Hi there,
I had the same problem. Your 5DIII is not broken or different. You just have to go to the “first dot in the Orange menu section” The first option is to change the EXPOSURE LEVEL INCREMENTS. Change this from 1/2 to 1/3. Voila! You now have 1/50 shutter speed to work with. 🙂
Samuel
samuelrichards.com.au