If you focus a camera set to a wide aperture (eg f2) on some nearby object out of doors then this throws more distant objects out of focus. The same happens if you focus on things reflected in a mirror, presumably because the light from more distant objects is travelling all the way to the mirror as well as from the mirror to the camera.
But why is this? In the first example the light from everything hitting the lens has travelled all the way from the sun before being reflected off the various objects, near and far. Why are they not all out of focus or in focus? Alternatively, the last point of departure for the light from the mirror is identical in all cases, so why isn't everything in focus.
There seems to be a different rule depending on whether the last point of departure for the light was an object or a mirror. I'm genuinely puzzled.
Answers to pwkeeble@btinternet.com please.