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Today's auto-everything cameras have made taking snapshots easier... but not necessarily better. And that's because cameras can't think. They don't understand what part should be in focus.
That's where depth of field comes in.
Depth of field is the distance in front of your camera that's in focus. For example, if you focus on a person 20 feet in front of your camera, there's an area closer and further than 20 feet that's actually in focus. This is your depth of field.
Three factors play major roles in determining depth of field.
First, there's the distance between your camera and your subject. All other things being equal, the closer your subject is to the camera, the narrower your depth of field. So, if you move the person in the example above to just 10 feet from the camera, there will be less in front and behind them that's in focus.
Second, there's the focal length of your lens. (This is the "lens length.") The longer the lens's focal length, the narrower the depth of field tends to be. So, all other things being equal, a 50 mm lens generally has a greater depth of field than a 200 mm lens. (And the 70mm setting on a zoom lens has a greater depth of field than the 200 mm setting on the same lens.)
Finally there's the "f-stop" or "f-number." This is a measure of how much light the lens allows in. Although f-stops are actually ratios, they're expressed as single numbers, such as f 1.4, f 2.8, f4.5, etc. The smaller the f-number, the narrower the depth of field.
Now, here's where all this knowledge comes in.
Let's say you're shooting an outdoor portrait, but the background is a busy street. To blur more of the background, you can shoot with your subject standing closer to your camera. Or, you can switch to a telephoto lens (or zoom your lens out). Or, you can set your camera to a smaller f-number.
On the other hand, you may be shooting a landscape scene, and want to get as much in focus as possible. In that case, you may not be able to move your subject. But you could still switch to a wider angle lens (or zoom out) or use a larger f-stop setting.
After you've practiced these tricks a few times, they'll become second nature - and your pictures will stand out from the pack.