The best way to get candid photographs is keep the camera down/away from your face until the split second before the “decisive moment”. People start altering their behavior (e.g., “stiffening” their expressions, turning away from you) as soon as they see you aiming that camera at them; so you have to be like a “gunslinger” and only reach for the camera to bring it up and aim it just before pushing the shutter button.
The other “trick” is to get a steady, in-focus image when only giving the camera a split-second to respond. The latter takes good equipment, knowledge of how to use that equipment, experience and well-honed technique.
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