Depth of Field

The following is an except from an article in, which was itself an excerpt from a book about digital photography. The text below is quoted from (link was active on Nov 21, 2005) but is preserved here, because I thought it was an excellant explanation of depth of field. The whole book excerpt begins here. The book is titled Guide to Digital Photography written by Daniel and Sally Weiner Grotta. If the whole book is as well written as this excerpt, it would be worth reading... you can find it on

Reality Check: How Much Difference Will f-stops Make to Your Camera?

There's a direct relationship between the size of your image sensor or the film you use and the size (or focal length) of the lens in front of your camera. You get more depth of field with a wide-angle lens, even when it's not stopped down (narrowing the aperture), because of its shorter focal length. On the other hand, depth of field becomes progressively more limited as your lens' focal length becomes greater, even when you use a small f-stop.

In a 35mm film camera, wide angle is defined as any lens with a focal length between 24mm and 35mm, a normal lens is 45–55mm lens, and telephoto is between 105mm and 150mm. (18mm–24mm is considered ultra-wide angle, below 18mm is usually a fisheye lens, and greater than 300mm is extreme telephoto.)

Digital camera image sensors are much smaller than film (see Chapter 1), so lens focal lengths also are correspondingly much smaller. A typical 4 megapixel consumer digital camera with a 3X zoom lens has as its wide angle focal length anywhere between 5mm and 6.5mm, its normal focal length is about 7mm–8.5mm, and telephoto focal length runs between 17.5mm and 21mm. (However, you may see your camera's focal lengths displayed or represented in terms of their 35mm equivalents.)

What this means is that your consumer digital camera will likely have great depth of field, no matter what its f-stop and focal length. That's why background softness is more limited, even in portrait mode, with the lens wide open (using a low number f-stop) in consumer cameras. Prosumer and professional digital cameras usually have physically larger image sensors and bigger lenses (though most are not as large as 35mm film lenses), so their focal lengths will be correspondingly longer, giving users a greater range of depth of field options. So, if you really want pro-quality portraits or soft, out-of-focus backgrounds, spend a bit extra and get a prosumer or professional model with a large image sensor and a longer focal length lens. (See Chapter 2 on types of cameras.)