Canon EOS "EF" lenses (no suffix after the "EF") can be used on ANY Canon EOS camera body... film or digital... full-frame or crop-frame.
Canon "EF-S" lenses can be used only on cameras that have an "APS-C" size sensor (such as your 70D).
A few years back Canon launched a mirrorless line and there is a small number of lenses which are designed to be used exclusively with those mirrorless bodies. They are labeled "EF-M" lenses.
A Canon "mirrorless" camera can use any of the EF-M lenses, and via an adapter it can also use Canon EF-S lenses or Canon EF lenses. In short, it can use any Canon EOS lens.
A Canon camera with an APS-C sensor can use any Canon "EF-S" lense AND can also use any Canon "EF" lens.
A Canon camera with a full-frame sensor (or a Canon EOS 35mm film camera even though these haven't been made in years) can use any Canon "EF" lens (but cannot use the EF-S or EF-M lenses.)
There are a few specialty lenses such as the four different tilt-shiift lenses (designated as "TS-E") and one speciality macro photo lens called the "MP-E". These can be treated like "EF" lenses which means any Canon EOS camera can use them.
So what's the difference?
The sensor inside a full frame camera measures 36mm x 24mm. That's a diagonal measure of just over 43mm. So as long as the lens projects an image circle into the camera body which has a diameter at least 44mm across then the image can completely cover the sensor (no vignettting in the corners). Canon "EF", "TS-E", and "MP-E" lenses can all do this.
The sensor inside an APS-C camera (like your 70D) measures roughly 23mm x 15mm. That's a diagonal of about 27mm. It turns out the "EF-S" lenses were specifically designed for these cameras. The lenses and it's optical elements are all smaller and it projects a smaller image circle into the camera body. This allows Canon to reduce size, weight, and even cost... WITHOUT sacrificing optical quality. However... since they only project a smaller image circle, the image projected into the camera body is not large enough to completely cover the size of a the larger "full frame" sensor. Canon also pushes the lens elements back closer to the sensor (they can get away with this since the reflex mirror inside the camera is smaller and doesn't need as much space to swing clear). A full-frame camera has a larger mirror which needs more space... and would likey crash into the rear-most elements. For this reason, Canon designed the mount so that an EF-S lens will not actually mate properly with a full-frame body.
The mirrorless cameras, of course, have no mirror at all. So the lens' optical elements can be even closer... allowing for an even smaller and lighter lens without sacrificing optical quality. So these "EF-M" lenses can ONLY be used with the Canon EOS M series camera bodies. However... since they're all EOS cameras and lenses, they know how to "talk" to each other. This means with a simple adapter that holds the EF-S and EF lenses at the proper distance from the sensor (each lens is optimized for a specific back-focus distance) it is actually possible to use any EF-M or EF lens with a Canon EOS mirrorless camera (but that adapter module is needed.
There are also 3rd party lenses. Often the 3rd party lens maker will indicate if the lens is intended for use only with APS-C cameras or if it is designed for full-frame or APS-C cameras.
If you do NOT intend to go to full-frame then there's no advantage to buying full-frame lenses (in fact there's a disadvantage because focal length ranges are a bit different.). If you know that you definitely DO plan to go full-frame in the future, then you may want to stick to only EF lenses.
Just be aware that a normal 1x zoom is achieved on YOUR camera at roughly a 28mm focal length. So things like the 28-135mm lens offers a "normal" focal length through a telephoto length ... but absolutely no "wide-angle" capabilities at all when used on a 70D body.
A lens such as a 24-70 or 24-105 offer only a very very slight wide-angle (not much) when used on your 70D.
A lens such as the 16-35mm or 17-40mm zooms WILL offer some wide-angle through a rather mild amount of telephoto when used on your 70D... but on a full-frame camera those lenses will range from very-wide to moderately-wide.
Tim Campbell
5D III, 5D IV, 60Da
