Depth of field is determined by the amount of subject matter that remains in focus both in front of and behind what you are focusing on.
For example, in a portrait, if your subject and the background are both in sharp focus, then that image would have a “long” depth of field.
On the other hand, if your subject is in focus and the background is blurry, then that image would have a “short” depth of field.
To control the depth of field in your image, you must use the manual settings on your camera. In other words, you will have no control over depth of field if you are using automatic settings.
Note: Apertures (or f-stops) are those funny fraction numbers. You can see them on your lens, in your digital LCD screen, or in your camera’s view finder. If you know nothing about cameras or photography, just remember…with f-stops, small apertures are the higher numbers, and large apertures are the smaller numbers. This can be confusing.
Just remember this: the smaller the aperture, the more depth of field you will get. So, if you want as much of your image in focus as possible, use the higher numbers (smaller apertures like f/16 or even f/22).
However, if you want a blurry background, use the lower numbers (larger apertures like f/1.8 or f/2 – f/3.5 or f/4.5 on a standard zoom).Once you have your aperture set, you’ll need to then set your shutter speed. If you have a digital camera, then you can see your image right away and make necessary adjustments to the shutter speed or aperture. It’s important to set both accordingly to get the best lighting results.
Here is an example of an image with a short depth of field (where the subject is in focus and the background is blurry):
You can see that the baby’s feet are in sharp focus and the background (the baby’s face) is out of focus.