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.