That's because the point on the photo where you tap on the screen is the place that decides the focus, light and all the other stuff that makes the photo. When you use the button, it will pick the closest thing in the frame or something else and measure out from that. It's the same on real cameras where you either choose all of the focus spots, or just one that you point at the object in the photo. You can even, on a really good camera, use the spot function.
On landscape photos or similar, use the button. If you take photos of one special item/object, use the "tap on screen" function![]()
