Focal lengths: cm vs. mm vs. in?

[Michel Hardy-Vallée]

Is there any reason why some older lenses have their focal length in centimeters rather than in millimeters? For example, my pre-war Contax mount Triotar is 8,5cm, but everything else I have is in millimeters. And does anybody know at what point inches were not used anymore for focal lengths?

Just feeling curious today...

 

[Nick Zentena]

Inches imperial countries. UK/US. Cm/mm metric countries. I don't know when US companies stopped using inches. Did they ever?

http://www.cameraeccentric.com/html/info/goerz/p1.html

1967 still showing inches.

 

[AgX]

An interesting question. Actually some time ago I raised the same question to myself… Did not get an answer, and the situation has not changed.

The only, very vague, idea would be that by time a general trend towards more precision has evolved in society. The people were surrounded by more industrially manufactured goods with smaller tolerances. And that trend had its effect on the photo optical industry too, though they were actually applying the same high precision as before.

 

[Ole]

The focal length of (many) German lenses went from mm to cm around 1905, then back to mm after WWII. With lots of variations, of course...

 

[Richard Kelham]

I wonder whether the change to (or back to) millimetres has anything to do with the fact that the centimetre is not an officially recognised ISO unit of measurement?

I seem to remember Nikon were still using cms in the early 1960s.



Richard

 

[Monophoto]

An interesting question. Actually some time ago I raised the same question to myself… Did not get an answer, and the situation has not changed.

I frequently don't get answers to questions that I ask myself.

My sense is that the US manufacturers tried to stay with inch measurements as long as they were commercially viable. But by the early 1950s it was obvious that their commercial relevance was fading rapidly, being replaced initially by the Germans and later by the Japanese. And by the early 1960's, the dominance of European manufacturers meant that for the sake of industry standardization, lens specifications were almost universally metric.

As to cm versus mm - that's a good question. I have a suspicion (shear speculation) that it was ascendance of the 35mm format that drove that change. In the smaller format, the difference between 35mm and 50mm and 100mm was a big deal, and while those differences could be expressed in cm, mm was more convenient. And by the early 1960s, the world had adopted the 35mm format and both LF and 6x6 were left to specialists. Therefore, with the center of gravity now on 35mm equipment with mm specifications, everyone else had to conform.

Frankly, it has always struck me as the ultimate in lunacy that the US claimed to the the world's leading economic power, and yet we shared the inch-pound system of measurements with such leading economic powerhouses as Trinidad. I would speculate that when the next "History of the World" is written 100 years from now, with China identified as the most powerful nation on earth, the reason for the decline of the US will be attributed to (among other things) our stupid insistence on that archaic and arcane system of measurements.

 

[Nick Zentena]

How much of the drop off with US lens companies had to do with companies like Kodak outsourcing lenses to cheap labour countries?

 

[AgX]

Monophoto, I thought of that convenience issue too. Tried it in several languages (though not in French) dit not `hear´any difference between the point-version (decimal version) and the other. I must admit I am mumbling…
And as Ole pointed out that there was even a mm period before the cm one, this whole thing seems quite weird.

I won’t comment on that Imperial measurement system. I never really understood it. And those parts of inches which I still often come across, really irritate me.

 

[Pinholemaster]

'cause

 

[Sirius Glass]

I won’t comment on that Imperial measurement system. I never really understood it. And those parts of inches which I still often come across, really irritate me.

Try the wet ounces versus the dry ounces, the peck, bushel, drams, fortnights and farthings!

Steve

 

[bdial]

...I won’t comment on that Imperial measurement system. I never really understood it. .

At least you don't have to live with it!

There's also furlongs/fortnight...

 

[AgX]

Yes, we Continental Europeans have been lucky people...

But what about that mm/cm issue? Will the answer ever be unveiled? Is there an answer anyway?

 

[Curt]

The distance between two points is whatever you want to call it, the distance is still exactly the same.

Come on, any mathematicians out there?

 

[Steve Smith]

In the UK , both the metric and imperial system are in use at the same time. People of my age group (35-45) had the metric system imposed on us at school but our elders were still using imperial.

I work as a design engineer and anything with precision is drawn in millimetres. When I do some woodwork or building work for myself though, I use feet and inches.

The UK's road signs are all measured in miles (and yards) and last week the European Union reversed its ruling on selling produce in imperial weights. We are now allowed to display the price per pound, ounce, etc. along with the metric equivalent. It still has to be worked out by the metric figure though but a pound of potatoes is the same weight and price in whatever system you choose to weigh it.


Steve.

 

[David H. Bebbington]

What I like best are the hybrids (for no apparent reason), such as tire sizes: For example, 195/65 15 is a tire 195 mm wide, aspect ratio 65%, wheel size 15 inches. Even when Britain had a real indigenous motor industry, with cars on which every nut and bolt was Imperial, engine sizes were always in cc, never cubic inches.

