I suspect he may be trying to use an old Sigma lens designed for 35mm Canons. The link between camera and lens is all-electronic in Canon, and Sigma didn't have a license to use Canon's technology, so developed lenses that worked by analyzing the signals and power etc. between camera lenses and bodies. Which was fine until digital came along and it turned out that they used a different signal protocol that was built into the chips etc. in Canon lenses but singularly lacking in Sigma designs... You get Canon's Error Code 99, which basically means "Not invented here" - it used to be possible to get Sigma to upgrade them, these days it's a DIY job needing soldering, programming and electronic skills, and I for one wouldn't want to touch it, even though I often have nice lenses that sell for peanuts because of the incompatibility.
We'll see what happens.
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