In plain English, depth of field is the area in front of, and behind, your subject that is in focus. Your subject can be anything from a mountain to a person’s eye.
There is no sharp delineation between a photograph being “in focus” and “out of focus.” Instead, a picture will gradually go from sharp to blurry. What causes something to look out of focus is often referred to as the “circle of confusion.” This is when a point, whether it’s a pixel or a dot on a print, slide or negative, becomes so blurry that the blur can be detected by the human eye. The blurrier the dot is, the more out of focus it looks.
If you’re taking a picture of a person, you can have a very shallow depth of field, and only their right eye may be in focus. Or you could have a deep depth of field and not only would their entire body be in focus, but so would the mountains behind them, and most of the ground that’s between them and the camera.
It’s important to understand depth of field so you can get the photograph you want. Let’s say you are photographing a person in a crowded situation, like a Renaissance Faire, and you want them to stand out in the photograph. In this case you would use a shallow depth of field. This would throw the crowd behind him out of focus, and since people are drawn toward objects that are in focus, he would stand out in the photograph. On the other hand, if you are standing on the edge of a meadow, and beyond the meadow is the Rocky Mountains, then you could use a deep depth of field, so both the meadow and mountains would be in focus.
Depth of field is a tool, and you need to learn how to use it because it’s in every picture you take, but you can control it to meet your photographic needs.
Here’s how you can control Depth of Field.
- Aperture – The smaller the aperture in a lenses diaphragm, the sharper the picture. Shooting at F 16 will give you a greater depth of field than shooting at F 2. However, you must also consider light diffraction, which occurs in all lenses. Light diffraction is how light scatters in the barrel of a lens. Most light is focused by the lens elements to go directly to the film or sensor in the camera, but some light does just bounce around in the lens. The more light diffraction you have, the less sharp an image will be. Other things affect sharpness too, but that’s another article. As you close down the diaphragm, going from F 2 to F 16, this diffraction decreases, because less light is going through the lens. But there is an optimum F stop where diffraction is at a minimum, and it may not be F 16, it may be F 11 or 8. You will need to test your lens to determine this. But as a rule, F 16 (or 22 or 32) will give you your sharpest image.
- Focal Distance – The closer you are to your subject, the shallower you can make the depth of field. Shoot a person that’s just a few feet away, and you could only have one eye in focus if you want. Shoot that same person with the same lens when they’re 100 feet away, and their entire body will be in focus, and probably much more.
- Focal Length – A shorter lens will give you more depth of field than a longer one. Shoot with a 50 mm lens, and you can get more depth of field than with a 200 mm lens at the same distance.
How do you determine the depth of field for a picture you are taking? It can be simple or complex, but you have three options.
- Experience. Shoot with your camera, a lot, and you will be able to have a pretty accurate idea. With a digital camera you can see the results instantly.
- You can use the Preview Button. This button can be found on all 35 mm film cameras, and most 35 mm DSLRs (Digital Single Lens Reflex) cameras. When you push the button it closes the diaphragm to whatever F-stop you have the camera set for. I know people that use this technique with great success, but for me it only makes everything dark.
- Your final option is to use a somewhat complex mathematical formula. I’ve never know anyone to use this method, but if you want to put your high school algebra to use, do an online search and you will find the formula.
To get a feel for depth of field, put a yardstick or tape measure on a table, with one end pointing toward you. Focus on the mid-point of the tape measure and taking pictures at F 2, 3.5, 5.6, 8, 11 and 16. You can put a piece of paper in the picture with the F-stop you are using so you can easily compare the images. It may be a little hard to see the change from say F 2 to 3.5, but you will easily see the difference between F 2 and 16. Once you have a good handle on this, go outside and do the same thing, but use what you find outside instead of a tape measure.
Notice how the sharpness of the 25 changes with the f-stop?
With experience, controlling depth of field will become second nature to you, so get out there and start shooting.
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