It looks like it was the EF 50mm f/1.2L used to film in the video if you go with what On a Hazy Morning posted on their blog. It was fun to dream wasn’t it?
…we’ve tried the 50mm f/1.2 lens on the EOS M3 camera while filming and we were so impressed with the results; super crisp quality with a beautiful and dreamy depth of field. We like that!
In the above promotional video for the Canon EOS M3, we see what appears to be an EOS M3 shooting a lens that is set at f/1.2. During the entire promotional video, there’s no real mention of shooting with the EF to EF-M adaptor. Are we going to see a fast aperture EF-M lens coming soon?
You can see the f/1.2 setting at 2:21 of the video.
The video is worth a watch, as it shows a lot of the improvements in the EOS M3.