Restricting Depth of Field, Not Always Easy

A photograph always works best when it has a single clear subject that is well defined. That isn't to say that you can't have a complex scene (like this one) and still emphasize just one subject (the Christmas decorations in the tree), but you have to find a way to make that subject dominate its background. One reliable way to make a subject pop, of course, is by limiting depth of field (near-to-far sharpness) and thereby restricting focus to just one zone of the photo. Three factors go into how much depth of field any photo will have: the lens aperture you're using (the wider the aperture the less will be in sharp focus and vice versa), lens focal length (the longer the focal length, the less that will be in sharp focus and vice versa) and subject distance (the nearer you are to your subject, the less will be in sharp focus with any given lens at any given aperture). OK, that's the mini lesson in depth of field.

But even restricting depth of field with two of those factors doesn't always have enough impact. I took the shot above of a neighbor's tree after an ice storm last week. It wasn't a serious photo, I was just chatting with a few neighbors and popping off some shots. But what I wanted was for the decorations to really jump out against the background and not get lost in the tangle of ice-covered branches. And to some degree, I got that--but not quite as dramatically as I would have liked. Two depth of field factors were at work to give me shallow focus: I was using a Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor Zoom Lens for Nikon Digital SLR Cameras set at 112mm (equivalent to 168mm in 35mm terms--which is a pretty long lens) and I was shooting nearly wide-open at f/5.6. For most subjects that would have restricted depth of field to a very narrow zone--and, in reality, it did--the sharpest focus is limited to the ornaments. But because I was about 30' away from the tree, I didn't have the third factor at work: I was a little too far away from the subject to really knock the background out of focus. Unfortunately the snow was about four feet (yes four feet!) deep between me and the tree and there was no way I was wading into that mess!

So, while I had the right lens and the right f/stop, the long lens compressed the space (a trait of long lenses) between the main subject (the ornaments and foreground tree) and the background trees and branches into a bit of a visual mush. Had I gotten physically closer (probably 10' closer) and isolated just a few of the ornaments I could have easily tossed the background completely out of focus. Or I could have zoomed out more, but from where I was standing I couldn't find a good composition using more lens.

The photo is OK, it's pretty, but the photo I was really seeing in my mind's eye would have required the third factor: getting closer. And that would have required a fourth factor: a photographer willing to stand up this his chest in snow to get the shot. That day, it wasn't me!

By the way, not to throw in a book plug (oh hey, why not!) but there is lots more about depth of field and related issues in my book Exposure Photo Workshop: Develop Your Digital Photography Talent.

Minimize Depth of Field for Selective Focus

A lot of depth of field is a great thing if you want everything in your pictures to be in sharp focus. But there are times when you might want to limit or restrict depth of field so that only your main subject is sharply focused and the background and/or foreground is less sharp. The main reason for doing this is to accent your primary subject--in a head-and-shoulders portrait, for example--where you want your subject's face in sharp focus but want to toss the background into a soft blur.

You can reduce the amount of depth of field by simply reversing the things we talked about in the previous tip. Again, here are the primary factors:

• Lens focal length: The longer the focal length of the lens, the less depth of field you will have at any given aperture (f/stop) and at any given distance. As you zoom a lens from wide to telephoto, for example, you quickly lose depth of field (provided you are keeping the aperture the same, see below). I used a 70-300mm Nikkor AFS lens at the 300mm position (450mm in 35mm terms) to shoot this horse on the King Ranch in Kingsville, Texas partly because I needed the long lens to bring the wild horse close, but also so that I could throw the foreground scrub and background out of focus.
• Lens aperture: The wider the aperture (again, assuming focal length and subject distance are constant) the less depth of field. You will get significantly less depth of field at f/4 than at f/16, for example. Think "smaller" f/stop numbers = larger lens openings = less depth of field. Remember, this does not affect how sharp your subject will be if you focus carefully, the larger aperture only reduces the near-to-far sharpness.
• Subject distance: The closer you are to your main subject, the less that will be in sharp focus in front of and behind the subject. Want less depth of field? Move closer. Want more? Get farther away from your subject.
  

So, if you were shooting a portrait of a friend sitting on a bench near a harbor and just wanted the person sharply focused with the boats in the background recorded as a soft impressionistic blur, you could choose a wide aperture and a long lens. You'll be able to see the results of your depth of field experiments on the LCD when reviewing the shot, but you won't see them if you're composing on the shot live on the LCD because the lens is not stopped down to its shooting aperture until you actually shoot the photo. (If you're using a DSLR with a depth-of-field preview button, you can use that feature to check it, but the viewfinder gets so dark that it's often hard to tell the difference.)

