Depth of field refers to the slice of the photo that is in focus.

A shallow depth of field has a narrower band of focus. For example, everything from 4 feet away from the camera to 8 feet away is in focus. Objects outside of that band are increasingly out of focus as they get closer or further away. Think of a wedding photo with the focus on the bride while the background dissolves out of focus.

A deeper or greater depth of field has a wider band of focus. For example, everything from 3 feet away to infinity is in focus. Think of a landscape photo where everything is sharp from the foreground to the background.

The Mechanics - How to Control Depth of Field

Focal Length - The longer the focal length (higher numbers, like 200mm) the more shallow the depth of field. The shorter the focal length (lower numbers, like 18mm) the deeper the depth of field. So zoom in to shrink the depth of field, zoom out to expand the depth of field.

Aperture - The wider open the aperture (lower f/stop) the more shallow the depth of field. The smaller the aperture (higher f/stop) the greater the depth of field. To remember the rule think of squinting your eye to see far away.

Distance from Subject - The closer the distance from the camera to subject the more shallow the depth of field. The further from the subject the greater the depth of field. So get close to shrink depth of field, move away to expand depth of field.

To get the most shallow depth of field open the aperture, get in close and zoom in. The get as much as possible in focus with a deep depth of field step back, zoom out and set a small aperture.

Artistic - How to Use Depth of Field

The artistic use of depth of field comes down to one thing: controlling what is in focus. Decide what is important about what you see and use the depth of field to get rid of the rest. In a close up portrait the details of the leaves on the tree behind the subject just take away attention from what you want the viewer to focus on.

That's a good way to think about it. What do you want the viewer of the photo to focus on? That's what should be in focus. Simple enough, but tricky to make second nature as you shoot.

This Shot
The view of San Francisco from Coit Tower. This shot uses a deep depth of field to keep everything from the houses in the front to the skyscrapers in the back in focus. I made one big blunder shooting this - ISO 1000. That's why the sky looks a bit grainy. Oh well, reason enough to go back.

Supporting Photos - Click to on Each Photo to Open
The butterfly demonstrates a shallow depth of field. The focus is on the body of the butterfly while the greenery in the background dissolves out of focus.

The carousel horse demonstrates a middle of the road depth of field. You can still make out what is behind the horse but with less definition. This carousel is inside the Please Touch Museum at Fairmount Park in Philadelphia. The expressions on the horses caught me as full of tension, disturbing and odd as part of a kiddie ride.