Understanding Exposure – Part 1: ISO

by Anthony Morganti


This is part one of our 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 refers to the sensitivity of their camera’s image sensor. The higher the number, the more sensitive the sensor is.

Every time the number is doubled, the amount of light needed to record the scene is halved.

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.

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 but using higher ISO’s comes with a catch — image noise increases with higher ISO’s. The higher the ISO, the more noise the image will have.

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.

Unfortunately we can’t always shoot at the lowest ISO’s. Even at the widest aperture, in very low light situations, one might find that they’re dealing with shutter speeds that are too slow for the subject they’re trying to photograph or the shutter speed is too slow to be handheld steady enough without encountering camera blur. You are forced to make the decision to increase the ISO so that you can photograph the subject with a fast enough shutter speed thereby no camera shake will blur the image.

 White Tailed Deer

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.


Owl Butterfly

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, accordingly, 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 grain in the shot, use the lowest ISO you can for the scene you’re dealing with.


In Part Two of Understanding Exposure we’ll be discussing Shutter Speed.