joeswe
Well-known
I do own Lens serial # 2025XX in the Canon 50 f1.8, and have measured this filter thread to be 40.5. A 40mm filter will go on with resistance as we say in machine shop practice. After I measured; I went out and purchased the Fuji X10 hood, with a 40.5mm thread which flies on smoothly with no resistance. This is a great vented hood replacement for the original S42 hood.
All I can say is that my Canon 50mm f1.8 (vers. II black/chrome, don't have the serial number at hand right now) has a nominal 40mm thread. I have several filters for it, some vintage Canon ones (very thin) and some modern aftermarket (Marumi etc.) and they are labeled "40mm" and all fit smoothly without binding. I have no experience with the Fuji X10 accessories.
I realize this is an older thread and rfaspen has probably sorted out his questions or problems by now, but to anyone who is looking into the same issue, my humble suggestion would be to get the original lens hood (S-42), it shouldn't be too difficult or expensive to find on Ebay with a bit of patience. It works perfectly and as it is a clamp on type, it is also less fiddly then any threaded solution if you are working with filters at the same time.