Focal lengths: cm vs. mm vs. in?

copake_ham

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Of course, hybrid systems can also lead to disasterous consequences such as when one of NASA's Mars explorers was programmed in feet and inches and then some engineer made a course correction in meters.

Oops - another pile of space junk got strewn across the Martian surface!
 

John Koehrer

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I sort of agree with david Bebbington re: the change to metric in the UK.
We did pretty much the same asinine thing here in the states.

Anyway, rather than dividing an inch by two by two etc. & involve different denominators what's so hard about using mm? Don't have to change values unless you moving by relatively gross amounts do you? 1M=100CM=1000mm.

Need finer than that and you're at .5mm=.020"=a change from fraction to decimal if you want to use anything but a yardstick.
 

Ole

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In the oil industry we use an atrocious mix of units.

In some cases, pipe diameter is measured in inches, and length in feet. But instead of "29ft 2 1/4in", it would be measured as 29.19ft!
 

Richard Kelham

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Just to further muddy the waters I could point out that 19th century ship's registers here in UK used a metric foot to measure ships: thus an old sailing brig could be 81.04 feet long with a beam of 24.62 feet. The inch was "decimalised" (ie divided into tenths rather than multiples of 2) in the 1890s.

But then the Germans can still go to the market and buy ein Pfund Äpfeln...





Richard
 

Ed Sukach

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It may be of interest to note that the united States *IS* officially "on the metric system".

In 1888 (not a typo - the year Eighteen Hundred and Eighty-Eight) the National Bureau of Standards adopted the Meter as the "Standard Unit of Length" - specifically, the distance between two lines inscribed on a platinum-iridium bar known formally as the "International Prototype Meter, Nr. 9" ... and at a "standard temperature" of 20C - 68F.

Michaelson, in the year following (~ sometime around there) counted the number of "lines" in the spectrum of a mercury vapor lamp contained between those two inscribed lines, using an intereferometer of his design - that task, in itself, took nearly one year.

Since the 1888 adoption of the "Standard Meter", the United States has, officially, been a Metric Country. Practical acceptance of MKS is another thing ... we, as a nation, still have not done that.
 

bdial

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I sort of agree with david Bebbington re: the change to metric in the UK.
We did pretty much the same asinine thing here in the states.
Click to expand...

At least it was done in a more or less planned way. What has happened here is that suppliers and manufacturers are using metric because of practical necessity, but either simply referring to measurements in inches, or not referring to it at all. For example, if you go to a lumber yard or home center much of the plywood is in metric thickness, and 3/4 inch plywood isn't that, but 20 MM. And most domestic cars, as I understand it, use metric fasteners.
Speaking for myself, I wish we'd get over the reluctance and just go metric overtly. I suppose there will always be remnants though, like 15 inch wheels on our cars.

Barry
 

Petzi

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NASA should know better than to use feet and inches for anything. It is unscientific. Like the ancient Romans, who never achieved anything in math due to their irregular numbering system. I mean, nobody uses a kilofoot or a milliinch for anything, right? So how can you use these units in a scientific context? How about a yard? Or a furlong? You have to standardize to one unit for distance.

Boeing have adopted the metric system for their recent aircraft AFAIK.
 

AgX

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But there are still relicts of inches over here in Germany: you still find it at installation pipes and their fittings and with some hoses. If you go to a DIY/garden store you will find those standardized hoses for watering only in ½, ¾ inch sizes. Most probably the customers won’t know about inch or even the German equivalent `Zoll´, but just refer to `size ½´.

But we still have the `Zollstock´, that folding scale. However metric...
 

David H. Bebbington

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Example of metric versus imperial chaos: Reid and Sigrist, a firm of aircraft instrument makers in Leicester, UK were asked by the British Government to produce a camera based on the Leica IIIb patents and machine drawings seized by the Allies at the end of the Second WW.

The camera was announced at the British Industries Fair of 1947 but was not actually on sale until 1951. The reason for this was believed to be that R&S insisted on converting every metric dimension to imperial units, which threw out numerous tolerances, particularly those relating to the shutter. It took four years to even begin to fix this problem, the Reid camera always had shutter reliability problems (and was also very expensive) and faded away to nothing as soon as real Leicas were available (which was not until the mid-1950s in Britain).
 
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Kilgallb

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In Canada, officially metric, I travel the 3 km to the home depot at 60 km/hr to buy 4 foot x 8 foot sheets of 8mm plywood. The new home depot is on a range road (Rural one mile grid road).
 

Steve Smith

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UK sheets of plywood (and plasterboard/drywall) are 1220x2440mm but everyone refers to them as 4'x8'

Steve.
 
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Michel Hardy-Vallée

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I still haven't found an answer from the switch to mm, except maybe the following, speculative ones.

