What I don't get is how this focus shift happens.
Wide open and focussed, all the rays passing through all parts of the lens focus (I think this is a pretty sharp lens?) to a sharp point on the sensor plane. Occlude some of the rays by stopping down the iris and the unoccluded rays seem to have their paths changed so they focus off the sensor plane.
Don't see how that could happen - maybe there's something different going on. Can soemone educate me on that?
It occurrs on all lenses, but the construction of the 50mmL and the wide aperture makes it a little more apparent. You cam mitigate it once you know whats happening and why.
is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
Quite literally, the optimal plane of focus moves, depending on aperture! With every lens I’ve tested to date, the focus moves farther away. For example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it might now be centered at 1.02 meters. That apparently small difference means sparkling-sharp eyes versus not-quite-there eyes—it matters, especially with high-resolution digital cameras.
Focus shift is caused by spherical aberration
for an excellent technical discussion). Instead of a sharply-focused point of light a spherically aberrated lens produces a point of light with a “halo”. This is visually confusing when focusing by eye (because of the lowered contrast) making it difficult to find optimal focus. It also is confusing to autofocus systems. In spite of these issues, accurate focus can generally be obtained—but it’s no longer accurate when the lens is stopped down."http://diglloyd.com/articles/Focus/FocusShift.html