define a good lens. Optical quality is the key. DxO has objective ratings of lens accuracy and image quality, if you get into that. Low f-stops mean bigger lenses and bigger costs, all other things being equal.

You need to balance f-stop vs. the planned usage, your budget and your DSLR's ISO noise rejection capacities. I use Canon's L-series lenses (their "best"), but opted for f4 vs. f2.8 because my 5D MkII has very low noise at ISO 3,200. If I shot a lot of indoor sports and/or had lower ISO camera performance, then I might have opted for the comparable f2.8. Going from f4 to f2.8 results in a MUCH larger, heavier lens.

My zooms are 24-105mm and 70-200mm IS (image stabilization) lens. The 24-105 is a little soft at 24 and wide open, but I can fix that with DxO Optics Pro software that applies corrections for my lenses at each specific focal length and aperture, so that IQ is right there with the best prime lenses.

So, a fast lens doesn't equal a high quality lens, per se.

Dave

Thanks for the info -