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DEPTH OF FIELDWhy is this important? This is what draws the viewer’s attention to what it is that you want him or her to look at when they look at your photos. It should be that which is in focus.

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What’s it mean?

Depth of field is simply the zone in front of and behind the point of focus that is acceptably sharp. Simply, how much of your picture is in focus. We try to impress people with the beauty of our pictures, so we have to know what effect focus has on the viewer. Depth of field is what we use to control what the viewer looks at.

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What affects depth of field?

Depth of field is controlled by your choice of Aperture, Lens and Working Distance. It is also controlled by your vision of the scene.

Aperture as you probably know is expressed in f-stops. There are two numbers you must memorize to master f-stops. They are 1 and 1.4. All f-stops are then a mathematical progression of those two numbers. It goes like this: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, and 64.

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Working Distance

The closer you are to the subject the shallower the depth of field.

So if you want to shoot a bee on a flower you could use a 28mm lens which will give you better depth of field but not much closeness or you could use a 200 mm lens and stand farther away. But you would have to stand 10 feet back. Do you know why?

Orrrrrrrrrrr, you could use a 200 mm lens with a 25 mm extension tube and get up about 5 ft closer.

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But what is an Aperture?

An aperture, usually referred to as an “ap”, is a hole through which the light from a scene passes to land onto the film or sensor plane. It represents the diameter of the opening as a fraction of the focal length of the lens. So for example, f/5.6 on a 100 mm lens is a larger opening, 1/5.6 100mm than, f/11 which is 1/11 the opening of a 100mm. So the larger the f/stop number the smaller the lens opening.

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Aperture

In a visual sense this means that the apertures are related by size. So, an “ap” of f/2.8 is twice as wide as an “ap” of f/4.0 and an “ap” of f/4.0 is twice as wide as an “ap “ of f/5.6. 5.6 is twice f/8, f8 is twice f/11, f11 is twice f/16,f/16 is twice f/22, f/22 is twice f/32, f/32 is twice f/64. How many times bigger is f/4 than f/16? See if you can figure that out. The upshot of all this is as you go from f/2.8 to f/16 you get half as much light each time you change f/ stops.

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Check it out.

You can check out the depth of field if your camera has a depth of field preview button. When you push the preview button it actually changes what you see. When you focus a shot before you press the shutter, the lens is actually wide open so you can see to focus. Pushing the preview button stops the lens down to the actual setting you have chosen for the shot. That allows you to see what’s in focus and what’s not.

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Lens

So how does lens figure in? There are a lot of factors in your choice of lens. What is your subject? Time of day? Wind? Stationary object? Subject to distance? You are standing on an overlook in Yellowstone N.P. using a 28mm lens to get a vista shot. If you choose f/5.6 as your aperture and the closest object is 500 yards away you may get an acceptable shot. But if you choose f/32 you’ll get a great shot, all other things being equal. Secret: your exposure time is controlled by your choice of aperture. Thus your depth of field is a function of time. The smaller your aperture the longer your shutter is open; the longer your shutter is open , the greater your depth of field.

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Lens So if you are trying to isolate a flower you

might chose a 100 mm lens or a 200 mm lens because you want to isolate the flower from the other flowers or other foliage. The longer the lens, the shallower the depth of field. Conversely, the shorter the lens the greater the depth of field. So if you use a Macro Lens you will get a very small depth of field. Adding tele-converters will cut down your depth of field more as will the use of extension tubes.

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Extension Tubes

Extension tubes are used for magnification purposes and follow a simple formula. That is:

ExtensionMagnification = ------------------------------

Lens length in MM

They also limit Depth of field

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Working distance

Speaking of lenses, an important statistic to know is what is the focusing distance of the lens you will use for this shot? I bring this up because it will help visualize the shot. My 70-200 f/2.8 lens stops focusing close up at 10 feet. If I want to get closer I have to add extension tubes. So read the papers that come with your lenses. The extension tubes ad magnification. This affects depth of field in the reverse of a grand scenic shot. You use this to isolate a flower, a bug, or some other small subject.

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Summary

Depth of field isolates a subject for the viewer, be it a grandios scene, a single object within a wider view or a close up .

You control this by a). Lens b). Aperture c). Working

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