User Activity

Recently got my Extender EF 2x III in the mail and loving what I can do with it. Out of morbid curiosity, is it possible to stack the extender with another 2x or the 1.4x for added multiplication of range? 
Been really wanting to pick up some alternative eyecups and adter careful inspection of my camera, I don't see any means of removing the eyecup from the viewfinder. Is it possible to remove/replace the eyecup from the M50 Mark II?
I have a M50 Mark II with the EF-M - EF-S adapter, so I'm fairly sure I can use the extender with my camera. My question is, is any EF/S lens compatible with the extender itself?The lens in question is the Canon EF 75-300mm f/4-5.6 III