Travel, one-lens day

The constraint is readiness for anything with no time to think. On a travel day you meet wide streets, quick portraits, food, and details in the space of an hour, so you want one lens and one baseline that says yes to all of it. The 17-70mm covers the whole range; aperture priority at f/5.6 gives a dependable depth of field, an Auto ISO floor of 1/250 keeps candid people sharp, and Wide AF with recognition means you rarely move a focus point. Set it once and walk.

Watch out for dithering over gear instead of watching for pictures. The point of one lens is to remove a decision, not add one — resist second-guessing whether the Sony would be wider or the aperture should be f/2.8. Keep the camera on and settings fixed, and spend your attention on light and moments. You will come home with more keepers than a bag of lenses would have given you.