Have you ever wondered why some photographs are in focus and others are not? You really thought that you focused your picture properly through the camera lens but the result turned out blurry or out of focus. Maybe you were shooting a picture from a distance and tried to capture the subject somewhere in the middle of the photograph at a sporting event. You thought that through the eye of the photography lens that it all looked great but the camera lens did something else. Yes folks, I have been there all too many times and still make mistakes using my photography lenses improperly.

Why does this happen? Especially if I am using an auto-focus photography lens that is supposed to assist me?
Have you heard of depth of field? You camera lens on your digital SLR camera has options for controlling the amount of light entering the lens like the iris in your eye. This is adjusted using the aperture setting on your lens. The more light you let into the camera lens the more shallow your focus area is and the less light the larger your focus area is.

F-stops on the aperture ring on the photography lens can typically run from F:1.4 through F:22. The larger the number the more subject area of focus you can get in your photographic detail like you may want in shooting landscapes. The smaller the number might be used when shooting portraits.

Rob Barron takes you through this very simple, comprehensive explanation of depth of field as it is used with Photography Lenses.