This is part one of the four part series, Understanding Exposure. This series of articles will explain everything a photographer needs to know about how the light gets into their camera and how their camera uses that light to record the scene.

Today we’re going to talk about ISO. In digital photography, ISO is often said to be the sensitivity of the camera’s image sensor. More accurately, it is a numerical representation of the gain applied to the signal reaching the sensor. When you raise the ISO on your camera, you’re applying more gain to the signal.

In everyday terms that a photographer should know, every time the ISO number is doubled, the amount of light needed to record the scene is reduced to 1/2.

So, in other words, an ISO of 200 requires half as much light to record the same scene if the ISO were set at 100. Carrying along on that line of thought, an ISO of 400 requires just a ¼ of the light. 800 ISO requires 1/8th the light. I could go on but I think you get the point.

Every time the number is doubled it is said that the “exposure increased by one stop”. So, when you go from ISO of 100 to ISO of 200, you’ve added 1-stop of exposure. When you go from ISO of 100 to an ISO of 400, you’ve added 2-stops of exposure. Conversely, if you reduce ISO from say, 400 to 200, you’ve reduced the exposure by 1-stop. From 400 to 100, you’ve reduced exposure by 2-stops.

Pop Quiz

If you increase the ISO on your camera from 100 to 3200, how many stops of exposure did you add? (Answer is at the end of the article)

You might be thinking to yourself, why don’t we just shoot with the highest ISO that is available on our camera? Well, increasing the ISO does allow you to shoot at faster shutter speeds and/or smaller apertures and/or in scenes with less light but using higher ISO’s comes with a catch — image noise increases as the ISO increases. The higher the ISO, the more noise the image will have. Additionally, depending on the camera, as ISO climbs, color and contrast may be affected in an adverse way.

Sometimes photographers want noise in their photograph. If they do, it’s usually for artistic purposes, but most of the time, we photographers want our images clean and as noise free as possible so we try to keep the camera’s ISO setting as low as possible.

Unfortunately we can’t always shoot at the lowest ISO’s. Even at the widest aperture, in very low light situations, we might find that we’re dealing with shutter speeds that are too slow for the subject we’re trying to photograph or the shutter speed is too slow to be handheld without encountering camera motion blur. We’re forced to make the decision to increase the ISO so that we can photograph the subject with a fast enough shutter speed to eliminate subject or camera movement that will ruin the image.

Later, when we discuss Aperture and Shutter Speed, you’ll learn that there will be situations where our shot will demand a specific Aperture or a specific Shutter Speed and in order to have a resultant image that is properly exposed, you may be forced to increase the ISO.

In the photograph of the deer, the sun had just risen and it was still very dark. I was hand holding my camera and wanted to get a good crisp shot. With an ISO of 100, my shutter speed would have been about 1/4 of a second long. There wasn’t anyway possible, at that shutter speed, to be able to hold the camera still enough to get the sharp picture I wanted. I boosted the ISO to 1000 and was able to shoot at 1/60th of a second which was more than fast enough to hold the camera steady and get a crisp shot.

As you can see in the photo, the tradeoff was that there is some noise in the shot — i.e., it’s a bit grainy.  Click on the image to make it bigger.


In the picture of the Owl Butterfly, I used a ring flash. The added light of the flash allowed me to use a very low ISO, 100 in this case, and the resultant image is noise free with excellent detail. Click on the image to make it bigger.

Auto ISO

Many cameras come with a feature called Auto ISO. With Auto ISO your camera will adjust the ISO automatically, to the scene. For example, when I was taking the picture of the deer, if I had Auto ISO enabled, my camera would have automatically increased the ISO until the resultant shutter speed was fast enough to allow handholding without camera shake. Most experienced photographers don’t use Auto ISO preferring to adjust their camera’s ISO on their own.

So remember, unless you want noise in the shot, use the lowest ISO you can for the scene you’re dealing with.

In Part Two of Understanding Exposure we will discuss Shutter Speed.

Answer to the Pop Quiz

When you increase the ISO on your camera from 100 to 3200, you’re adding 5-stops of exposure to you picture.