Two threads under almost every tripod

What the standard defines

ISO 1222 defines the tripod connection used in photography: two threads, one of 1/4 inch with 20 threads per inch and one of 3/8 inch with 16 threads per inch, together with the depth of engagement. They are the two screws found under camera plates, binocular adapters, star trackers, phone clamps and the heads of light tripods.

The smaller thread is what small accessories carry; the larger one carries heads and plates that have to hold real weight. Many tripods provide both through a reversible stud or a supplied bushing.

The standard fixes a thread and a depth. That is genuinely useful, because it means an adapter always exists — which is not true of any other fitting on this site.

What it does not promise

A matching thread does not promise a tripod will hold the load. The thread is the same under a photographic tripod that damps for four seconds and a steel one that damps in one, and nothing in the designation distinguishes them.

It does not promise a stable interface, either. A single central screw allows rotation under load unless something else locates the head — which is why telescope mounts use a flat top with a pin or a bolt circle rather than relying on the thread alone.

And it says nothing about the head above it. A ball head rated for a camera held close in daylight behaves very differently with a tracker and a lens cantilevered out for four minutes.

What it works with

The tripod thread is where astronomy meets the photographic world, and it is the reason a binocular, a small tracker or a phone clamp can be put on equipment that was never made for the sky.

On this site it appears on telescope tripods, star trackers, binocular and monocular adapters, spotting scopes, lighter alt-azimuth heads and the phone adapters that sometimes carry their own thread.