to make this a bit easier, the more expensive cameras such as most
digital SLRs allow you to change this method a little by allocating half-press shutter for locking AF to a FUNCTION button on rear top right of the camera. Doing this means you can lock the AF and then take your finger off the camera while you wait to re-compose or for the perfect moment, then all is ready to take the shot as usual as long as subject has not moved further fvrom or closer to the camera.