UNDERSTANDING DEPTH OF FIELD

Have you ever seen a photograph that was taken through a window, with a screen on it, and wondered why the screen does not show up? Or, have you ever compared snapshots with friends, taken at the same time, and wondered why the garbage can shows up in the background of your picture but not in your friends? The answer lies in the concept of "depth of field". 

Depth of field is defined as that area in front of, and behind, your subject, which remains in focus in your photograph. And this concept will allow you to have all sorts of seeming special effects with very little effort. Your pictures can look very professional with a few simple tricks.

To take advantage of depth of field, you need to be able to adjust your aperture, or "f-stop" setting on your camera. Almost all single lens reflex (SLRS) allow you to do this, and many point and shot cameras, along with the more modern digital cameras offer this feature as well. This is often called "shooting in aperture priority". 

The aperture is one of two ways that light gets into your camera, exposing the film. The second way is through the shutter speed. These two methods combine to properly expose your film. When one method is letting in a lot of light, the other compensates by letting in less light. Most cameras make the calculations automatically, so there is no guesswork for you in this combination.

The aperture, also called f-stop, is similar to the pupil of your eye. When it opens up very wide, in a low light situation, your eye (or in this case the film) receives more light. During the daylight, your pupil contracts, letting in less light so that your eyes do not get burned by the sun. If the sunlight is too bright, you may have to wear sunglasses. This is much like adding a filter to your camera!

To help you understand depth of field a little better, try this experiment. The next time you wake up in the middle of the night, look around. Let your eyes adjust to what little light there is in the room. Notice how you can see a few things around the room, but you can't see very far. If you turn on even a low light lamp, your ability to see more further in the room will improve. You may also notice then when you are driving at night. Even with the headlights on and the streetlights aglow, you do not have the distance vision you would have in the daytime.

So, basically what you need to remember is that the bigger the aperture opening is, the less depth of field you will have. There will be items in the foreground and background that appear out of focus. 

SO HOW DO I USE THIS TO MY ADVANTAGE?

I've got one more piece of information on how to set your f-stop and then you'll be ready to shoot like a professional. The aperture or f-stop is rated in numbers. The numbers are basically fractions. So, an f-stop of 2 is a bigger opening than an f-stop of 4, and an f-stop of 5.6 is a bigger opening than an f-stop of 8. (These are all common f-stop or aperture setting numbers.) So the smaller the number, the more light that is being let in. But, remember that the bigger the aperture opening (and smaller number) the LESS depth of field you will have. Now you are ready to get started.

Let's say you are at a picnic and want to capture that great picture of your friends being silly. The only problem is the clump of weeds in the background. Choose an aperture setting with a relatively small number, say 4, or 5.6. Focus the picture on your friends. The end result is that your friends should be in focus but the background should blur.

But, what if you want EVERYTHING in the picture to be in focus. Maybe you have taken your kids to see Mt. Rushmore. And you want their faces, and the faces of our presidents to be in focus. Chose an f-stop with the largest number you can. This will let in less light (so your camera will use a slower shutter speed to compensate) but the depth of field will increase. The result should be a picture where everyone and everything is in focus.

There will be times when your ability to choose your f-stop maybe limited. If the light is low, it is sometimes hard to get an f-stop that lets in enough light. So, these are the times you may want to use a flash unit. Depth of field still applies to pictures using a flash.

Next month I will discuss choosing shutter speeds. Since the shutter speed determines how well you are able to stop action (is your little leaguer a blur when she runs to first base or is she crystal clear? This is determined by the shutter speed). So, sometimes you will make a choice regarding whether the ability to have great depth of field or the ability to stop action is more important. 

Below is a chart showing some common aperture setting numbers. Since most cameras will automatically select the correct corresponding shutter speed, you won't need to make that adjustment. I've included the shutter speed information here for you, however. I want you to be able to see the relationship between the aperture setting and the shutter speed. Basically as one decreases the amount of light allowed to the film, the other increases the amount of light allowed to the film, resulting in a proper exposure. The chart reflects correct exposures I measured when taking pictures of my back yard. (Shutter speed is measured in fractions of a second.)

Good luck and happy shooting.