I think part of the reason that the metric system is unpopular with some people in Britain is that we never changed to metric products, we just converted the Imperial sizes, for example we did not give up 4x5" and use 9x12 cm, instead we called 4x5" 10.2 x 12.7 cm, which makes the metric units seem stupid and clumsy. It gets even better at the DIY market, where you find bags of nuts and bolts labelled "6 mm / 1/4" ", which makes me feel ill - I am not aware of any thread pitch which is both Imperial and metric.

 

[Ole]

... I am not aware of any thread pitch which is both Imperial and metric.

Try the threads on Compound shutters: A Compound #3 has a cell thread of 55.69mm x 40 t.p.i, a flange thread of 60mm x 29 1/13 t.p.i.

Those are not results of some kind of strange conversion either; those are Compur-Werke's original specifications!

 

[Steve Smith]

I am not aware of any thread pitch which is both Imperial and metric.

The British Association (BA) thread is usually assumed to be an Imperial system (except for those who know better!) as it dates to around 1890 but it is actually metric and based on threads which were commonly used in Europe at that time.

A 0BA thread is the same as an M6 thread in diameter and pitch. (I don't know if the angles of cut are the same though).


Steve.

 

[Steve Roberts]

In the UK , both the metric and imperial system are in use at the same time. People of my age group (35-45) had the metric system imposed on us at school but our elders were still using imperial.
Steve.

Being a little (but not much!) older than the age bracket above, I started my school life with imperial measurements and ended it with 50/50 (or should that be half and half?) imperial/metric. Never got to grips with metrificated stuff, though, and whilst I can estimate reasonably a gallon of oil or a stone of spuds, ask me what a litre or a kilogramme is and I'd have to covert mentally to imperial to be anywhere near.

However, in these days of digital everything the imperial system has a lot going for it. The inch can be divided repeatedly by two in a binary manner until an increment is reached that gives the desired accuracy, ie it is possible to measure to a quarter of an inch, an eighth, sixteenth, thirty second, sixty fourth, etc. until the required degree of precision is reached. Anyone keen to go metric can, of course, quite legitimately, do so by dividing the inch by ten a couple of times and working in "thous". Having said all that, if I'm measuring something that turns out to be nine and seventeen sixty fourths inches, but by flipping the ruler over it turns into 55 metric doodads, then I'll swallow my pride and go with the doodads for convenience!

Rumour had it a couple of years ago that because of the big jump in size between a centimetre and a metre, there were plans to introduce a "metric foot", equal to 30cm. Didn't hear any more about that, though!

Yours nerdily,

Steve

 

[Steve Smith]

Rumour had it a couple of years ago that because of the big jump in size between a centimetre and a metre, there were plans to introduce a "metric foot", equal to 30cm. Didn't hear any more about that, though!

Wood used to be sold in 'metric foot' lengths. If you go to a timber yard now, you can buy timber in multiples of 0.3 metre lengths which is basically the same thing.
I will still ask for 2.4 metres of planed 2" x 2" instead of 50mm x 50mm though.

My favourite measurements are the type you can occasionally find in a house being built or repaired. Sometimes you can find a mark on the wall with something like "two metres, three and half inches" written next to it!

I don't know about the rest of Europe or the US but the centimetre is not used much here other than in schools. Building plans are always in millimetres except for large, outside dimensions which are in metres.
The ISO standard states that the unit used should be chosen such that the measurements shown on a drawing will be within the range of 0 to 1000 of that unit. Therefore, the centimetre would be the ideal unit to be used on building plans but it is never seen - Perhaps it seems to be 'not professional enough'.

Steve.

 

[Struan Gray]

A variant on the metric system that used centimeters, grams and seconds as its base units was popular for a while, especially among astronomers and Electrical Engineering types. It was originally proposed as a standard by Gauss, which might explain its popularity in German-speaking nations. Wiki has an article on it here:

http://en.wikipedia.org/wiki/Cgs_system

The dates where it was most widespread and popular don't really fit with those for 'cm' markings on lenses. So I suspect the main reason for using 'cm' was lack of space for engraving on the small lenses common on hand cameras for amatuers in the first part of the C20th.

 

[Ole]

I doubt that theory too...

While "5cm" is shorter than "50mm", what about "10,5cm" vs. "105mm"?

 

[Sirius Glass]

A variant on the metric system that used centimeters, grams and seconds as its base units was popular for a while, especially among astronomers and Electrical Engineering types. It was originally proposed as a standard by Gauss, which might explain its popularity in German-speaking nations. Wiki has an article on it here:

http://en.wikipedia.org/wiki/Cgs_system

The dates where it was most widespread and popular don't really fit with those for 'cm' markings on lenses. So I suspect the main reason for using 'cm' was lack of space for engraving on the small lenses common on hand cameras for amatuers in the first part of the C20th.

You are referring to the cgs [centimenter-gram-second] measures as opposed to the mks [meter-kilogram-second] measures. Electrical engineers use cgs units.

Steve

 

[Tom Hoskinson]

Angstroms are one of my favorites

 

[AgX]

However, in these days of digital everything the imperial system has a lot going for it.

I forgot to say, I'm not into digital either.

Ooops, this unwilling computer of mine read that too...

 

[Struan Gray]

I doubt that theory too...

So do I. I just wanted to have an excuse to think about dyns and ergs.