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Almost everything about cameras, lenses, sensor and film sizes are specified in metric units. So why are common tripod mounts (e.g., 1/4"-20) not metric?

I'm just curious if there's an interesting bit of history to explain this.

  • In WWII Britain & the US were still on variants of 'Imperiial... ermm... not metric, USC, USF, BSW... Wikipedia can fill the depth of my 27" screen with 'current' non-metric standards - en.wikipedia.org/wiki/Screw_thread#Other_current_standards – Tetsujin Jun 24 at 19:02
  • Interestingly [or not ;) cameras, light stands & in the audio field, mic stands seem to have settled on .. well.. a pair of standards... – Tetsujin Jun 24 at 19:03
  • The auto industry still uses imperial, the world over. 18" rims are just that, 18", not anything metric. I used to own a [fancy, shhh] BMW 7-series with metric wheels, done specifically so you could only buy very specific tyres for it. – Tetsujin Jun 24 at 19:05
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    @Tetsujin: Is that still true? I was under the impression that German and Japanese cars are almost completely metric (rather than SAE). – Adrian McCarthy Jun 24 at 20:33
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    At least on Japanese cars sold in the U.S., the lug nuts are metric (17mm, 19mm, 21mm, etc.), but the rim diameter is inches (13", 14", 15", 16", etc.). Even the tire sizes are metric in terms of width, but the rim size they fit is in inches (e.g. 185/65R14 means a tread width of 185mm, a sidewall to tread width ratio of 0.65, and a rim size of 14 inches. – Michael C Jun 24 at 21:29
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First of all, tripods are things you really want to be compatible across cameras & between tripods: no-one cares if the screws which hold your Leica together are compatible with the screws that hold your Nikon together, because you're not about to screw bits from your Leica onto your Nikon (actually: camera repair people kind of do care as it means they have to keep stocks of fewer different sorts of things). But no-one wants to have to buy a new tripod each time they buy a new camera, other than camera makers, and very few camera makers have ever been dominant enough to drive that kind of awfulness onto people. (It's perhaps significant that in many cases they have been able to do this with lens mounts and still do to a great extent, although even here consortia have appeared (μ4/3 for instance). But lenses are things that camera makers can argue really make a difference, while a tripod is, well it turns out they matter too but they're a lot harder to sell as being somehow special). And not all lens mounts really can be compatible: SLR mounts & can't really have flange distances as short as mounts for cameras without mirrors, and if you try and use SLR mounts for mirrorless cameras you get weird things like the recent Sigmas.)

Secondly tripods are expensive & last a long time: unless you damage it you can use a tripod for many, many years.

The result of this is that, once a tripod screw thread standard exists, there's a really strong incentive for it to persist for a very very long time.

Tripod mounts in fact use a standard called Whitworth which was standardised in 1841, in Britain. For the purposes of mounting a tripod it's close enough to UNC which was standardised over a century later (the thread angle is slightly different, but the diameter & threads/inch are the same).

And this, I think, answers the question. In the early days of photography in the late 19th century if you wanted to sell your tripod the biggest market in the world, by a large margin, was the British empire (in 1870 the British empire was about 25% of world GDP, while the US was about 9: the US caught up sometime during the 1914-18 war), and they used Whitworth (and a number of other standards, but Whitworth was pretty dominant for most purposes). And most tripods would have also been made in the British empire. So if you were outside the British empire, then you made your cameras and tripods with Whitworth threads, and this has stuck ever since.

[Apologies if this reads like some kind of misty-eyed wasn't-the-empire-great romanticism: it's not meant that way.]

  • Thank you. I'd been unable to find anything definitive about when tripods were standardized beyond a footnote about 1901. – Adrian McCarthy Jun 25 at 16:44
  • I think it would be interesting to know when the threaded mount was standardised. A bunch of early field cameras came with dedicated tripods which did not use a screw mount and which may have been idiosyncratic to the camera. The oldest camera I can easily lay my hands on is about 1913-1917 and has a 1/4in tripod screw (I just checked it will sit on my tripod, which can have both 1/4 and 3/8!). – tfb Jun 25 at 17:04
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[Consider this re-framing: What would be gained by having a metric tripod mount?]

The core of the reason why metric tripod mounts are uncommon is a combination of:

  • Legacy Standards
  • Readily available parts/tooling
  • Lack of a good reason to change

The early part of the 1900's saw a wide range of tooling standards established early on before metric tooling became popular/common. This established a long lasting snowball effect in some industries like camera gear.

Once a large portion of gear on the market was already using the 3/8th and 1/4-20 standard, it became far less practical to introduce new gear that used metric threading.

Little to no new near that used metric threading meant that there was little to no economic reason to make a change.

  • Who is going to want to buy a tripod that needed an adapter plate to use with their cameras?
  • Who is going to want to buy a camera that doesn't fit on their existing tripod?

Screws for the camera assembly had a far lower barrier to entry into metric - It doesn't matter what standard you settle on, or even if every screw was machined to a different standard in the camera body. If no one is expected to replace the screw themselves or have it screw into something else outside the camera, then it really didn't matter what you used.

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    This explains why metric sizes are not used today...not why imperial was chosen in the first place. – Hueco Jun 24 at 18:17
  • I honestly don't know enough about screw threads [who really does] to know if the current choice is actually Imperial, or whether it's a US standard. That Japan [camera makers] would adopt a US standard [to please one market at the expense of others] is actually a reasonably well-known meme. Japan had a very very US-centric business model after WWII. [way off topic for here]. – Tetsujin Jun 24 at 19:15
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    ermmm... "before metric tooling became popular/common" Where? It was certainly standard everywhere except UK & US by then. – Tetsujin Jun 24 at 19:16
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    That said, the standard does have a set size in mm too [ISO 1222:2010]. If you can get into the ISO standard to confirm the specifics even better photokonnexion.com/photography-thread-size – AthomSfere Jun 24 at 19:33
  • @Tetsujin the mass of industry from the US/UK during the period translated into European tooling being a rather mixed bag of standards for items like screws even into the 1st quarter of the 1900's. Even if you built a product to Metric standards, you often used US or UK screws [or bought machines to make your screws from them]. The 3/8 and 1/4-20 standards hit critical mass first, and there has never been a good reason to change away from it. – TheLuckless Jun 24 at 19:46

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