LEARN: How To: Photography Exposure Guide
Photography exposure guide
In This Guide
1. What is a camera?
2. Aperture & Shutter
3. Sharp pictures

Other Resources
Histogram Guide
Photo Glossary
Discuss Digital Cameras
More Photo How-To's

Photography Exposure Guide

Controlling Light
by Edward Ginsberg

There are big cameras and there are little cameras. There are cameras that allow you to take pictures of a cat's nose, and there are cameras that allow you to take movies. Some researchers, rumor has it, are even developing a camera that will make coffee while telling you how great your last picture was. But despite all these wonderful features, the camera basically hasn't changed since it was first introduced in 1839. This is true for both film and digital cameras.

This mini-tutorial will cover these exposure basics:

1. What is a camera?
A camera is a light-proof box with a hole in it. It is designed to hold either film or an electronic imaging sensor. If it holds the former, it is a regular, or film, camera; if the latter, it is a digital camera. When the hole is closed, no light reaches the film or electronic imaging sensor. (I'll use the word "film" to indicate both.) The purpose of the hole is to allow light to enter the box and reach the film. If no light gets to the film, you will have a picture of a coal mine at midnight. Too much light gives you snow at noon on a sunny day. So the light reaching the film has to be just the right amount. In other words, a camera has to have a mechanism to open and close the hole for a precise amount of time.

The simplest camera is literally nothing but a light-proof box with a piece of tape in the rear of the box to hold the film in place and a second piece of tape to close the hole. The hole is made with a pin, which gives the pinhole camera its name. By knowing the size of the opening, the photographer can make a reasonable guess as to how long to keep the hole open. This primitive camera is still used and can make some beautiful pictures. But there are easier and more useful ways to control the amount of light entering the box.

2. Aperture and Shutter
There are two ways to control the amount of light passing through the hole. The size of the hole can be changed or the length of time the hole is open can be adjusted. The hole itself is called the aperture. The size of the hole is called the f-stop. Let's look at the first illustration. It shows a camera lens. Around the top of the lens are strange numbers such as 5.6, 8, 11, 16, 22. These numbers are the f-stops. The smaller the number, the larger the aperture. So f5.6 lets in more light than f8 which, in turn, lets in more light than f11 and so on. Also, the numbers are picked so that each f-stop allows half as much light to enter the camera as the f-stop with the next larger number. So, f8 allows twice as much light to enter the camera as f11 and four times as much as f16.

Depth-of-field sample photos
Left: Lens aperture inside in an old manual focus Nikon 50mm lens
Right: Aperture ring and f/stop numbers on the same 50mm Nikkor. Rotating the aperture ring changes the size of the aperture you see in the left image.

The second method to control the amount of light entering the camera is by regulating the length of time the aperture remains open. This is done with a shutter. Hold your hand in front of your eyes for five seconds. Your hand just acted as a shutter. It came between your eyes and the light source and blocked the light for a given period of time. In the 19th century, film was slow and exposures took several minutes. There were no mechanical shutters. Photographers simply removed the lens cap for a given amount of time. With modern film only a small amount of light needs to reach the film so a moveable cover whose movement can be timed is built into the camera system.

If you look at illustration two you will see a knob with numbers such as 15, 30, 60, 125, 250, 500. These represent fractions of a second that the shutter remains open when the picture is taken. So 15, for example, is actually 1/15th of a second. Since these numbers are actually fractions, the larger the number, the shorter the time the shutter will be open. Again, as with f-stops, the relationship of these shutter speeds is two to one, meaning that 30 (1/30) will keep the shutter open twice as long as 60 (1/60), four times as long as 125 (which is always 125 and never 120 as would seem logical), and so on. Shutter dial on a Nikon F 35mm SLR
The shutter control dial on a Nikon F 35mm SLR

Okay. I'm about to explain something that has to do with numbers. Don't panic and we'll get through it together. First the concept. I walk a mile in an hour, and my friend Charlie walks a mile in two hours. That is, I walk twice as fast as Charlie, and he walks for twice as long as I do. But we both walked a mile. I walked twice as fast as Charlie for half the time, and Charlie walked twice as long as I did at half the speed. If your eyes haven't glazed over yet, keep reading.

