Aperture is a mechanism inside every lens that opens and closes just like your iris in your eye to let more or less light through.  Aperture is measured in f-stops and unfortunately a smaller # is more light and a larger # is less light.  If you think of it as a fraction it is easier to remember 1/2 is more than 1/16.


A few thing will happen when you close your aperture:

  • Less light will hit the sensor – if you want to maintain the exposure you have you must have a longer shutter speed or a higher ISO
  • Depth of field will increase (more of the scene will be in focus)

When you open your aperture:

  • More light hits the sensor – if you want to maintain the exposure you have you must have a shorter shutter speed or a lower ISO
  • Depth of field will decrease (less of the scene will be in focus – background will blur)

Controlling your depth of field is the fun part


Three things affect Depth of Field


Focal length

Subject distance

Lets take a look at each of those individually

Here we have a subject and a camera that don’t move and all we change is the aperture.  The gray portion is what would be considered to be in focus


You can see at f/1.8 just the woman would be in focus and the man standing slightly behind her would not be in focus.

At f/5.6 both subjects are in focus.

At f/11 both subjects and most of what is behind them would be in focus.

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Next we are going to leave our aperture at f/2.8 and change the focal length.  We will also have to move the camera farther away to keep the subject the same size in the frame.


At 16mm even though our aperture is wide open the background is relatively in focus

At 50mm we are starting to get the nice blurry background that is great for portraiture

At 200mm the background is so out of focus that we can no longer tell what is behind our subject

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Last we will be using the same focal length and aperture; changing only the subject distance.


When the subject is close to us our depth of field is very shallow

As the subject moves farther away more of the background comes into focus.

At the farthest distance even though we are at a wide open aperture the whole scene is pretty sharp.

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Now that you understand the three things that will affect your depth of field get out there, experiment and have some fun with it 🙂

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All images and text by Melinda Walsh


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