Keep in mind, if you're shooting with a point-and-shoot camera, you'll have less control when it comes to restricting depth of field because they tend to have inherently smaller apertures which (you knew this already!) creates more depth of field.

Finally, if you're shooting in bright light and the camera won't allow you to use a wide aperture, even at a high shutter speed, you can use a neutral-density filter in front of the lens to reduce the light. These filters won't affect color but reduce the amount of light entering the lens. I'll write a tip of "ND" filters in the future.

Maximize Depth of Field for Ultimate Sharpness

There are really two kinds of sharpness in a photograph. One is the sharpness of your main subject, say, a person standing on the beach and that sharpness depends on how steady you were at holding the camera, the shutter speed you used, how carefully you focused on your subject and how still your subject was standing. If you are careful with your technique, your subject should be quite sharply focused.

The other is depth of field or "near-to-far" sharpness and that describes how much of the beach in front of and behind your subject is also in sharp focus. Depth of field sharpness is not an absolute thing that has exact starting and stopping points, rather it's a zone of what is called "acceptable" sharpness. Ultimately how sharp a photo appears also depends on where you're seeing it and how far away you are from the image. If you're looking at an online version it will often look sharper than it will in a print, largely because a print is made of paper and ink and has a surface that can interrupt our interpretation of sharpness. Also, images online are rear-illuminated so they will always look brighter and, within reason, sharper. And if you're looking at a print, the farther you are from the print, the sharper it will appear (particularly if you refuse to wear your reading glasses).

But you can maximize depth of field, that zone of acceptable sharpness, if you know what factors affect it. Basically three things (in combination) determine how much depth of field sharpness an image will have: lens focal length, your distance from the subject and the aperture you're using. Here are some things to keep in mind when you're trying to maximize depth of field:

• Wider lenses have inherently more depth of field. That means that for a given f/stop at a fixed lens-to-subject distance, the shorter the focal length of the lens, the more that will be in sharp focus. Wide-angle lenses provide the most depth of field and telephoto lenses the least.
• With any given lens, the smaller the aperture (f/16 as opposed to f/4, for example) the more depth of field you'll get.
• With any given lens at any given f/stop, the farther you are from your subject, the more depth of field you will have. If you were shooting a picture of a friend with a 28mm wide-angle lens at f/8, for example, your photo would have inherently more depth of field if you stood 10 feet away than if you stood five feet awy.

Remember, it's these three factors in combination that really determine depth of field. If you're photographing a landscape and want lots of near-to-far sharpness, choose a wide lens and a small aperture and you will increase depth of field. I shot this photo of Las Olas Boulevard in Fort Lauderdale using a combination of a wide-angle lens and a very small (f/20) aperture to wring the maximum amount of sharpness from the image. Because you're usually using small aperture to get a lot of depth of field, you'll also be using relatively slow shutter speeds. In order to be able to shoot this photo at such a small aperture, I had to reduce the shutter speed to 1/30 second. You can see the motion blur caused by the slow shutter speed in the pick up truck. Using a tripod helps to steady the camera, but you'll still get blur with any moving subjects.

Of course, there will be times when you want to minimize depth of field--to limit sharpness in a portrait, for example, and to do that...you'll have to read the next tip!

Extend Depth of Field with the Landscape Mode

In an earlier posting I talked about the benefits using selective focus and a shallow depth of field. Depth of field is the near-to-far distance in a scene that is in acceptably sharp focus. There are three factors that contribute to how much depth of field a shot will have: lens focal length, lens aperture and distance to your subject. All other things being equal, a wider lens (shorter focal length) and a smaller aperture create more near-to-far sharpness.

One fast and easy way to get a lot of depth of field is to put your camera in the "landscape" exposure mode. In this mode the camera automatically selects the smallest possible aperture giving the amount of existing light and the ISO you have set. (The camera will also try to select a shutter speed that is safe for hand holding.) In this rural Iowa scene, for instance, I wanted to keep everything from the grass in the foreground to the distant hills in sharp focus. I could have taken manual exposure readings and then choosing the smallest available aperture--and that's usually what I do. But there are times when I'm just taking a quick snap of a pretty scene and in those circumstances, I simply flip the camera to its landscape mode and shoot.

Read your manual for more info about these specialty exposure modes (often called "scene" modes in manuals); they can provide some very useful technical shortcuts.