How does focusing affect Depth of Field?

As we learn about f-stops and depth of field, we often make one major mistake; we assume that aperture is some kind of a constant. I can't tell you how many photographers I have met who think that f/11 will have similar depth of field with all lenses, at all distances, with all styles of cameras. Photograph for an afternoon at f/11 and your photos will prove the fallacy of this idea. 

There are charts showing depth of field with different lenses, cameras and distances; but, who wants to memorize? 

My recommendation is to use the "worry or not?" test
  1. Is your subject more than 30 feet away? Unless you are using a telephoto lens, depth of field is probably not much of a problem. Try using f/5.6 or f/8; switch to smaller aperture if needed.
  2. Is your subject closer than 10 feet away? Depth of field will be more of an issue.
  3. Is your subject 3 feet away or closer? You need to consider depth of field. See the example photos below!
  4. How close is your subject to the background and other objects in the image? If your subject is near the background, the background will probably be distinguishable. If there is a great distance between subject and background, you may need a high numbered aperture to make the background identifiable.
  5. Are you using a compact camera (point and shoot, 4:3 camera, cell phone, etc.) ? As sensor size decreases, depth of field at a given f-stop increases. Cell phones have amazing depth of field because of their tiny sensor size. Depth of field is going to more of an issue with DX sized and full (35mm) sensors
How does focusing affect Depth of Field?

These examples demonstrate several principles:
  • At close distances, depth of field is often a concern.
    The total depth of this image is less than 1 foot and yet
    a relatively small aperture such as f/11 is unable to bring
    the entire image into focus
  • Telephoto lenses, such as the 200mm used here, magnify
    the subject and make depth of field more of a concern at
    short distances.
  • In the example images, more of the 'gained' depth of field
    is coming behind the focusing plane than in front. Generally
    speaking, 2/3 of the additional depth of field comes behind
    plane of focus. The exception is macro images, where
    half the extra depth of field is in front of the plane of focus
    and half in back. 
All images taken from about 4 feet away
Canon 30D slr (APS-C sized sensor)
200mm lens at f/11

Click on any image to enlarge
Yellow dotted line indicates plane of focus
200mm lens f/11
focused about 1/4 of the way into the image

200mm lens f/11
focused about 1/2 of the way into the image 
200mm lens f/11
focused about 3/4 of the way into the image

200mm lens f/11
focused near the back edge of the image

Photo of the Week - September 27, 2014

Julie's Vases in the Sunlight
(where have the flowers gone?)

September 27, 2014

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