ISO Speed. Let’s Throw In The “Third Leg” of Exposure

When I discuss exposure, I talk alot about shutter speed and aperture and how they are inversely related.  (As you choose faster shutter speed numbers, the aperture numbers  go down.)  More speed=less depth at each setting.

So far, so good. But if we want to totally understand exposure, we need to  stop hobbling around on two legs and throw in the third leg.

Wait a minute… we only have two legs, and we don’t hobble around on them unless we just finished the Nastar course at Steamboat and our knees feel like jelly.

Let’s stay on topic here. To refresh our frozen brain…there are three “legs” to achieving a correct exposure. Shutter speed, aperture and the ISO speed.

Note that ISO does not refer to “I’m So Over” this lesson and I’m headed back to the slopes, unless you are using a point and shoot camera, in which case get back on that Nastar course and come back when you decide to upgrade to a camera with manual settings.

The ISO setting controls how sensitive your camera’s sensor is to the light that reaches it. A typical digital camera will have ISO values of 100, 200, 400, 800, and 1600 as a minimum. This means that if you have your shutter speed and aperture combination set and you still don’t have enough light to take the photo, you can switch to a higher ISO setting to make images in low light without a flash or a tripod.

However, remember to set your ISO back to 100, because the rule is to keep your ISO setting at 100 and only increase it if necessary for a proper exposure. That’s because each time you choose a higher ISO setting, you increase the digital noise in your images. Noise is the digital equivalent of film grain and it shows up as tiny colored dots which are particularly noticeable in the darker areas of your photo.

The noise reduces the detail of a photo by making the image appear grainy and uneven. So just remember:

Lower ISO number = Lower sensitivity = Finer quality photos

It’s a stretch, but we can look at this fun picture of a giraffe to illustrate the point. I took a picture of this picture at both ISO 100 and then ISO 800. Here it is at ISO 100.


Next I enlarged the eye section of both pictures (the one taken at ISO 100 and the one taken at ISO 800).  I have purposely enlarged them past the point where there isn’t much sharpness left, but we can clearly see more detail in the ISO 100 crop.


 ISO 100


ISO 800

If you need to review the three legs of exposure, now would be the time. You can find information about the first two (shutter speed and aperture) in my previous blogs, or you can go whole hog, buy my latest book and find out everything about all three. The blogs are free, the book isn’t, but if you buy the book you will further my goal to become a full time ski bum, which is an oxymoron because skiing is now so expensive that there is no such thing as a ski bum. But hey…I can always dream….

Good shooting!