DSLRs operate in that way, and it makes sense for a professional photographer to be using one. There's a mirror that directs the light coming from the lens to the viewfinder, and then when you take a picture, the mirror flips down and the light goes to the sensor instead. So whatever is on the screen is the last picture taken, not current, because the sensor doesn't even get any light unless you're in the process of taking a picture. Unless it's taking video or in "live view" mode, then the mirror is down, the viewfinder gets no light, and the screen is the live view.