Fuji X-T1 ISO dial.
For many years film speed, and more recently the amplification level of of a digital sensor, has been described by the ISO number. I have always pronounced ISO as “eye-ess-oh” and this is how I have generally heard others pronounce it.
Recently, I noticed some people using the one-word pronunciation “eye-so”. One such person is Tony Northrup, who states in his book Stunning Digital Photography that this is the correct pronunciation, and explains why in this video.
ISO is the abbreviation for the International Organization for Standardization (so it is not an acronym, which would have to be “IOS”). This abbreviation was chosen because the translation of the title into different languages leads to different abbreviations, so one was declared official, with “ISO” justified on the ISO website as being “derived from the Greek isos, meaning equal”.
This choice of abbreviation has led to confusion. Looking at photography books on my shelves, I find that The Complete Photography Course (by M. Joseph and D. Saunders, Weidenfeld and Nicolson, 1993), Digital Food Photography (L. Manna, Thomson, 2005), and The Art of Black and White Photography (J. Garrett, Amphoto, 1990) all state that ISO is an abbreviation for the “International Standards Organization”. This is what you would guess if you reverse engineer the abbreviation, but it is not correct.
But this is besides the point. There is no reason to pronounce ISO letter by letter. It is a pronounceable word, just like GIFF, NATO, and UNESCO. So “eye-so” it is.
Nick Higham 
Many of us are old school, and used the ASA rating long before it was replaced by ISO. We pronounced the individual letters in ASA and did the same with ISO when it appeared. Hard to break old habits. Just as some people feel that the trend of just typing “RIP” instead of taking the extra few seconds to type “Rest in Peace” isn’t as respectful. Certainly the “RIP” usage has been around for centuries, but it was mainly due to peasants not being able to afford to have the complete phrase engraved on headstones, and saved money by abbreviating it. There was a real reason for it back then, but not so much these days when the extra few letters come free on a keyboard.
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The ISO Story
International Organization for Standardization
https://www.iso.org/the-iso-story.html
The Founding of ISO
‘The first question that had to be settled in London was that of the name of the new organization. There were different proposals. The English and the Americans wanted “International Standards Coordinating Association”, but we fought against the word “coordinating”. It was too limited. In the end ISO was chosen. I think it is good; it is short. I recently read that the name ISO was chosen because “iso” is a Greek term meaning “equal”. There was no mention of that in London!’
Willy Kuert
Swiss delegate to the London Conference, 14 to 26 October, 1946
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Prior to 2013, I was an engineer at Aptina Imaging, a company that manufactured CMOS image sensors. I was a junior member of the Technical Leadership Program and as an additional duty, I wrote a column for our monthly newsletter composed of interesting relevant bits I could find by exploring the web. The idea was inspired by Robert Silverberg’s column in Asimov’s Science Fiction magazine. At some point I had become confused about what people were talking about when they mentioned ISO. On one hand, there is the well known reference to film speed. But some people talked about a set of standards that this mysterious international organization created. I ended up writing a column to disambiguate the term and talked a bit about the various ISO standards ranging from information security to food safety, to concrete curing times. I’m reminded of the wonderful humorous discussion between a grocer and a buyer in one of the main color science references about the standard for vegetable coloration in CIE xyz color space!
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