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Diplomacy in Action: In Search Of ISO

"ISO" is a measurement of how sensitive a camera's recording medium is to light. Based on historical usage, it's still commonly referred to as "film speed." But why is it called "ISO"?

The place to start with to answer this is, of course, the letters "ISO" themselves. As might be apparent, they form an acronym. What might not be apparent is what that acronym stands for. Curiously, the "S" doesn't stand for "speed" or "sensitivity." Indeed, none of the letters have anything to do with cameras or photography at all. In a less then helpful manner, "ISO" stands simply for "International Organization for Standardization," the group responsible for the standard, but not in any way what the standard is for.

So if that's the organization's name, why is the acronym "ISO" rather than "IOS?" Turns out it results from a compromise decision by those involved. Being an international organization, its members don't all speak English as their first language. ISO has in fact three official languages, English, French and Russian. In French, the group is known as "Organisation Internationale de Normalisation" which would be "OIN" as an acronym. I'm not going to begin to try spelling the Russian name, but suffice it to say it wouldn't become "ISO" as an acronym either. Rather than favoring any one language in the official designation, they chose to use "ISO" as an abbreviation for the Greek word "isos," meaning "equal." That's diplomacy in action for you. Everybody is happy because nobody lost.

As you might expect, a group this diverse has interests beyond just film speed. Indeed, they have published standards for countless subject areas, and many of the more popular ones are also known simply as "ISO." For example, the most common CD and Blu-ray disc image file format is known simply as "ISO." It can be tempting to say that this format is known as "ISO" because the file format suffix is itself spelled as ".iso" but that's basically the same as saying that film speed sensitivity is known as ISO because the camera knob that controls it is labeled as that.

This tendency to name standards generically after the controlling standards body didn't begin with ISO either. Back in ye olden film days, sensitivity ratings were known as "ASA" here in the United States, with "ASA" standing simply for "American Standards Association." You may be more familiar these days with ASA under its new acronym ANSI, the American National Standards Institute. As with many things photographic, the ASA standard had its origin with Kodak who first published a standard for this back in 1943. The by now familiar progression of doubling numbers for increasingly sensitive media began here.

A similar country-sponsored standard for sensitivity was created in 1934 by Germany's Deutsches Institut für Normung, thus beating the United States in the standards race by nine years. As seems to be the way of these things, the standard is commonly known simply as "DIN." Although similar to ASA, the DIN standard differs in that it is logarithmic rather than arithmetic in its progression of numbers. By this I mean the scale increases by three (known as degrees) with each effective doubling of sensitivity. The numbers thus relate to the exponent of the powers of two, not the actual result of that calculation. ASA 100 corresponds to 21 degrees DIN. ASA 200 would be 24° DIN.

Lesser known country standards also got defined including the British Standards Institution (BSI). Other, even less well known sensitivity standards also got proposed over the years, mostly predating the country standards. The need to measure and label film speed pretty much goes back to the beginnings of film, as you can probably imagine.

One of the earliest ISO standards, known technically as ISO 6 (these things are numbered sequentially) combined the American ASA rating and the German DIN systems back in 1974. The ISO standard can be expressed as either an arithmetic progression (the same as ASA) or as logarithmic (as was the case with DIN). Again, diplomacy in action.

Mainly due to the worldwide reach of major camera makers who would no doubt prefer to keep things simple, the degree nomenclature is often omitted and the ASA values tend to be more common worldwide these days. If you see merely ISO and some number such as "ISO 100" you can assume the number refers to the ASA arithmetic scale. To unambiguously refer to the logarithmic DIN values it is necessary to use the degree symbol as in "ISO 21°."

The ISO standard has since been revised a few times and thus has several other associated ISO numbers including 5800, 2240 and 12232. Each of these gets revised periodically as well so to correctly refer to a specific version of a specific standard it is necessary to append the year revised to each of these numbers. For example, the current standard for measuring sensitivity of digital still cameras is technically known as ISO 12232:2006.

Perhaps it's best after all if we just stick with calling it ISO.

Date posted: January 26, 2014


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