What's Depth of Field?
Notice in the above photograph, the grass at the bottom of the picture in front of the river otter is out of focus. The grass at the top of the picture behind the otter is also out of focus. Even the back of the otter is a little out of focus. My depth of field for this photograph was just wide enough to cover the otter's head and neck.
The depth of field of your photograph will change depending on three factors:
- F/Stop. The above photograph was shot at f/5.6. If I had chosen f/16, the depth of field would be much greater, and more of the frame would be in focus.
- Distance from subject. The greater the distance between your camera and the place you are focusing, the greater the depth of field you will have in your photograph.
- Focal length of lens. The longer your lens, the less depth of field you will have in your photograph.
If you shoot with a large enough f/stop, a wide enough lens, or are far enough away from your subject (or some combination of the three), your depth of field can be so great that it will extend behind the focus point to infinity. That is, the entire image behind the plane of focus will be sharp.
Note: For more detail on the affects of distance, focal length and aperture on depth of field, if you shoot the same subject at the same f/stop many times, varying the focal length of the lens and your distance from the subject so that the subject is always the same size in the frame, the depth of field will be exactly the same in every photograph. However, there is still significance in choosing whether to shoot your subject with a wide angle lens close up or a long lens far away.
- With longer lenses, it's easier to get your subject separated from the background. Longer lenses have a narrower angle of view, so the background of your image will cover much less area than shorter lenses. Wide angle lenses have wider backgrounds that cover a greater area. This means that it is easier to compose your photograph with a background that is not distracting to your subject. If I shot the above photograph at 100mm, I would have had to include much more stuff in the background, stuff that I wouldn't want in my photograph, like trees and water. Because I shot at 400mm, I was able to shoot this picture with only grass in the background, making (for me) a more pleasing and simple composition. And, because the background covers less space at 400mm, the background grasses are larger in the frame than they would be if I had shot at 100mm. Even though they would be out of focus whether I shot at 100mm or 400mm (with the otter the same size), the out of focus grass in the background is larger in the frame at 400mm, so it's blurriness is more apparent to the viewer. The subject will appear to stand out from the background a little more when shot with a longer lens.
- Wide angle lenses create a sense of drama and depth. If you use a wide angle lens and make a small subject large in the frame, you may still have an entire mountain in the background. But since the photograph is a flat, 2-D artifact, the disparity between the relative size of the small subject in the foreground and the large mountain in the background can create drama and interest. Beyond this, it will also be easier to ensure that the distant background is in focus along with your subject.
Love this article, especially the part about narrow and wide-angle lenses and their effects on the background of the image. I did not know this.
One suggestion: You should explain how f/Stop affects light on the sensor and what it means if you move to a higher f/Stop (shrinking the aperture). You might also draw a diagram that shows why depth of field is affected by the aperture size.
As I understand it, by increasing the f/Stop, you can get much more depth of field, but you trade the amount of light let in. If it's really bright out, or you have a tripod and can leave the shutter open for a long time, you don't have to worry about the loss of light. If, on the other hand, you are shooting an action shot or are working in dimmer lighting, you may not be able to afford the longer shutter time required to get enough light without blurring the shot. In this way, f/Stop (Aperture Adjustment) and Shutter Speed are a trade off with one another.
This means that the hardest shot to get is one in low light, with a moving subject, where you want a long depth of field. These elements are all in tension with one another. You can have 2 of them with a good enough lens/camera combination, but you can't have all three.
Thanks! And I think you just gave me an idea for my next post, "The Art of Compromise."ReplyDelete
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