It's 120 format, it's approx 6cm. It's for a medium format camera and it's the tits mate. I got myself a mamiya rb67 and I'm telling you when correctly focussed you can see the arm hair on a fly... You can also get a more shallow depth of field.
Yeah that mash tun looks great. I'm struggling on a 5us gallon atm... Gonna have to dig under the car seat for some gold coins haha