An exposure is simply a reflection of the ISO, aperture, and shutter speed used to take a given photograph. For any given aperture setting for a scene, there is a set amount of time the shutter needs to remain open for a good exposure. The smaller your aperture, the longer your shutter speed. The wider your aperture, your shutter speed can be very high.
Generally speaking a good exposure shows highlight detail (details in the brightest parts) and shadow detail (detail in the darkest parts) in an image.
Each camera has a light meter, which measures how much light is reflected off of your subject and into the camera. The camera’s meter displays a graph based on the ISO you have selected. By adjusting the aperture and shutter, based on what you want to accomplish, the meter moves to the middle showing proper exposure.
When too much light (called over exposure) is allowed, the bright areas of the image have no detail and the darkest parts of the image become lighter shades of black. When there isn’t enough light (under exposure), the opposite happens, the bright areas become gray and the darker areas become black with no detail.
A few examples: Good exposure.
Under exposure. Notice how the white siding on the building has turned gray.
Over exposure. Notice how the white wall has no detail in it.
Both the over exposure and under exposure examples are off by one stop (one full setting away).
For any given photograph there are many different exposures that are all acceptable. All acceptable settings for a given scene are called equivalent exposures. If you let a little bit of light through the aperture you need to let more light through the shutter speed to compensate. Move both setting in opposite directions to reach equivalent exposure. You can use this idea to accomplish what you want with your aperture and shutter settings. If you want a lot of depth of field, set the aperture to f/16 or so, and you’ll have the shutter open for a long time. If you want to stop someone running, a high shutter speed would balance with a wide aperture, f/2.8 or so.
Now that we’ve gotten ISO and aperture taken care of, the easiest control to explain is shutter speed. This is simply the amount of time that the shutter is open. The shutter is a mechanism that allows light to hit your film or sensor in a digital camera. Think of it like blinds on a window that you can tilt to open and shut.
The typical shutter speed scale is: 1 sec, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000.
When you adjust the shutter speed you are adjusting the fraction of a second the light passes through. When it reads 250, it’s actually 1/250 of a second. That’s a good starting point if you’re photographing a moving subject and you want to stop them in place. The faster and object is moving the higher shutter speed you need to stop it in place.
The slower your setting the more blur in your picture. You use these settings for low light situations as well as artistic situations where you may want to show motion.
The aperture of a camera is a measure of how wide the opening is that lets light into the camera. In most point and shoot and SLR cameras this setting can be manually adjusted for every lens. Each setting is called an f-stop and the core scale is: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. Just like ISO each full stop allows twice as much light through the lens or half as much, again depending which way you go. Unlike ISO the larger the number the less light allowed through the lens.
On the above scale f/1.4 allows in the most light and f/22 the least. F/8 allows in half as much light as f/5.6.
Aperture controls not only how much light is allowed through a lens but also how much depth of field is in your picture. Depth of field is the amount of a picture that is in focus from the foreground (or front of a picture) through the middle ground to the background.
An aperture of f/22 allows more of the picture to be in focus than f/2 for example. F/2 is said to have a shallow depth of field (like the shallow end of a pool) while f/22 has a wide depth of field.
A shallow depth of field allows you to really focus attention on the subject of your picture because the background will be out of focus. A wide depth of field can be useful for landscapes where virtually everything is in focus.
In this first example you can see how the flower in the middle is in sharp focus while the flower closer to the camera in the lower left is out of focus and the background is a blur of other flowers.
As we increase our depth of field the foreground and middle ground flowers are becoming more clear in the photograph.
The foreground and middle ground are in good focus. Notice how the leaves in the background are clearer but not totally in focus.
Now at f/22 the background is in greatest focus possible for the lens.
Whether using a film or digital camera a photographer needs to understand how to manipulate ISO/ASA. The ISO/ASA number is a reflection of how sensitive the film or digital sensor is to light. The lower the number the less sensitive a medium, while the higher number reflects a greater sensitivity to light. This basic scale of ISOs is 100. 200, 400, 800, 1600, 3200. Each whole difference in setting is equal to twice or half the sensitivity difference depending on which way you are going. For example ISO 100 is the most common low setting which means it is not highly sensitive to light. ISO 1600 is a common high setting. ISO 1600 is sixteen times more sensitive to light than ISO 100. ISO 200 is half as sensitive as ISO 400.
This understanding is important because it allows you to chose the right ISO for any given situation. If you are at a dinner party you can set the ISO to a high number which will allow you to take images at a high shutter speed to avoid blurred images. If you are outside, you can use a less sensitive setting because there is plenty of light to capture an acceptable image.
Now choosing an ISO is pretty simple, but you need to know one more thing. The higher and ISO (i.e. the greater the number) the more noise in the picture. If your using film this is grain on the negative from large silver crystals. Noise is the same idea. It makes the image look blocked up through pixel distortion.
In this image I used an ISO rating of 200. Notice that despite the tight cop, you can see the detail pretty well and the shallow depth of field, especially at the top, make the cloth appear smooth.
In this image with the same aperture but an ISO of 1600, the sensor is 8x more sensitive to light. You can see the noise in the darker area and at the top, where the pixels look gritty as opposed to the smoothness in the other image.