Depth of Field, as Dictionary.com explains, is a noun and defines it as follows; "The range of distances along the axis of an optical instrument, usually a camera lens, through which an object will produce a relatively distinct image."
HUH? Lost? Well if you are, don't fret. Most people don't truly understand what that means until they've studied it. If you think about it more practically it becomes much easier to understand.
What is Depth Of Field?
It is the portion of a scene that appears acceptably sharp in the image with the rest being relatively out of focus.
A lens can only precisely focus at one distance, but the sharpness of everything in front or behind your sharp subject point is gradual and can be more or less in focus. The "more-or-less" out of focus is described as Shallow or Deep Depth of Field.
Shallow Depth of Field - More out of focus and less in focus.
Only the leaf is sharp and in focus. Everything else is out of focus.
Deep Depth of Field - More in focus and less out of focus.
Everything from the back tires of the bicycles to the trees behind the building are sharp and in focus.
Creating Shallow or Deep Depth of Field
Subject distance plays a big part determining the depth of field in an image.
Try this little experiment.... Hold your arm outstreched with your finger up and focus on your finger. Notice how you can see your finger and pretty much everything else in front and behind it in focus. Now bring your finger closer to your face, while still focusing on it. Notice how your finger remains in focus but everything else begins to blur the closer it gets to you.
So the conclusion is, the closer you are to your subject the less depth of field you'll have - (Shallow depth of field). The further you are from your subject the more depth of field you'll have - (Deep depth of field)
In the article, Back to Basics: Apertures, FStops, Shutter Speeds and ISOs we described how apertures work. The relationship between apertures and depth-of-field and is an important one to understand - they inevitably control Depth Of Field.
Quite simply, the smaller the aperture (i.e. F16, F32 etc) the deeper your Depth-of-Field will be, and more of your image will be IN focus.
The larger the aperture (i.e. F1.2, F2.8 etc) the shallower your Depth-of-Field will be, and your image will concentrate on a much smaller focus point.
Depth of Field in Composition
Depth-of-Field is a very powerful composition tool. It directly affects the viewers sensory perception and gives the illusion that there is more to the image than initially meets the eye. By focusing on something very specific you force the viewer to pay attention to the area of the image you choose, perhaps something interesting, something beautiful or a very rare occurance.
It is important when using shallow Depth-of-Field to make sure that your main subject is in sharp focus. The out of focus background or foreground will help by inevitably making the subject look much sharper, even if it is a little out of focus.
Practice with this online Depth-of-Field calculator.