Three things control how great the depth of field will be in a photograph: lens focal length, aperture, and distance from the subject. Other things being equal, shorter-focal-length lenses, smaller apertures, and greater lens-to-subject distance all increase the range of sharp focus, while longer lenses, wider apertures, and a shorter distance to your subject shrink depth of field.

When you look through your lens, however, you will not be seeing what’s really sharp and what’s not. With point-and-shoot cameras, this is because you’re not looking through the lens–you’re looking through a separate viewfinder. With SLRs, you’re looking through the lens at its widest aperture; it closes to a smaller f/stop only at the instant you press the shutter button. This is a problem if you’re using a medium telephoto lens to take a portrait of Mom in gay Paree. As you look through the lens, it appears that the traffic and crowds in the background are out of focus. But because you have a small aperture set, when you get the picture back, zingo, Mom is lost in all that junk behind her.

The solution? Many SLRs have a depth-of-field preview button; when you press it, the lens briefly closes to the shooting aperture, showing you the real depth of field. Be aware that when you press this button, the image in the viewfinder will temporarily darken. If too much is in focus, simply open the aperture, put on a longer lens, or move closer. Or all three. If too little is sharp? Well, you can figure that out.

In photography, using this The Rule of Thirds keeps the main subject off center, away from the middle of the frame. As a result, a photo looks more dynamic and interesting.

The Rule of Thirds envisions two horizontal and two vertical lines trisecting an image with four intersecting points. You place your main subject where the lines intersect rather than centered in the frame. For example, placing the horizon on an upper or lower line helps create a well-composed landscape photo. For portraits, placing person where lines intersect vertically produces a more compelling photo.

A Rule of Thirds grid is built into some digital cameras. It can be turned on and viewed on the LCD. Use it as as a guide when taking a photos. If you don’t achieve this layout just right with a camera, apply the principle when cropping an image.