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The lens in question has an aperture range of f/2.8 to f/4.5...... which will mean that you can only get f/2.8 at the short end of the zoom range. There would be a 'crossover' point between 17 and 70mm where it would switch between the two widest apertures, but usually it's going to be just a little higher than the shorter end... perhaps at around 28mm. It won't be a gradual change, either.... one moment you'll be on f/2.8 and the next at f/4.5.
I see there's also a Sigma 17-70mm f/2.8-f/4....
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