Since both the f-stop and the shutter speed are arranged in a series of 2:1 ratios, by halving one and doubling the other you allow the same amount of light to enter the camera. So a setting of f8 at 60 will allow exactly as much light to pass into the camera as f5.6 at 125. This is called the Law Of Reciprocity, and understanding this principle is basic to knowing how a camera works.

Why all the choices? Wouldn't it be easier if there were only one f-stop and a variety of shutter speeds or one shutter speed and a variety of f-stops?

In order to answer that, we have to define a few more things. So, back to the pinhole camera. If you look at photographs made with this type of camera, you will find that they are fuzzy. Why? When the light from an object enters the aperture and strikes the film, it doesn't create a point but rather a small disk. To a certain extent, the smaller the hole, the smaller the disk. But no matter how small the hole, the disk can never become a point. The result is a fuzzy image.

Enter the lens. The way around this fuzziness is to use a lens to focus the light on the film. The lens consists of one or more (usually more) pieces of glass or plastic that are arranged in a metal barrel. The barrel is placed - you guessed it - in the hole. The barrel also contains a series of overlapping metal plates (the iris) that are arranged in a circle and can fold onto each other. They can be manipulated to control the amount of light that passes through the glass. The size of the iris opening is called - you guessed it again - the f-stop.

Exposure settings on Olympus SP-350 LCD display
Left: The LCD on an Olympus SP-350 with the aperture and shutter speed indicated at the top of the screen, in green.

We used a traditional 35mm film SLR (Nikon F) to illustrate exposure control because the controls are most obvious. Your camera will likely have different controls. Compact digital cameras, like the Olympus SP-350 (left) usually have very different controls from SLRs. So check your camera manual to find out how to access the exposure controls on your camera.

3. Sharp pictures: do you always want them?
Be patient, we're almost finished. Let's say that you want to take a picture of your cousin Betty. You also want your uncle Harry and his horse Tony to be in the picture. Harry and Tony are closer to you than Betty is. If you look through the lens (or at the digital camera display) and manipulate the lens until Betty is in sharp focus, you see that Harry and Tony are not in focus, that is, their images are not as clearly defined as Betty's is. They are a bit fuzzy. So you ask Harry and Tony to move closer and closer to Betty. Eventually they all look sharp. The distance between Betty and Harry is called the depth-of-field (see illustration 3).

Depth-of-field sample photos
Depth-of-field samples: The photo on the left was taken at f/3.2 and 1/1250th of a second. The image on the right was taken at f/13 and 1/80th of a second. Notice how much more of the right image is in focus.

To put this more formally, the depth-of-field is the area of the photograph in which objects look acceptably sharp. The smaller the opening, the greater the depth of field. This means that instead of moving Harry and Tony, you could have made the aperture smaller. Of course then you would also have had to change the shutter speed in order to let in the same amount of light.

But there are restrictions. If you have ever held a rifle, you will have noticed that it was impossible to keep it steady. The same is true when you hold a camera. And it is even harder to keep from moving the camera when you click the shutter. In other words, there are limits as to how slow a shutter speed you can use when hand-holding your camera. (Some cameras have an image-stabilization system built in, but that is too complicated to go into now. Also, since the ability to hold a camera steady varies depending on the lens you are using, I won't give you any numbers, but they are easy to find.) If you handhold your camera at too slow a speed, you will get a fuzzy picture.

You might not want everything in your photograph to be in sharp focus. Consider. You're going to take another picture of a relative. The sun is in her face, she's smiling and looking at the camera, and you are about to snap the shutter when you happen to notice that there is a parade passing about 20 yards behind her. If everything in the picture is in focus, she (let's call her Veronica) will get lost against the people in the parade. However, you can set the f-stop (and, of course, adjust the shutter speed) so that everything behind Betty is out of focus.

We have ignored one question. How do you know how much light to let into the camera in the first place? Almost every camera has one or more light-measuring devices built into it. You can also buy handheld light meters. But that's another story.

      - end -

About the Author
Edward Ginsberg is currently writing a book to help the beginning photographer find true happiness as he embarks on the road to creativity. Ed's photos can be seen on his website, www.line17.com, where he also offers a free tutorial.

Edward Ginsberg Profile >>