Between subminitature formats and ULF, most focal lengths fall below 1000mm, and just a few go above. For 90% of your lenses, you need only two or three digits to express their focal length. That makes very clean tables to list available products, much cleaner than inches+fractions, and cleaner than cm, because with cm certain numbers will have digits after the comma, while others won't. It's harder to align properly on a table, because you have to align the last digit of whole cm, not simply the last digit.

For most camera lenses, advertised focal lengths are not expressed with a precision greater than the mm. In reality, I'm sure that a lens advertised as a 50mm can be a 50.5mm, but for all practical purposes, it does not make a difference to the average photographer. On top of this, the last thing you'd want is a customer trying to decide between a 50.5mm and a 50.7mm one.

So if you standardize focal lengths to be rounded mm, you get: enough precision to make a sale (get the 40mm instead of the 35mm!), but not too much that it would impact choice (50.5 or 50.7 ?).

Plus, at that level of number precision in that unit, you will never have a decimal point/comma. That's one less character to have in your font to engrave numbers on the barrel, and you do not risk alignment problems (commas are always a little below the line of type).

In sum, I would go for the clearer, cleaner, and simpler, as motives to eventually settle down on mm.
 

Kino

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I have always assumed that it grew out of the experiences of the US and the UK and the rest of Europe during the Lend Lease programs during WWII.

Poke around a bit on Google and somewhere is a webpage that delves into exhausting detail the amazing headaches everyone had to deal with when US manufactured machinery poured into Britan and the USSR.

That and the overwhelming flood of Japanese and German cameras into the USA in the 1950's.

Of course, its all speculation on my part... That and 5 bucks will get you a cup of coffee at Starbucks...
 

AgX

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Well, that this comma issue is not related to speaking as we already discussed but could be to making lists is interesting.

To those who have (had) a hard time with strange measurement units, here in Europe until few decennia ago (I did not encounter it) national time zones were off those 15° longitudes, which resulted when crossing borders in strange differences in time e.g. 19m 32,13s ...
 

dslater

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Richard Kelham said:
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
Click to expand...

The centimeter is indeed an officially recognized unit - However, it is not a base unit in the SI (The International System of Units) unit system - it is a derived unit as is the millimeter. And what you ask is a "base" unit? - A base unit is defined in terms of some physical constant - for example a meter is defined as the distance that makes the speed of light in a vacuum exactly 299,792,458 meters per second. Derived units are defined in terms of base units - For example a centimeter is 0.01 meters.

There is no ISO unit system - ISO is the organization that defines many standards - among them the SI unit system.

Dan
 
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Michel Hardy-Vallée

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Thanks for pointing that out, I was also wondering whether a mm would be a more "base" unit than the meter, but I see it's not the case. So I'll stick for now with the idea of convenience in communication as the driving principle behind the standardization on mm.
 

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"The metric system is the tool of the devil! My car gets forty rods to the hogshead and that's the way I likes it." - Grandpa Simpson
 

benjiboy

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Twelve inches to a foot,12 pence to a shilling, 240 pence to a pound ( before we went metric) counting in dozens, and using twelve as a unit of measurement I was taught at school came from the Viking invasion of Britain. I have sometimes wondered if you buy eggs France, Germany or Italy, do you get them in tens or twelve s ?
 
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David H. Bebbington

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In Germany at least, eggs come in 10s (and the plastic egg-rack inserts for fridge doors do, too). Very widespread use of "Pfund" (metric pound, 500g).
 

Ole

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But in Norway, you get them in twelves. Or rather in sixes, but two and two packs are linked together so you can get the full dozen.
 

Struan Gray

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Twelves are Babylonian. Vikings count in twenties. In modern Danish fifty is 'half threes', meaning half way between two twenties and three twenties (or, equivalently, half a twenty less than three twenties). I blame the mead.

Humans seem to have a deep-seated need to work in whole numbers. In my work I used to use Ångströms the whole time, because I was working with atomic dimensions. Now that I regularly deal with objects which are tens or hundreds of Ångströms in size, I have switched to nanometers. Don't get me started on the chemists and their cm^-1s.
 

AgX

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Well, in my true German fridge I've got an insert for 7 eggs...
And here common are packages of 6 or 2x6...
 

benjiboy

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David H. Bebbington said:
In Germany at least, eggs come in 10s (and the plastic egg-rack inserts for fridge doors do, too). Very widespread use of "Pfund" (metric pound, 500g).
Click to expand...
Thanks David ,I was just curious I have been to France and Germany, but I stayed in hotels, and never needed to buy any, anyway I've never been very keen on them since I found out where they come from
 
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Michel Hardy-Vallée

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In Québec and Canada, eggs come by the dozen, but everywhere in France I saw, eggs were sold in large square crates of 36? or more.

Québec is another weird case of metric/imperial mixups. At the stores, the prices must be labelled with $/kg, but there is also a $/lbs price in smaller type. People compare their height using feet/inches, but measure land distances in kM.
 
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