ISO Speed was, in the “olden days”, the sensitivity to light of the roll of film you loaded in the camera. Each roll of film had an ISO Speed. The higher the speed, the more sensitive to light the roll of film was. A low speed of 100 was great for bright daylight. 200 was a good general purpose speed for most uses. As you went up in the ISO Speed range, 400 was the typical black and white speed for general use. A Speed of 800 was used for low light conditions.
Notice the speed went up by doubling the increments. This was by design. A roll of 200 was twice as sensitive to light as a roll of 100. A roll of 400 was twice as sensitive to light as a roll of 200; and so on.
While having a roll of high ISO speed film may be great, there were some downsides and trade-offs. One downside was that you were stuck with the whole roll being at one speed. If you had a roll of 100 film in your camera to take pictures of a bright sunny day, and then that day drew into evening, you were stick with a low sensitive film and that made taking pictures in low light virtually impossible.
The Down Side
A downside was that the higher the speed was, the grainier the pictures were, too. A high-speed film could show significant grain, or little specks of color, degrading the image. So, you wanted to use the lowest possible ISO speed film to do the job.
ISO Speed Today
What, you may be asking yourself is the relevance of this ancient history to today’s world of digital photography?
Today, it works exactly the same way, except with some remarkable improvements. Your camera still uses ISO speed, except that rather than applying it to a whole roll of film, it is applied to the camera’s sensor. And it is applied to each picture individually, rather than a whole roll of film. But, the concept is exactly the same.
Think of it as turning up the volume on the stereo. To make the sensor more sensitive to light, the computer in the camera simply cranks up the volume.
The Up Side
Today’s cameras also have a much broader range of ISO speeds. Most can go up 6400 and even higher! This can give you the ability to take one picture in broad daylight at ISO 100, and the next at night at 6400. A beautiful thing! However, there is still the trade-off of image quality, although not quite so pronounced as in the “old days”. Today, rather than image artifact, or grain, it’s “digital noise”, or pixelated remnants on the image. Think of it as if you were turning your stereo up as loud as you can, as it gets louder and louder, the quality of sound is distorted and not as clear. This works the same way.
Set it and Forget it
You may be asking yourself: “So, what speed do I use and how do I know when to change it?” That’s easy. I tell my students, especially in the beginning when they are just learning about photography and how to use their camera, to just put it on Auto. Set it and forget it. Let the camera’s computer figure it out for you. That’s the great safety net of ISO speed today. The camera will always try to use the lowest possible ISO speed to match the shutter speed and aperture selected. So, if you want to take a very low-light picture, just be aware that it may have some “noise” in it. Otherwise, you play with the aperture to get the depth of field you want, and the shutter speed to get the motion effect you want -and let than camera decide what ISO speed to use to match.
Later, as you become more proficient with your camera, you may want to manually set the ISO speed, but for now you have enough on your mind just figuring out shutter speed and aperture. That’s why I say the ISO speed settings is the great safety net, the unsung hero of digital photography!
As I mentioned, ISO speed is the 3rd point in the Exposure Triangle. Shutter Speed (Part 1 & Part 2) and Aperture (Part 1 & Part 2) are the other two. I’ll discuss just how the three work together in a future post, but you already have the pieces of the puzzle if you’ve read my posts on the other two topics.