My first question about photography was What does ISO mean?
When I was 18, my father gave me my first camera, a Canon-A1. It was his old one, but it was the best 35mm film camera you could buy in the 70s. I still use it if I have time to photograph outside my work hours.
I became obsessed with all the inner-workings and history of photography almost as soon as I touched it.
This camera showed ASA as a photographic film classification, and it sparked my curiosity.
No doubt, from using your camera or smartphone application, you have come across something similar. Either ASA, ISO or even DIN, depending on your cameras’ (and your own) origins.
The name stands for International Organization for Standardization. I know this doesn’t offer much help. We need to look back at photographic history a little bit to understand Where it came from, What it means, and What it does?
Where Did ISO Come From?
As soon as photographic emulsion came around, each area or country tried to create classifications. People needed the classification to know what they could do with the film.
There were many different groups of people who tried to come up with a method that classified their film in the best way. This is the early process of trying to work out what we now call film speed.
Hurter and Driffield were one of these people. They were working on emulsion (silver halides dispersed in gelatin) sensitivity since 1890. Their system had an inverse system of classification and exposure. The higher the number, the lower the exposure the emulsion needed.
This was used until 1928. Changes in light sources and chemicals they developed the glass plates with led to a need for a different system.
From 1928, the Soviet Union used this as their standard until 1951, when they replaced it with GOST. The GOST system was similar to ASA in classification and was overpowered by ISO in 1987.
During this period, DIN (Deutsches Institut für Normung) was introduced in 1934. This system was based on an earlier version called Scheiner. The name came from a former German astronomer who tried to photograph the sky at night.
The Scheiner system used degrees to classify their emulsion. A 3° change would either double or halve the sensitivity. He based his findings on the least exposure to produce a visible darkening upon development.
After the 80s, emulsion and film ratings were shown as ISO 100/21° for a while.
ASA (American Standards Association) came around in 1943. Kodak and General Electric’s own film classification deeply influence this. They created a linear scale, where 200 ASA is twice as fast as a film with 100 ASA.
After some revisions, ASA changed its name to ANSI in 1988 and withdrew its name from film classifications. They then also adopted the ISO standard.
Since 1974, ISO has combined DIN and ASA. This system uses both an arithmetic and a logarithmic scale. Its arithmetic scale corresponds to the arithmetic ASA system, where a doubling of film sensitivity is represented by a doubling of the numerical film speed value.
ISO 400 is double 200, and 800 is double 400.
To show the equivalents of each of the three most important film speed classifications, see the table below:
ASA – DIN – ISO
50 – 18 – 50/18°
100 – 21 – 100/21°
200 – 24 – 200/24°
400 – 27 – 400/27°
800 – 30 – 800/30°
1600 – 33 – 1600/33°
We all now use ISO for our photographic film. It is also what we work with in digital photography. But other systems are still out there.
While travelling across the Balkans, I even came across film that still showed GOST (or ГОСТ in Cyrillic). It didn’t work, as the film was over 30 years old.
Some cameras I still use today still show DIN ratings, such as the Smena-8M and Smena-Symbol.
What Does It Mean?
So, as we saw, ISO is just a way to classify your film speed and how sensitive to light the film is. They are usually in the range of 50, 100, 200, 400, 800, 1600 and 3200. There are other films that are outside this range, but they are more specialised.
These increments seem a little strange, but what you will notice is that the numbers either double or halve. This is because moving from ISO 100 to ISO 200 halves the sensitivity of the film. Moving from ISO 200 to ISO 100 doubles it.
It is linked directly to the quality of the film. When photographic film was used, the ISO rating of the film indicated how big the silver halides were in the emulsion.
Photographic emulsion is what we covered (first glass plates) a roll of plastic with to make it sensitive to light. The emulsion contained silver, which is what ‘recorded’ the light information.
Film with a low ISO number held very small silver halides. Films rated as ISO 3200, for example, had large pieces of silver. These helped capture more light in low light conditions.
The bigger the pieces of silver, the least amount of light you’d, but the halides were visible. This is where we get film grain or noise.
Effectively, the silver pieces were twice as big in ISO 200 than in ISO 100, but it could capture twice as much light.
In the digital world, the same numbers continue to be used. The only differences are that we use digital sensors, not photographic film. And ISOs on DSLRs tend to go way above ISO 3200.
There are also added increments in the above range. 1/3 stops would be found between ISO 400 and 800, for example.
What Does ISO Do?
The ISO of your photograph depends on many factors. It comes down to how much light is available, and what you want to capture.
We tend to try and shoot with a low ISO as that gives us the best quality and a smaller amount of noise/grain.
What you are capturing determines the value you should use. If you’re photographing outside in the middle of summer, you should use ISO 100 due to the abundance of light. But if you’re photographing inside, this value might jump to ISO 800, or even higher.
One of the best ways to explain the use of ISO in photography is by using the photographic exposure triangle. This triangle helps you to capture the perfect exposure.
Using the three interchangeable areas that control light, you can photograph anything, anywhere.
For example, photographing a scene with ISO 400 at f/16 and a shutter speed of 1/400 and then wanting to create a smaller depth of field. You would first lower your aperture to f/8 (two stops).
To keep the same correct exposure, you would have to decrease either the ISO 2-stops or increase the shutter speed by 2-stops. Decreasing its value from 400 to 100 would give you better quality.
If you choose Av (Aperture Priority) or Tv (Shutter Priority) shooting modes, the ISO will change automatically. To get a more in-depth look at how ISO can be used, have a look at our article here.
ISO has come a long way, through the world of analogue and digital photography, but the idea stays the same. It is one value that focuses on how much light it lets your sensor or film capture but also focuses on the quality of your image.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
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