trying to understand how my camera focuses

[CatherineA]

When I see diagrams showing how a lens bends light, it seems to me that once I set my focal length and aperture, then the plane in which everything would be in focus would be completely determined. The shutter speed would influence what could actually be seen, but not what is in focus. Today I tried manual focus for the first time and I don't understand why, when I pressed the focus button, I saw a different set of peaking lights from what I saw without pressing the button. Wouldn't any difference in the two mean that the lens shifted position?

[Manfred M]

Not quite Catherine.

The camera will focus where it calculates the focus plane should fall. Those are the areas that are highlighted by the focus peaking.

When you focus manually, the focus plane changes, based on where you manually tell the camera where the focus plane should be. This area will show up in focus peaking and show you where you told the camera to focus.

[CatherineA]

Thanks Manfred! I clearly need to read more about this to it understand better.