SteveR Loc: Michigan

This illustration best explains the relationship between crop and full frame. The large circle is the image projected by the lens. The large rectangle is the image projected onto a full frame sensor, and the small rectangle is the image projected onto a crop sensor. Note that the objects in the center are the same size for both full frame and crop sensors.

http://www.google.com/search?q=crop+vs+full+frame&tbm=isch&imgil=r9SOPeD1yoAKnM%253A%253BExiRO0XZIbPnSM%253Bhttp%25253A%25252F%25252F%3Ca%20href=#imgrc=r9SOPeD1yoAKnM%3A

Here is also a good article that incorporates that illustration.

http://www.meetyourcamera.com/2010/full-frame-lens-on-cropped-sensor/

cambriaman Loc: Central CA Coast

I have both and intend to stay that way. They each have an advantage. I am sorry to throw ambiguity into the discussion bit as they say: " ....It's complicated."

BebuLamar

I think the cameras that use phase detection sensor can examine the out of focus image and know how far the lens has to move to achieve focus. The inaccuracy comes in when it try to move the lens by a certain distance but the lens moves more or less. The mirrorless has contrast detection sensor thus it has to move the lens until the image is sharp. It can't predict how far the lens has to move to achieve focus. It's slower but there is no error.