Saturday, January 21, 2012

Challenges with depth of field

Warning: this post is probably only interesting to people who like photography....

In photography, depth of field (DOF) is simply the distance between the nearest and farthest objects in focus. It is controlled by the distance to the subject, the lens' focal length, the camera's sensor size, and the aperture (f-stop). 

DOF can be a great tool for creating better photos.  In this one I took in the summer, I wanted the flower and the bee to be sharp, but I didn't want all the distraction of what was behind it. By using f5.6 and my 105mm macro lens, I was able to make the background appear as some vague greenery.  You know what it is, but there isn't any detail to grab your attention.
This week, I got a lesson in how bird photography can have some real issues with depth of field.  I took this photo at f8 and 1/640th of a second.  My zoom was set at 270mm. If you don't know, your camera stores all this information in the digital picture, so it's always easy to go back later and see how it was shot.

The problem is trying to get all the birds in focus.
This little guy was pretty sharp.  He's the one that is second closest to the top right corner.  Even zoomed in tightly, you can see
This blurry little guy was near the top left. Notice how you can't make out any of the feather detail.
I have a little iPhone app called Simple DOF.  When I stick in an estimated distance of 75 feet, focal length of 270mm, and f8, I get a depth of field of 7.5 feet, with focus from 71.4 to 79 feet.  That's not much.  If I was zoomed in to my max of 400mm, the DOF would have only been 3.4 feet. While not as convenient as a phone app, you can find a nice quick DOF calculator here.


To get all the birds in sharp focus, I have several choices:
  1. Zoom out. By zooming out to a focal length of 100mm, the DOF changes to 63 feet!  That's great, but I don't want a picture of the marsh, I want a picture of the birds!
  2. Set the aperture higher, like f16.  This would double the DOF to 15 feet.  However, closing the aperture also reduces the amount of light being let in.  Instead of shooting at 1/640th, I would have to use 1/160th of a second.  This is a problem because I was lazy and shooting without a tripod.  A long lens and that slow a shutter can cause blur, even with all the fancy vibration reduction techology.
  3. I could also increase the aperture to f16 and bump the ISO from 400 to 1600 to compensate for the loss of light.  However, on my D300s, ISO 1600 starts to look grainy.  Perhaps on my next camera?
So the real answer, and the one I need to grind into my photography brain, is to:
  • Check my app to make sure I have the right DOF before I start shooting.  At some point, I will know it by heart, but not yet.
  • Set the aperture to get the DOF I need
  • Get the damn tripod out of the truck and use it like I am supposed to.
This is often described as one of the most common photographer failings: either not using a tripod or using a crappy one that isn't stable.  I believed in this enough to buy a very good one and slowly but surely, I am using it more.  This is just an easy visual lesson.

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