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ISO setting on canon camera
ISO Setting on Canon Camera

You have probably have heard the term ISO in regards to photography. Even your phone has an ISO setting that is set to auto by default. More than likely, you have never touched it but did you ever wonder what ISO is? What does it stand for, and why should you care?

What the ISO acronym means.

First, ISO stands for International Standards Organization, and it has to do with the sensitivity of digital sensors to light. The standard is fairly simple. When the ISO doubles, the sensitivity of that chip to light doubles. We’ll talk more about that later. For now, a little history.

The history of film speed.

The ISO standard was adopted in 1974. Previously, two world standards were in use. The US was using ASA (American Standards Association) for the sensitivity of a film to light. Also referred to as speed. Its algorithm was the same as ISO. When a film’s ASA doubled, it’s sensitivity to light doubled. An ASA of 400 is today’s ISO of 400. Europe before 1974 was using a system called DIN (Deutsches Institut fur Normung), and this was not as straightforward. With DIN, when the sensitivity of a film doubled, the DIN number would increase by three. A film rated at 15 DIN was twice as sensitive to light as a film with a DIN of 12. Yeah, I know, weird.

What ISO has to do with your photos.

So what does ISO have to do with my photos? For most of you using your phones for family photos, you don’t even have to think about it. The phone is set to auto ISO by default. Most phones and all cameras allow you to manually set the ISO, but why would you want to? Why not just let the camera take care of the tech.

This image, copied from a B&W negative taken in 1973 looks good at this magnification.

Well, there is a reason. The higher the ISO, the more pixelated the image will become. Conversely, the lower the ISO setting, the smother, less pixelated the image is. If you want to make an enlargement to hang on the wall, an image taken at a lower ISO will make for a smoother, sharper print.

A close up of that image shows the grain (or pixilation) of the image.

The phone’s image is so small that you will most likely not even notice the pixelated image. Enlarge it on your laptop, even a little, and you see how degraded that photo is. Try and make an 8×10 print, and you may not be happy with the outcome.

Film speed back in the days of film

In the days of film photography, an ISO of 400 was considered a fast film, a high sensitivity to light. When I was shooting concerts and working on the newspaper at COS back in the 70s, I shot hundreds of rolls of Kodak Tri-X. Tri-X was a B&W film with an ISO of 400. If that was too slow for the light available, I would push process, (increase the developing time in the darkroom) to an ISO equivalent 1200.

If I needed to shot color, my options were even more limited. The fastest color available was a slide film called Anscochrome, having an ISO of 500. It was grainy (pixelated), and the color was not very good. Today, the highest ISO speed available on high-end digital camera goes all the way up to 102,400. Even with my phone, I can set an ISO of 10,500. A setting unheard of back in the film era. In fact, we would have laughed our heads off if someone would have said they were shooting at that ISO back then.

As the years progressed, film tech got better, and ISO 400 color film became common. By the early 90s, I was shooting Fuji NPH 400 ISO film for my wedding photography. It was a beautiful film for wedding and portrait photography.

It’s not always about “fast” ISO speeds.

In the days of film, you would also have very slow speed films. ISOs of 25 or 32 were not uncommon. They had what was termed a tight grain structure. Thus having the ability to make big enlargements without that graininess showing up, like in the second photo on this page.

So there you have it. ISO is the basic building block of the image that is captured by your camera. ISO controls how sensitive your camera’s sensor is to light and how pixelated your images will be.