The compatibility of Canon lens is only one way. You cannot mount a Canon "S" lens to a full frame camera. The reason is that the "S" lens is made to focus the image on a cropped sensor. The cropped sensor is smaller than the full frame sensor by a factor of 1.6. Canon lenses which carry a "EF" designation (without the "S") are made for full frame sensors, but will mount to a Canon cropped sensor body, like the 80D. When mounting a "EF" lens to the 80D, cropped sensor, the image that will be projected on the sensor will be larger than the sensor. The sensor will only produce the portion of the image that is from center to the edge of the cropped sensor. This gives the illusion of a larger subject in the image. Some think the "EF" gives better "reach" or possibly some magnification but that is not so. Example would be using a 50mm lens on a full frame camera, take a shot. The shot will show the full frame view of the lens. Now, mount the 50mm lens to a 80D, and take the same shot. When you look at the 80D shot you will notice the center subject will be the same, but the outer edge will be cut off. This is due to the angle of view of the cropped camera sensor. So, on your 80D you can use any Canon EF lens, including any Canon "L" quality lens, with either the red ring, or white telephoto lenses. When you mount a EF lens to your 80D, you can calculate the crop factor by multiplying the lens mm by 1.6. (50mm X 1.6= 80mm angle of view) Now, for third party lenses. With the 80D, you'll not have a problem, all the third party lens makers have both type of lenses in their lineup. Good luck with your 80D, and any lens you mount on it.

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