Does it fit a canon L lens
Not sure I understand your question. This unit does not attach directly to ANY lens and has an inner diameter of 90mm.
"The R-300 Ring Light comes packaged with an L-bracket. The L-bracket has a 1/4"-20 thumb screw that allows you to mount your camera for mobile use with the R-300 system. The bracket also has a 5/8" female receptacle. For hand held shooting the 5/8" female receptacle can also accept a hand grip to allow for a mobile, compact shooting solution. The 5/8" receptacle also allows you to use the light off-camera and mount the Ring Light directly onto a light stand."