I am new to photography. I bought very recently a starter level Canon Rebel SL3 (200D) that came with a kit lens (Canon zoom lens EF-S 18-55mm 1:4-5.6 IS STM). A friend gave me a Sigma zoom 70-210mm 1:4.0-5.6 UC-II lens he had since 2001 when he had a film SLR Canon camera EOS 3000V with EF lens.
Will this Sigma lens be compatible with my new SL3? If yes, any suggestions as to which adaptor I should get?
Probably not the way you want it to. The lens has an EF mount. Your camera accepts EF mount lenses. So the lens will mount to your camera just fine with no adapter needed. But it probably won't work the way it should.
Older third party lenses, such as your Sigma 70-210 f/4.0-5.6, are reverse engineered to work with whatever camera bodies are already on the market when they are being designed. When the camera makers, in this case Canon, release newer bodies they sometimes use provisions in their lens-to-camera communication protocols that weren't used in earlier models. The reverse engineered third party lenses can be less than 100% compatible with the newer bodies. Sometimes they'll work under specific scenarios, but not under others.
For instance, I have a Sigma 70-300mm f/4-5.6 lens that I bought in the late 1990s for use with a Canon EOS film camera. The lens works fine with that camera, and any other Canon EOS bodies that old.
When I try to use the same lens on my Canon digital bodies, even older ones like the EOS 50D that came out in 2008 or the Rebel XTi/400D from 2006, it has problems. If any aperture other than the brightest setting (lowest f-number) is selected, when I try to take a photo it completely locks up the camera. I have to turn the camera off, remove and reinsert the battery, and then turn the camera back on to unlock it.
If I have the aperture set to f/4 at 70mm in manual exposure mode then I can use the lens to actually take a photo. Of course as the lens is zoomed to longer focal lengths the maximum aperture (lowest f-number) increases. By the time it is zoomed to 300mm the largest aperture is f/5.6. If the aperture was set to f/4 at 70mm and I zoom to 300mm it will take a photo with the aperture at f/5.6. So far, so good. If the aperture is set to f/5.6 at 300mm the camera will take photos without locking up. But if I then zoom out to 70mm and the aperture is still set to f/5.6 it will lock the camera up when I fully press the shutter button without first setting the aperture back to f/4.
Newer third party lenses from Sigma and Tamron have provisions for owners to update the lens' firmware themselves after downloading it. If an issue with one of those lenses crops up with a newer camera model, Sigma or Tamron will usually (but not always) release a firmware update that allows their lens to be fully compatible with the newer camera body.
For older lenses like yours, though, firmware updates required a trip to the lens maker's service center. After a certain period of time they usually would no longer continue updating older lenses. It's far too late in 2021 to get a Sigma lens from around 2000 updated to the latest firmware for that lens model that was probably written somewhere around 2005-2010 which would allow it to work with Canon cameras made by 2005-2010.
Put the lens on the camera and try. If it doesn’t seem to fit look at the parts closely and try again without forcing things.
If they fit together, try using the combination and see what happens.
Then you will know if your particular camera works with your particular lens.
