I suspect this is because of the way it is being taught. You see, ISO started out as a property of film, and it was much easier to visualise it in terms of the old technology. So that's where I want to start my explanation, before bringing you into the 21st century with ISO today.

ISO actually started out as ASA, which stands for American Standards Association. Decades ago, a commercial film manufacturer came up with a set of numbers to define the sensitivity of different types of film. That set of numbers was accepted by the American Standards Association, so all American manufacturers could use the same system. Later, the American standard was adopted by the International Standards Organization, so ASA became ISO.

What does all that mean? Well, it means that the letters ISO didn't really stand for anything except for the name of an organization.

What is important is what ISO referred to, which was the sensitivity of the film. The emulsion on some films reacted quite slowly to light, and on other films much faster. Slower films had a smaller ISO number, like 25, 64, 100. Faster films had a higher number, like 200, 400, 800.

A slow film needed a relatively high level of light to create a well-exposed photo. That meant that to take a photo in darker conditions, you would need to use a fairly wide aperture and/or a fairly slow shutter speed to get a result. On the other hand, a faster film reacted to light a lot more quickly, so it needed much less exposure to light to take a photo.

Fast film sounds pretty good, doesn't it? A chance to take a photo in any conditions without a tripod, and to freeze moving subjects with very fast shutter speeds. So why didn't everyone just use fast films all the time?

The answer is that the advantages of fast films came with a trade-off; loss of image quality. The grains of emulsion on a fast film were larger, so a photo taken on a film with ISO 400 or 800 had a rougher, 'grainier' look. This may not have been a problem in a small print, but became quite apparent with big enlargements. Consequently, most professional photographers preferred to use slower films of 100 or 64 ISO for most of their work.

So is this just a lesson in ancient history? After all, you have a digital camera, so what does all this have to do with you. Well, it may surprise you to know that despite the huge revolution in technology, the essentials of ISO have not changed one bit.

Your camera should allow you the option of adjusting your ISO setting. Just like in the days of film, if you set your ISO to a low number like 100, you will need more light to create a correct exposure. That means that you may need to keep a tripod handy for cloudy days, and in certain low-light situations you may not always get the aperture and shutter speed settings you want. If you set your ISO to 400 or 800, your camera will become much more sensitive to light; you will be able to shoot in exactly the same conditions without a tripod, and with greater flexibility to choose the aperture and shutter speeds you want.

But here is the amazing part. Higher ISO settings still come with the same trade-off that once existed with film. Along with the speedier sensitivity to light, you can also expect the image to have a grainier finish. I don't know if it is pixellation, or digital noise, or a combination of both, but it is generally understood that for all their advantages, high ISO photos come with a reduction of image quality that becomes more obvious the more you enlarge the image.

So there you have a quick introduction to what ISO is all about. Perhaps I am just showing my age, but I find this subject easier to explain in old-technology terms. For many people it is easier to visualise when related to something solid like film, rather than something that happens on a computer chip. Anyway, I hope this helps you if you have had trouble understanding what ISO is all about.