You can use any EOS "EF", "TS-E", or "MP-E" lens on the 6D II (or any Canon full-frame DSLR body).
You cannot use the "EF-S", "EF-M", or "RF" series lenses on the 6D II (or any Canon full-frame DSLR body).
There are two fundamental differences between EF vs. EF-S lenses.
Originally EF lenses were designed to work on Canon EOS 35mm film cameras. A single frame on the film was 36mm wide by 24mm tall. A "full frame" DSLR, such as the 6D II, uses a digital sensor ... but it's the same size as a single frame of film ... 36mm wide by 24mm tall. As such, Canon EOS "EF" lenses work on these cameras.
While the photos are rectangular (becuse the sensor is rectangular), the lens itself is round. This means it projects an image "circle" into the camera. That circle needs a diameter large enough to fill the sensor from corner to corner... just a bit shy of 44mm across.
In the early days of digital, the sensors were very expensive. A way to reduce cost and make them affordable was to use a physically smaller sensor. Canon came out with DSLRs that used "APS-C" size sensors... which are smaller. These sensors are 22.5mmwide by 15mm tall. The sensor measures about 27mm diagonally.
Since the sensor is smaller, Canon realized they could produce lenses which are *also* smaller and project a smaller image circle (large enough to project a 27mm image circle, but does not need to provide a 44mm image circle). This advantage means you can get lenses for these APS-C cameras that are smaller, lighter, and cost less ... but optically are still as good as the larger lenses.
ONE technique employed by these smaller lenses is to shift the lens closer to the image sensor. EF-S lenses have a rear optical element that projects INSIDE the camera body when attached. Since DSLR cameras have an articulated reflex mirror that needs to "swing clear" when you take a shot, they need to make sure there is enough clearance for the mirror to swing clear. But since the sensor is smaller, the mirror can also be smaller ... that means there is still enogh room even with the protruding rear-element on the EF-S lens (the "S" in "EF-S" stands for "Short back-focus")
By making cameras with smaller sensors and offering smaller lenses ... the cost was at a point where they could sell cameras that mere mortals could afford. This create a huge market for DSLR cameras among emthusiast photographers. Full frame DSLRS were financially out-of-reach for most people. (The 6D was the first "full frame" camera with a price below $2000. Prior to the 6D, you would expect to pay more than $3000 for a full-frame DSLR.)
But now there are two sensors sizes and two lens sizes. (there was actually a third, but I'll leave that out for simplicity).
If you were to attach an EF-S lens to full-frame body (which you can't do by design -- but hypothetically lets just say you can) TWO bad things would happen.
#1 The image circle projected by the EF-S lens isn't large enough to fill the frame for a full-frame sensor camera. This means you get a round image with black corners and sides -- which you probably wont like.
More importantly...
#2 Remember that mirror that needs to swing clear? On an APS-C camera it has enough clearance because the mirror is smaller. But on a full-frame camera where the mirror is larger... there's not enough room. The mirror would strike the back of the EF-S lens and either jam or break. This is NOT a good thing!
To protect you from damaging the camera, Canon designed the bodies so that even though they use the same lens bayonet design on the mounting flange... the APS-C bodies have a recessed area to allow the EF-S lens to seat flush to the mounting flange. The full-frame bodies do NOT have a recessed area ... this way if you attempt to mount the lens, the rear element ont actually be able to fit on the body.
So the two differences are:
1) EF-S lenses have a smaller image circle (not large enough for a full-frame sensor) and
2) EF-S lenses have a protruding rear-element (not enough mirror clearance for full-frame body).
While you cannot use an EF-S lens on a full-frame body, you CAN use an EF lens on an APS-C body. That's no problem.
I should also mention that "EF" stands for "Electro-Focus" -- in other words these are all auto-focusing lenses. Canon makes a few lenses for full-frame bodies which are not auto-focus. These include all of the tilt-shift lenses (the "TS-E" series lenses) as well as one special extreme macro-photo lens (the "MP-E" lens).
There are now two other lens lines designed only for mirrorless bodies. Those are the "EF-M" and the "RF" series lenses. Those lenses will not work on any camera body other than the mirrorless bodies for which they were designed.
Tim Campbell
5D III, 5D IV, 60Da
