Some - not all - of the EF lenses have a distance encoder as well. It's fairly approximate; I think it exists mostly for the benefit of the flash system, which needs a rough starting point for the distance in some of the open-loop modes.

For the current Olympus ones, I think there's a broadly similar encoder on the ones with proper manual focus scales, and the pure fly-by-wire ones reset focus at connection so the camera can work it out.

There's a list of the EF lenses and which forms of distance information they provide here: https://web.archive.org/web/20130425064359/http://www.lenspl...