DOF for Newbies

Yesterday, while perusing photography blogs, I came across a post that described depth of field. And although it was well written, and explained the topic nicely – and although I already have a basic understanding of depth of field – it was still technical enough to make my eyes roll back in my head.

So, a picture being worth a thousand words, I’m going to visually explain the concept of depth of field for my non-technical friends:

Tiny forest

Macro with shallow DOF

Tiny Forest

Macro with wide DOF

The difference between the two (I hope it’s obvious!) is that there’s only a little bit of the picture in focus, which in effect shuts out a lot of noise if I’m trying to highlight a specific area. It’s not even just the front – a closer look shows that there’s a twig right up in the front left that gets blurred out.

The blog author identifies 3 variables in determining the depth of field:

  1. the aperture you’re using
  2. the focal length of the lens
  3. the size of the sensor in your camera

Taking these items individually: for #2, lens: yesterday, since I was mostly interested in taking macro shots, the focal length of my lens stayed the same for the day: 60mm.

For #3, sensor size: I only have one DSLR camera, with the smallest sensor that Canon makes: a 1.6x “crop” lens (for a nice basic discussion on the effect of the crop lens, see Ken Rockwell’s article.)

So, all that I had left to control depth of field was the aperture – and in fact, the aperture setting for the “blurry” shot was f/2.8, and the aperture for the “noisy” shot was f/20.  Truth be told, fairly often small aperture (a higher f number) is not an option for me because it requires more light.

There are various calculators (online and mobile) for calculating DOF. Using DOFMaster after yesterday’s pictures were loaded up, I calculated that the “blurry” picture above has about a 1 inch DOF, and the “noisy” one has about a 1 foot DOF. To do things right, of course, I’d do the calculations before taking the shots… so I have this nice Android app all ready for next time.



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