Take a look at the following two photographs.  They are both of the same subject and taken from the same place, with the same camera and lens.  Do you notice anything different between them?



Canon 50D, 70-200mm f/4L
ISO 200 f/4 1/1000sec.




















Canon 50D 70-200mm f/4L
ISO 200  f/16 1/80sec.











Clearly the second picture has more of the subject in focus than the first, it has a greater depth of field.  Controlling depth of field is useful in many situations and is easy to do.  It's all about the f-stops!  A large f-stop, f/1.4, f/2.8, f/4 will give you a shallow depth of field like in the first picture.  A small f-stop, f/11, f/16, f/22, will give you greater depth of field like in the second picture.  Remember that a f-stop is a way of talking about aperture or how large you shutter opens when you take a picture.

Why is controlling depth of field important you might ask?  It allows you to determine which elements in your photographs are in focus and which will be blurry.  This is very useful when taking portraits where you have a distracting or ugly background.  You can have your subject tack sharp and the background so blurry that it is unrecognizable.

In this picture the subject is in sharp focus and there is good separation from the background.  The depth of field is also very shallow since only the subject is in focus and all of the background is blurred.  In this case the subject was standing in an alley that extended for four or five blocks.  You can just see a little blue sky above the subjects head.






Canon 50D 70-200mm f/4L
ISO 200 f/4 1/2000sec.




The simplest way to adjust your depth of field is to use the aperture priority mode on your camera (AV mode for Canon).  With this mode you select the aperture and the camera sets the shutter speed automatically.  It's very simple to use and anyone can do it.  If you want even better results, use your camera's manual mode.  With this mode you can control everything, ISO, shutter speed and aperture.  I always start by selecting my ISO speed and then my f-stop or aperture value.  With those set I then use the camera's meter to determine my shutter speed and set it accordingly.  Shooting in manual is much easier than you think.  Go ahead and give it a try and see what kind of great images you can create.