Depth of field is a simple concept - once you understand it.  And it is one of your most powerful tools in photography!

Depth of field refers to how much of an image "appears" to be sharp.  The word "appears" is important, because, in actuality, sharpness in an image is relative.  Take an image and enlarge it to a 4"x6" print and it may look sharp.  Blow it up to a 20"x24" print and now we're talking a whole different story!  The areas that appeared to be sharp in a small print may not appear to be sharp in a large print.  So, the topic of depth of field really is an inexact science.

With this understanding, we can continue on our quest.

Depth of Field is affected by three things:

	Camera to subject distance
	Lens focal length
	Lens aperture

Let's say you focus on a subject that is 10 feet from your you.  Theoretically, the only point that will be in focus is that point that you focused on. 

But in reality, more of the picture will actually look sharp.  How much more is up to you.  You are in control (to an extent) of how much of the picture will be sharp. 

Your lens and lens aperture has lots to do with Depth of Field.  The larger your lens aperture (i.e., f2.8, f4, etc.) the LESS depth of field you will have.  The smaller your lens aperture (i.e., f11, f16, etc.) the MORE depth of field you will have.  The longer the focal length of your lens (i.e., 200mm), the LESS depth of field you'll get.  The shorter the focal length of your lens (i.e., 28mm) the MORE depth of field you'll get.

So, if you focus on your subject at 10 feet, there will be a portion of the image both in front of and behind your subject that will be in focus.  This distance is referred to as the Depth of Field.  As you can see by the following chart, smaller the lens apertures yield more depth of field.  The Depth of Field will always be twice as much behind the subject as in front. 

Here are some sample charts of depth of field with for 28mm and 90mm lenses at three different focus distances:

As you can see, depth of field increases with wide-angle lenses (e.g., 28mm) and decrease with telephoto lenses (e.g., 90mm).  The difference can be quite spectacular. 

So how can I use this?
From a practical standpoint, Depth of Field charts can be misleading!  Their main use is to help you understand the basic concepts, which are: 

(by the way, you can calculate Depth of Field with my Depth of Field calculator)

There are two ways you can determine depth of field when taking pictures.  You can use the depth of field "scale" on your lens. 

On this particular 50mm lens, the depth of field scale is color-coded.  Each lens aperture is color coded.  F22 is brown, f16 is blue, f11 is yellow, and all other apertures are white.  There is no way to determine depth of field for these apertures (f8-f1.8). This lens is currently focused at 20 feet.  The middle ring is where you look to determine your depth of scale.  For example, you can see two blue vertical lines on the middle (silver) ring.  Right now, one of them points beyond infinity and the other pints to just under 3 feet.  That means that when focused at 20 feet at F16 (color-coded blue), your depth of field is between about 3 feet and infinity.  As you can see, this is a relatively simple way "rough-guess" your depth of field.

Most lenses over 50mm and most zoom lenses will not have these scales.  That's because on telephoto lenses the depth of field is very small and you wouldn't be able to read such a small scale.  Zoom lenses don't have these scales since depth of field varies by focal length and the depth of field scale could be very difficult to read.

The second way, and the better way to determine depth of field when you're taking pictures is to use the Depth of Field Preview feature of your camera, if it has one.  Normally when you view thru your lens, the aperture is wide-open.  Thus if you have a 50mm/f2 lens, the aperture is always set to f2 when you are viewing thru the viewfinder.  Only when you actually take the picture does the aperture close down to whatever you have it set to.  So, when you are viewing your picture thru the viewfinder, you are seeing the depth of field when your lens is at it's widest aperture.  The depth of field preview feature allows you to close the lens aperture down to whatever you've selected and view at that aperture.  When you do this, you will see the actual depth of field that you will get in your final picture.  You also will reduce the amount of light that you are viewing with as you are closing the aperture and letting less light in.  But that's the price you have to pay for this feature!  Be aware, however, that the depth of field feature is another "rough-guess" as you are viewing a small image.  Once blown up, you may see less depth of field than you thought.

Why is this so important?
This is a very important tool in your photographic arsenal.  With it you control how your pictures look. 

Take a look at this large picture (157k) to see the effect that depth of field has on an image.  On the top-left is the full picture taken at F16 - on the top-right is the same image taken at F2.  On the bottom are enlargements of the foreground and background bears at F2, F5.6 and F16 which shows the difference in sharpness.  You can see how much of a difference your aperture makes in the depth of field.

Depth of Field is a very important creative tool in your toolbox.  It allows you to help isolate your subject as it helps the viewer know what you wanted the subject to be.