An Introduction to ISO, The Foundation Of Exposure

When you practice photography as an art, you will begin to feel the need to take more control of what the camera is doing. Once you feel confident enough to do this, a good place to start is with ISO. It is a rating of how sensitive your sensor (or film) is to light. Not only does it have a major impact on your other settings, but it also changes the quality of the photo itself. 

At the heart of it, ISO is the basis of your exposure, and understanding how to use it will set you up for taking full control of your camera.


Where ISO Came from

To fully understand what ISO is, and how it works, we need to look back at the history of photography. When photographers were first practicing their art, they had to physically prepare the plates they used on site. This required mixing chemicals in precise ratios, often in a tent or carriage. As you can imagine the delicate nature of this process left plenty of room for mistakes. Their exposure calculations were based on the mixture being exact, and they wouldn't know if it was right until they developed the slide.

As photography continued to develop, and mass production became a reality, the need for a standardized system arose. Each company used their own proprietary system, and their own development process, so settings would not carry across from one manufacturer to another. A number of groups introduced standards including ASA, American Standard Association, and DIN, a german establishment. 

Ultimately ISO, created by the International Organization for Standardization, became the universal guide. It is still the system we use to gauge the sensitivity of our digital sensors. Thanks to this, settings on one camera will give (nearly) the same results as another no matter who made them.


The Base Of Your Exposure

Often the three main settings of a camera (ISO, aperture, and shutter) are likened to a triangle. This makes sense because the three are interconnected, but it obfuscates the true nature of how they interact. 

I prefer to think of them as a pyramid, with ISO as the foundation. A proper exposure depends on all of the settings balancing each other out, just like every step depends on the one below it to create a proper pyramid.

If you get the chance to browse film stock you'll find that it's available in a variety of ISO, or speeds, ranging from 25 up to 3200. You'd also notice that as you move up the scale, each number is double the previous one. This coincides with the sensitivity of the film: ISO 200 is twice as sensitive as ISO 100, ISO 400 is twice ISO 200, and ISO 1600 is four times as sensitive as ISO 400. 

Back in the film days you couldn't change how sensitive your film was between each shot. Because ISO affects the rest of your settings (aperture and shutter), you had to pick the best speed for your application. If you were shooting landscapes on a tripod, you would pick a low ISO because you could afford long exposure times; on the flip side, if you where shooting street or event photography in low light you would pick a higher ISO film that allowed for short exposures so you could freeze a moment in time.

This relationship still stands today, but with the flexibility of digital technology we're no longer locked in to a single ISO for 36 frames. It's much easier for us to pick the right setting for any given situation.


ISO And Noise

If you think like me, you may be wondering "why not just use a higher ISO so you never need a tripod?"

The answer to that is two fold: 
1. You may not want to use the settings a higher ISO requires
2. Noise

As you crank up the ISO, you're also increasing the noise of the image. I don't want to go into the specifics because it's really technical, but the reason has to do with the signal to noise ratio of the sensor. If you are curious to learn more, you can read a more exact explanation here.

Common practice is to use the lowest possible setting to avoid noise, but you may find yourself needing to use a higher ISO to achieve the image you want. It will all depend on what you are shooting, whether or not you have a tripod, and the final output for your image. 

In smaller images, particularly those used online, noise is less visible. Not to mention that each generation of camera improves the noise of high ISOs, so eventually there will be little issue with shooting at ISO 25600 and beyond.


The First Part Of The Puzzle

Now that you are ready to master your tool, there are three main settings you'll need to learn. ISO is just the first, but it forms the foundation of your exposure pyramid. We will cover the other two in the coming weeks, but in the mean time: get out and take some test shots. Experiment with your cameras ISO settings, and find out the upper limit of your equipment.