The number 40, well 40cm to be precise. I haven't really tried my ring flash out much since I finished building it, so I have had a bit of a play today. I decided to take some shots in a pitch black room and see how things come out. This was taken on manual settings on the camera (1/250sec and f9) the flash was on auto TTL hand held. No glare or bad bright spots, sharp detail on the subject, a decent amount of colour in the B/G so quite pleased so far, but I must use it on a human subject to see its full potential!!!!