Friday, February 25, 2011

Experiment: Aperture and Depth of Field


The following is a super simple demonstration of how aperture works and how to create depth of field in your photos. If you want a little more detailed information I’d recommend reading Aperture – Everything you wanted to know but thought it was uncool to ask” by idigitalphoto.com.

This was originally posted to the Start to Photograph Meetup group where experienced members were welcomed to add their knowledge in the comments.

For this experiment you will need:

  • a digital camera that has an aperture priority setting (either AV or A) If you are unsure about where to find the aperture priority setting, refer to the manual that came with your camera.
  • a standard 50 mm lens (works best, but any lens you have will do)
  • a bright, consistant light source
  • 3 objects

    First, I’ll cover some basic terminology and then get into some easy step-by-step experiments that you can do anywhere.



    What is Aperture?

    Aperture is the lens shutter that opens and closes allowing a certain amount of light to reach the sensor of a digital camera or, in the case of an analog camera, the film.

    The amount that an aperture opens is the aperture size and is measured in “f-stops.” F-stops can range in size from f/1.4 to f/36. A lens with an extremely low f-stop number (like 1.4) is considered a fast lens. There is a technical mathematical reason for this. Unfortunately, because I am numerically challenged I choose to remember that the lower the f-stop number the faster the shutter speed I’m able to use. Most cameras will default to the lowest f-stop when set to Program or Automatic.


    How does Aperture work?

    Camera apertures and eyes work similarly. It’s all about letting in light. In bright light pupils are smaller to allow in only the enough amount of light needed to see everything. A photographer will use the smallest aperture on their camera to get everything they can in focus.

    In darkness pupils grow large to let in as much light as possible. Pupils also grow larger with excitement, allowing the brain to selectively focus. Ever noticed a cat’s crazy eyes when she’s about to pounce on a target? A photographer can use a wide aperture to selectively focus on their subject, as well.




    What is Depth of Field?

    When aperture settings are used to selectively focus on a subject in such a way that the area surrounding the main subject is out of focus, this is called creating "depth of field." The lower the aperture used, the more area is out of focus, the greater the depth of field.


    Experiments

    The following experiments will illustrate what happens when you change aperture f-stops.

    First, let’s set up:

    1. Set your camera to Aperture priority – AV or A. The camera should automatically select the proper ISO and shutter speeds.
    2. Set up your three objects so that they are about five inches away from each other in a row, one behind the other, but all still visible through your camera’s viewfinder. (Distance apart will depend also on the size of the objects. Larger objects should be placed further away from each other for this experiment to work.)

    Experiment 1

    Question: What happens when you focus on a foreground object and change the f-stop settings?

    1. Focus on the first object.
    2. Set your aperture to the lowest f-stop available for your lens.
    3. Snap the photo then look at what you shot.
    • What do you see? What happened to the other objects? What shutter speed did your camera choose?
      1. Reset your aperture to f 5.
      2. Snap the photo then look at what you shot.
      • What do you see? What happened to the other objects? Did the shutter speed change?
      1. Reset your aperture to the highest aperture setting on your camera.
      2. Snap the photo then look at what you shot.
      • What do you see? What happened to the other objects? Did the shutter speed and ISO change?

      Experiment 2

      Question: What happens when you focus on a middle object and change the f-stop settings?

      1. Focus on the second object.
      2. Repeat steps from Experiment 1.

      Experiment 3

      Question: What happens when focused on a far object and change the f-stop settings?

      1. Focus on the third object.
      2. Repeat steps from Experiment 1.




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