Exposure has to be one of the hardest concepts to understand in photography. It is also one of the most important. In this lesson, I am going to break down exposure into two categories. Technical and artistic. To start off, I will cover the technical aspect of exposure as it is absolutely essential to understand how your camera records your images. I will then cover the artistic end of things, and show you how to use your exposure settings creatively in your shots. So, let’s get started.
Exposure is the process of controlling how much light enters your camera and exposes the image sensor. There are three settings on your camera that deal with exposure. Shutter speed, aperture (or f-stop), and ISO. The shutter speed and aperture control how much light your camera is letting in to expose the image sensor. The ISO controls how sensitive the image sensor is to the light the shutter speed and aperture combination let in. They all three work together to produce a properly exposed image. If you shoot in one of the auto modes, the camera automatically makes these settings for you based on comparing your scene to a preloaded data base of images, and selecting the one that is closest to your scene. Most often, it does a pretty good job. But if you want to take complete control over how your images look, you need to learn to control these settings yourself.
The shutter is located inside of your camera. Basically it is a curtain that seals off the image sensor compartment, making it light tight. When you press the shutter release button on your camera, the curtain slides all the way open and then closed, exposing the image sensor to light. The shutter speed setting on your camera controls how long the curtain stays open, and in turn, how much light gets exposed to the image sensor. A fast shutter speed such as 1/1000 of a second lets very little light in, whereas a long shutter speed like 1/2 of a second lets quite a bit of light in.
The aperture is located in the lens of the camera. It is a variable size opening through which light travels. It functions much like the iris of the human eye, by varying its size to regulate how much light it lets in. On most cameras, the aperture is represented by a series of f-stop numbers to change the size from a large opening to a smaller one. Depending on the lens that is being used, these numbers will typically range from about f/1.8 to f/22. Now this is where it get a little confusing for most beginning photographers. One would think that the smaller the f-stop number, the smaller the aperture opening. In reality it’s the exact opposite. The smaller the f-stop number, the larger the aperture opening. So, an aperture of f/1.8 is a very large opening, and lets a lot of light through the lens and ultimately to the image sensor. An aperture of f/22 is a very small opening, and lets very little light through. Used in combination with the shutter speed, this is how we control how much light is exposed to the image sensor. But we need some way to determine how much light we should let in to properly expose our image. In comes the light meter.
The light meter is your cameras tool to help you determine how much light to expose the image sensor to, and reproduce your scene in a digital format the way you see it. It works by evaluating the brightness levels (or luminosity levels) of your scene, and determines how much light is necessary to record it. The light meter is found by looking through your view finder. Some cameras also allow you to view your light meter on the LCD screen on the back of your camera. Typically, it looks like the image at the beginning of this paragraph. To use it, you need to select an f-stop and shutter speed combination that moves the indicator at the bottom of the graph to the zero position. This is the proper exposure. There are many different combinations that you can use for any given light source that give the same amount of exposure. You can see this by looking at the chart to the left. Sounds easy don’t it? Not so fast. In comes the artistic considerations of selecting f-stops and shutter speeds, and some limiting factors for both.
Your choice of shutter speed can add artistic quality to your images when you understand what is happening to your image at different speeds. A fast shutter speed such as 1/250 sec or faster will freeze any motion within your image such as running water, leaves blowing in the wind, or a animal walking through your scene. A slow shutter speed of 1/30 sec or slower will allow any motion in your scene to blur in the final image. These effects can be used quite creatively in your compositions. A fast shutter speed used photographing waves crashing on a rocky shoreline will freeze the motion of the water, giving a feel of action and force. A slow shutter speed used while photographing a flowing stream will give you a peaceful, almost surreal feeling such as in the image above. The choice is yours. Be creative.
F-stops ultimately control how much depth of field (area of your image that is in focus) you have in your image. The larger the number (ex. f16, f22) the more depth of field you will have in your image. The smaller the number (ex. f2.8, f4) you will have less depth of field, and areas of your image will fall out of focus. Choosing the right f-stop is a critical decision you make when taking a nature photograph. It is also one that is directly related to your artistic composition. Choose a high number like f16 or f22 if you are taking a grand landscape image, and you want everything from your foreground to infinity to be in focus. Select a smaller number like f2.8 or f4 when taking close up photographs of flowers. This will allow the background to go out of focus creating a bokeh effect, which is isolating your subject against a soft out of focus background. This image of a Lupine Bloom demonstrates this effect quite well.
As I mentioned before, there are some limiting factors to consider, especially when it comes to your selection of shutter speed. As a slow shutter speed will blur any motion within your scene, it will also blur your image if your camera is moving at the time the shutter is tripped. Trying to hand hold your camera with a slow shutter speed is a recipe for disaster. It is recommended not to try and hand hold your camera if your shutter speed is less than the focal length of your lens. For instance, if you are shooting with a 200mm lens, your shutter speed should be 1/200 of a second or faster to prevent blurring your image. Anything less, and you should be shooting on a tripod. This is, however, only a recommendation. If you are really serious about getting the best quality landscape images you can, you should always be shooting from a tripod. I never leave home without mine!
The final setting that deals with exposure is your ISO. In the film industry, ISO is a measurement to determine how sensitive the film is to light. A high ISO number such as ISO 800 is very sensitive to light. A low number such as ISO 100 is not very sensitive, and requires more light to properly expose it. The ISO setting on your digital camera works in a very similar way. Your cameras image sensor amplifies its signal to produce the same results as different ISO films. The higher the ISO setting, the more sensitive to light the image sensor becomes. This allows you to shoot at faster shutter speeds in low light conditions, as the image sensor requires less light to properly expose your scene. You have to be careful though. The higher you set your ISO, the more digital noise you’re going to get in your images. Digital noise is a grainy pattern of colored dots that show up in the evenly toned and shadow areas of your image. With most digital SLR’s today, you are pretty safe up to about ISO 600 – 800. The image of the Tule Elk above was captured in very low light. I set my ISO to 600 which allowed me to increase my shutter speed by about three stops to be sure any movement by the elk would be frozen. In landscape photography, it is best to keep your ISO around 100 – 200. Most of the time you will be shooting from a tripod, and a fast shutter speed is not necessary. You want to make sure you get the sharpest images possible, and a low ISO setting will help ensure that happens.
This pretty much covers the basics of understanding exposure, and should get you well on your way to taking control over your images. Obviously, the discussion can get much more involved, as entire books have been written on the subject. And I encourage you to further your studies on exposure. But for now, get out in the field and start putting your newly acquired knowledge to work.