Aperture and Depth of Field

It’s not only focal-length numbers that are different in the digital and film worlds. Aperture comparisons between a digital camera and a 35mm film camera—at least when it comes to the depth of field—won’t always give you the same results either.

Apertures, which are found in most lenses, work in tandem with shutters to control the amount of light that passes through to the sensor or film. Aperture size is expressed as f-stops, which represent the focal length (f) of the lens divided by the diameter of the aperture (the number following the slash). The notation f/2 therefore represents an aperture diameter that is 1/2 the focal length of the lens, and f/16 is an aperture diameter that is 1/16 the focal length of the lens. f/2 is wider than f/16 and allows more light to pass through the lens. Theoretically, an f/5.6 setting on any lens, on any camera—digital or film—will always let in the same amount of light.

Depth of field is the range of acceptable sharpness in front of and behind the plane of focus. As you’ll see throughout the book, the ability to control depth of field—either to isolate a subject from its background or to put more objects in focus—is an important creative technique.

As most of you probably already know, wider f-stops yield a limited depth of field, while narrower f-stops increase the depth of field.

But depth of field is not determined only by f-stop. It’s also determined by the focal length of a lens (longer lenses produce less depth of field); the distance from the lens to the object (the closer the lens is to the subject the less depth of field); and, just as important for the purpose of this discussion, by the size of the sensor—or for that matter, the film.

From a purely practical point of view, just as with small film formats, the smaller the sensor your digital camera uses, the more depth of field is produced for any given lens, f-stop, or distance. Depth of field, therefore, varies from one digital camera model to another—and rarely will you get what you might expect without some experimentation.

I’ll give you a way to test your digital camera and lens for depth of field, and I’ll show you ways to deal with more depth of field than you want. For now, just know that this is a potential issue with huge implications, especially, for example, if you are shooting a portrait and you want the background to go more out of focus and it doesn’t.

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