Friends and acquaintances of mine know that my Canon 550D is an extension of me. It’s never more than three feet away from me. I take it from room to room in the house, even to the bathroom. It’s in the passenger seat on my way to class to teach chess in the afternoons, and when I have to go to the classroom and the car is more than three feet away from that classroom, I simply put my trusty camera in the bottom of the cart that I carry my chess sets in.
Last week in one of my classes, I had some new students show up to class, giving me a total of 16 students. I have only seven company-supplied chess sets, but I have my 1994 Kasparov computer chess game at the bottom of my cart. I held the two best students back and told one of them to get the computer chess game from my cart. He did, but when he came back, his eyes were all aglow about the camera he saw in my cart, and his mind and mouth were running circles around me asking questions about cameras and photography.
The question that seemed most on his mind was about depth of field. “What is depth of field?” he asked several times while he was playing chess. I guess he didn’t like my answer each time: “It’s how much in focus everything is in your picture.”
I have that class again this afternoon, but this time I’m taking two pictures with me. These two:
See the difference? That’s depth of field, or how much of the picture is in focus.
Depth of field is controlled by the f/stop, usually with a control marked Av on digital cameras. The f/stop is the size of the opening, the aperture, that lets in light. I won’t get into the technical aspects of light, but the smaller the aperture, the sharper, more in focus, the picture will be. A larger aperture makes the picture out of focus.
Where it often gets confusing for people is that a smaller aperture is indicated by a larger f/stop Av setting, and a larger aperture is indicated by a smaller f/stop Av setting.
The first picture was taken at an f/stop Av setting of 2.8. The smaller number means a larger aperture, and more out of focus throughout the picture.
The second picture was taken at an f/stop Av setting of 18. A larger number means a smaller aperture, and more in focus throughout the picture.
The explanation using cans with text on them is a technique that I learned back in 1966 in sixth grade. Surprisingly to me, I have never seen the technique repeated in books or internet tutorials discussing depth of field. So my post here will bring that technique into the modern age.
I should also thank Miss Gustafson, the principal of my grade school. She is the one who got me interested in photography when she asked for a volunteer photographer. When she explained that the volunteer photographer would get in free to all sporting events, well, I was an easy sell.
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