Tuesday, February 10, 2009

Depth Of Field

This is a quick shot that I took the other night while I was preparing dinner. The cutting board was not setup, it's just how it happened to look when I was done cutting the green onions. All I did was setup a couple of flashes to light it. Not going to hang it on my wall, but not a bad shot either. I think this image is a great example of DOF control. What I mean by DOF or Depth Of Field, is the area in focus. In this example there is a very shallow DOF because the cutting board is the only thing in focus. Everything behind it is a nice soft blurred mesh of colors and shapes. This was the intention, it didn't happen by accident, or by luck. Today I'm going to explain how to achieve this DOF control, and examples of when to use it.

The first thing to say about DOF is that it is controlled by the aperture. The wider the aperture (the lower the number), the more precise your focus will be. Meaning if you have an aperture of say f/1.8 the area that you are focusing on will be in focus, but the foreground and the background will be out of focus. Your lens doesn't go to f/1.8, not to worry. You might have a lens that only goes down to f/3.5, this is ok. You will still be able to get the same effect, the background just won't be as blurred as it would be with a wider aperture. Remember from an earlier post, I said that the minimum aperture obtainable is dictated by the lens, not the camera. I can get the same wide open f/1.4 with my 50mm lens on a D40 as I can on a D3. It's not your camera it's your lens.

Now how to control the DOF. In the example above, I shot the scene with a 17-50mm 2.8 made by Tamron. I my aperture was set at f/2.8 and I was zoomed all of the way out. I selected single point auto focus because I wanted to be very specific on where I was focusing. It worked out pretty well. The knife is in focus, but the tea pot and stove in the background are out of focus. If I had gone the other direction, say to f/22, the entire scene from front to back would have been in focus. For example, I took this photo last year at Zoo Blooms here in Cincinnati. I wanted it to be sharp from front to rear of the photo, so I set my aperture to f/22.


As you can see it is in focus from front to back, and side to side. DOF is also important when doing portraits of people. Most times you want to blur the background into a nice soft swirl of colors, but you don't want to go too wide with the aperture because if you do, places on the face and body will start to get blurred. Most times the "sweet spot" on a lens is around f/8. Any wider of an aperture and the body starts to loose focus, and smaller of an aperture, and the background will begin to come into focus. These are just some rough settings for the aperture. Of course like everything else, you need to play around and see what works out best for you. As always if you have any trouble or questions, feel free to email me. Jason

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