1/4" 20t or 3/8" 16t

[2F/2F]

If I had to change the entire head on a tripod to change between 1/4 and 3/8, I would look for a different tripod. Unless every camera one owned used the same size tripod screw, what a pain in the butt it would be having the coupling screw be a permanent part of the tripod head. I have only seen this on studio stands, but not professional-quality tripods. You should be able to change by simply swapping quick release plates, or by swapping screws on a system that does not use quick releases. Some Bogen/Manfrotto quick release plates come with both sizes, either of which can be inserted into the quick release plate, and you can stow the one you are not using on the tripod itself. They screw right into the threaded holes near the top of the legs (the ones designed for screwing stuff into).

FWIW, I use 3/8 whenever it is an option, but the optional 1/4 screw (and a step-down adapter to 1/4, pilfered from my SINAR) are always screwed into my tripod so that any camera is usable within a minute or two. I stow any step down adapters I have removed from cameras in a drawer, labeled, so I know right where they are and what camera they match.

 

[djhopscotch]

Survey equipment use a larger bolt size due to the plummet lens that looks directly down from the center of the equipment being used. That way we can center the equipment over the point being occupied. Survey equipment is not tightened down as much as camera equipment, it is merely tight enough to prevent movement in wind or light hand touch that the devices require.

 

[Vaughn]

Survey equipment use a larger bolt size due to the plummet lens that looks directly down from the center of the equipment ...

We didn't have any of that fancy stuff back when I was in forestry engineering classes almost 40 years ago! Our assignments involved (stadia?) surveys in the community forest (90 to 100 year-old second growth Redwood, Douglas fir, Grand fir, Sitka spruce and Western hemlock) along with finding old section corners out in the middle of the green stuff. I enjoy it, though it was the yahooism of my fellow forestry members, chainsaws and axes in hand, that helped me to decide to eventually change majors (to Natural Resources Mgt). That and the fact that there were not many girls (as in one or two) who were forestry majors at that time.

Vaughn