Controlling Exposure

To produce the best image, the creative photographer relies on wide variety of skills. It isn’t enough to buy the latest and greatest camera and expect to produce perfect images with every shutter release. Regardless of how advanced our cameras have become, obtaining creative images still requires human intervention. One of the tools that the creative photographer must master is exposure control.

Many photographers find this concept difficult to master. This difficulty generally stems from a lack of understanding of the concepts that influence exposure and how they interact to produce a proper exposure.

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What is exposure? For the purposes of this writing, exposure refers to the amount of light that reaches the film plane or digital sensor. Understanding the relationship of aperture, shutter speed, and film speed (ISO) is essential to understanding how to get the right exposure for your image.

The two camera controls that regulate the amount of light reaching the film, and thus the exposure the film receives, are shutter speed and aperture. Film speed or ISO is the third element of the exposure triangle that must be taken into account as well.

The shutter speeds control the time the light is allowed to act on the film or sensor (For the purposes of this discussion, it can be assumed that when I say film, I am also referring to the digital sensor). Aperture (or f/stop) determines the amount of light passing through the lens. The desired exposure, determined from meter readings, is achieved by setting one combination of shutter speed and aperture. ISO is handled a bit differently depending on whether we are speaking of a film camera or a digital camera.

Aperture Stops

For the technically minded, an f/stop is a fraction that indicates the diameter of the aperture. The f stands for the focal length of the lens, the slash (/) means divided by, and the number represents the stop in use. Thus, if you’re using a 50mm lens with an aperture of f/8, the diameter of the lens opening would be 50mm divided by 8 or a lens opening of 6.25 mm. Fortunately, we don’t have to think about that when we are trying to get the correct exposure.

I won’t get into the technical discussion of the aperture scale, but instead offer a simplified approach to understanding the scale. There is no need to memorize the aperture scale, but if you remember two numbers you can begin to see the pattern. There is one stop between 1 and 1.4 and if you double each of those numbers, you will build the aperture scale 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Each step is one full stop. ½ and third stops exist between each full stop.

Each full stop on the aperture scale will halve or double the amount of light that enters the lens. It is important to note that the aperture values given are actually the reciprocal of the fraction. In other words, f/4 is actually ¼ thus, f/4 is larger than f/16. So, if we stop down from f/4 to f/5.6, we have stopped down one stop and thereby reduced the amount of light passing through the lens by ½. Conversely, going from f/4 to f/2.8 would double the amount of light available.

Shutter Speed Stops

Shutter speed stops are much more simple to understand. Recall that a stop either halves or doubles the amount of light available. In the case of shutter speeds, we are controlling the time that the shutter is open and thereby admitting light. It makes sense then that ½ second shutter speed would admit half as much light as a 1 second shutter speed. Similarly 1/1000 sec admits half the light that 1/500 sec does. So, the scale can simply be a matter of doubling or halving the previous number. 1, ½, ¼, 1/8, 1/16, 1/30, etc. Note that I used 1/30 rather than 1/32. This is for simplicity, particularly when we get into faster shutter speeds. 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30. Shutter speeds between these values would represent partial stops.

ISO

Film speed or ISO is the third leg of our exposure triangle. In the film camera, it is set based on the type film you have loaded and, with the exception of some creative processes, doesn’t change. In the digital world, it becomes a larger factor and can be changed by simply changing the dial. Again, it is a factor of doubling or halving the amount of light. ISO 200 admits twice as much light as ISO 100 and half as much as ISO 400. Of course there is a tradeoff for using a higher ISO and that is grain (film) or noise (digital).

Light Meter – Tying it all Together

The light meter is the heart of the exposure triangle. It determines the amount of light entering and gives values to render a properly exposed photograph.

Before we go too far, let’s discuss the different types of metering patterns. Most cameras will offer optional metering patterns, but not all cameras will offer all of them. Modern cameras rely on matrix metering (or evaluative). Matrix metering uses an advanced algorithm that measures many different parts of the scene and compares it against known scenes to return the exposure. It works well in many situations, but not all and it is important for you to be familiar with your camera to know how well it handles scenes.

Center-weight average meter is another method of metering. CWA reads the entire scene and averages that scene with more weight given to the subject that is in the center of the viewfinder. Again, it works well in most cases assuming that the main subject is in the center of your viewfinder.

Partial and spot metering measures the reflected light from smaller points and are much more useful for precise metering.

If the camera is in a programmed mode, little creativity is required. The meter simply determines what it feels is the best exposure settings and sets them accordingly. Typically, it will try to return a shutter speed fast enough to be handheld. While this may seem the way to go, it really isn’t. The photographer has no control over the aperture or shutter speed and therefore cannot control the depth of field or stop motion.

When the camera is placed in Av (aperture priority) or Tv (shutter priority), the photographer chooses one value and the the meter determines the other. If we’re taking a picture of a flower and we want to make a blurred background, we would chose a larger aperture and then the camera would determine the shutter speed. Similarly, if we are trying to stop the motion of a bicyclist, we’d chose a fast shutter speed and allow the camera to chose the aperture for us.

Manual mode allows the greatest flexibility because we simply set the aperture or shutter speed we want and turn the other dial until our meter indicates proper exposure.

But, does the camera always return the proper exposure. Hardly. The light meter is designed to render a reading based on 18% reflectance. Obviously, not every scene we meter is always neutral. There are times when we need to compensate the reading by adding or subtracting exposure. This can be accomplished by using the camera’s exposure compensation. This is a more complex subject that goes beyond the scope of this discussion and will be dealt with in another discussion.

Creative Exposure

So, now we have a better understanding of how the three elements tie together to give us a proper exposure. While a proper exposure can be obtained by selecting the aperture and corresponding shutter speed to control the light based on the meter’s reading, it is important to note that there are many possible combinations available to yield the same exposure. There are many “correct” exposures for a scene, but only one is the correct “creative” exposure for that scene. For instance, a scene that is properly exposed at 1/1000 sec and f/4 will have the same exact exposure at 1/250 sec at f/8 and again the same as f/16 for 1/60 sec. All three combinations will give the proper amount of light to give the proper exposure, but all three will return a completely different scene and determining what you are trying to convey will determine which exposure is the creative exposure.

Smaller apertures (f/2.8 to f/4) are great for isolating a subject, whereas larger apertures (f/22) are wonderful when you are taking a picture where you want much of the scene to be in focus. Choosing the right aperture will have a major impact on depth of field and what parts of your image are in focus.

Control of shutter speed is important when you’re dealing with motion. That motion can either be the subject or the camera. The slower the shutter speed, the more difficult it is to obtain a sharp image while handholding a camera. Stopping the wings of a bird flying overhead requires a fast shutter speed while conveying the fluidity of a gurgling stream may be more dramatic with very slow shutter speeds.

Creative exposure is where the photographer takes an active role in determining the impact of the photo. Although the photo may be properly exposed at a number of possible combinations, it is that one setting chosen that determines the mood the photographer wants the viewer to feel.

I hope that I have wet your appetite to look deeper into the concept of getting the proper exposure for your shots. It is a much more complex issue than I have delved into with this short discussion. There are many wonderful books on this subject and I urge you to pick one up and learn more about exposure. In my mind, exposure is the most critical aspect of photography. Surely composition and aesthetics are vital to artistic photographs, but an underexposed or overexposed shot that has wonderful composition is still a bad shot.