New answers tagged sigma
It shouldn't, PDAF occurs before any metering adjustments are made because the quality of focus is directly dependent on the available light. Focusing should occur with the aperture wide open before anything is stopped down.
I'd go with the constant f/2.8 version. I don't think I've ever used my Canon 17-55/2.8 wide open at less than 55mm, yet have used it wide open @ 55mm a lot, especially for pictures of people (including babies). The IS allows me to use a lower shutter speed at f/2.8 than my 50 f/1.8 wide open, albeit with wider depth of field.
Just measure it. To quote Wikipedia: "The specified diameter of the filter in millimeters indicates the diameter of the male threads on the filter housing." See picture: the diameter of the threads is 67 mm, so that's what the filter size is.
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