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You hear photographers talk all the time about depth of field. What they're talking about is simply how much of the photograph is in focus in front of, and behind their subject. This is called the zone of acceptable focus. Because of the physics of lenses, this zone extends forward a short distance from your point of focus and twice that distance behind. But in reality, it's not just a point of focus, but rather a plane of focus that is parallel with your camera's sensor.
If you're in focus in one point on this plane, you will be in focus throughout the entire plane. Depth of field is controlled by the aperture, lens, and the distance to the subject. When just a little bit is sharp, it's called a shallow depth of field, or selective Focus. When everything is sharp, it's called a great depth of field. Whether you have a shallow or great depth of field will depend on what you're shooting and the message of your photograph.
When I'm shooting landscapes I normally choose to have a great depth of field. I want everything to be sharp from the foreground to the middle ground to the background. This allows the viewer to enjoy the fine details in the landscape as well as seeing the broader composition of the scene. In this photograph I set my aperture to f/16. I didn't focus at infinity. I focused in the foreground. Remember that the zone of acceptable focus will extend forward and twice as far back from the point of focus.
I used a wide-angle lens. Wide angles increase the appearance of sharpness in photographs. But when I shoot portraits, a wide angle and great depth of field sometimes gives me too much information, and that distracts from the point of the photograph: the person. Instead, I want to emphasize the person and isolate him or her from the background. I do this by using a telephoto lens with a wide aperture, like 2.8 or 4, and I'll move in as close as the lens will focus, like in this shot of my daughter.
My focal length was 160, my aperture f/4. You can see that her eye and part of the tattoo is tack sharp, but then the focus falls off so that by the time you get to her shoulder, it's just a nice line in the composition. I did this shot for a magazine assignment about Apollo butterflies. I used a macro lens at f/8, but I am so close to the butterfly that my depth of field is about a 16th of an inch. I made sure the head of the butterfly was sharp by checking it with the Depth of Field Preview button that most SLRs have.
This, by the way, is a handy tool when you're working with shallow depth of field. In this one I needed to show the tiny transmitter antenna that scientists put on the butterfly so they could track it in the wild. I wanted everything in focus and it is. How come? The aperture is the same as the meadow shot. The distance is pretty close, too. The only difference is the camera sensor and the butterfly are now parallel.
Depth of field is all about controlling what the viewer sees in your photographs. It's the third dimension in composition, not up or down, left or right, but rather near and far. In the next movie we will demonstrate how depth of field can change the feeling of a photo and answer the age old question: what the heck is bokeh?
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