EFS 17-55 vs 15-85 in low light

Hi Peter,

I agree that the 17-55mm lens is better for low light because IS often prevents camera shake but not subject blurring due to movement. The IS of the 15-85mm lens is very capable and regularly allows sharp pictures at 1/8 sec at the wider end. However, people are mostly blurred by their own movements at this speed, so faster glass is better.

Could you please explain what you mean by the "extra-sensitive central AF sensor only activated at f/2.8 or faster". I shoot my 15-85mm and 55-250mm lenses exclusively only using the central AF sensor, but in both cases we are talking of f/5.6 at the long end. Sharpness is 95% perfect despite the small aperture. I have indeed heard that the central sensor is more sensitive than the others, but this refers to it being able to handle smaller effective apertures than f/5.6 (e.g. when using extension tubes) and not only f/2.8 or larger.

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