My "optix v6" was also advertised as working on 5D3. At first sight it's fine, but once you use it for thousands of pictures, you can start noticing all sorts of quirks, like entering program mode whenever it wants, or the reported focal length and aperture changes, so it's kinda annoying. I ended up removing the chips from my lenses and focusing with DIGIC peaking, high sharpness in picture style and 10x zoom on half-shutter.