The word “exposure” means light entering your camera through the lens and striking the sensor. If too little light enters, the picture is said to be UNDER- exposed, and dark areas — called SHADOWS will lack detail, or even be CLIPPED to pure black. If too much light enters, it’s OVER-exposed, and the brighter areas called HIGHLIGHTS will lack detail, or even be clipped to pure white. Clipped highlights are often said to be BLOWN highlights, or just BLOWN-OUT. Clipping can be likened to a circuit breaker in your home’s electrical system that pops off because of a dangerous excess of current flow. When more light hits regions of your sensor than they can record correctly, they essentially give up and clip to pure white – or to black in the case of dark shadows that receive no light at all. Clipping is almost always seen as a flaw in a picture, and because no detail information at all was recorded in those pixels, clipped areas usually can’t be repaired during post-processing with a photo- editing program.

Sadly, many photographers think so much about these side-effects that they lose sight of the MAIN purpose of each adjustment, which is to manage the light entering our camera in order to get good EXPOSURES. Let’s list each control’s side effect here, and then return to discuss each one shortly: The side-effect of shutter speed is that it controls MOTION-BLURRING. The side effect of theaperture setting is that it changes the DEPTH OF FIELD. And finally, the side effect of ISO is that it affects how much NOISE is in our pictures.Back to exposures now: A simple analogy is often used to illustrate how Shutter Speed, Aperture and ISO relate to each other: In a five-hour course that I teach covering basic photography skills, my slide show includes over 150 slides. I tell classes that one of those slides is by far more important than all the rest. It shows a picture of a woman standing at a kitchen sink filling a glass with water. Water flows into the glass just as light flows into the sensors of our cameras.
The faucet represents the shutter that turns the flow on and off. The longer the shutter stays open, the greater the amount of light that’s allowed to flow. The SIZE of the pipe represents the size of the opening, or APERTURE inside your lens that determines how quickly light can flow into your camera when the shutter is open. Both of those–the shutter and the aperture–control how much light flows into the camera. The third variable is different: The SIZE of the glass that we’re trying to fill with water illustrates ISO, which isn’t about controlling how much light FLOWS, but how much light is NEEDED. Low ISO numbers indicate that your sensor has lower sensitivity and requires more light to expose completely: High ISO numbers indicate more sensitivity — faster exposures — that require less light. As we said, that makes ISO different than the other two light controls, but it’s a mistake to separate them in our thinking. They are the three EQUAL PARTNERS for getting good exposures in our cameras, and every picture that we take involves getting the balance between these three correct.

Everyone can pretty much tell when a picture is too bright or too dark, but what objective measure can we use to when we’ve got a good exposure? Most photographers agree that a photograph is well exposed when it reveals good in each of the three LUMINANCE REGIONS: The highlights, the midtones, and the shadows. Consider a typical landscape scene that includes a bright sky, green shrubs, and dark shadowed areas underneath the shrubs. It’s easy to see each green leaf in the midtones, but what about the highlights and the shadows? If there are clouds in the bright sky, can you make each one out clearly? If there’s an animal sitting in the dark shade underneath one of those shrubs, can you see the animal? Go back and review shots that you’ve already taken. Practice spotting areas where you’ve lost detail in the shadows and highlights because of under or over-exposure. This is a great exercise! Don’t even worry about what you might have done wrong now: Just learn to identify these three luminance regions, and especially the extremes of highlight and shadow. You’ll find that just being sensitive to them while you’re aiming to take a shot will help you to get better exposures in the first place. For example, when framing to photograph a person indoors, you might spot a bright window in the background, and simply take a few steps left or right, until the offending window is no longer in the frame.

Exposures can also be measured by a device that’s built into most modern SLRs today called a HISTOGRAM. Histograms are a simple chart that shows us at a glance where each pixel in our image falls on a scale ranging from pure black on the far left, to pure white on the far right, with 254 shades of gray in between. The number of pixels that fall at each luminance level are represented by a vertical line at that level, and the resulting graph that includes every pixel in your image often resembles a mountain range. If most of the pixels are dark, the mountain will fall on the left side of the histogram. If the image is instead very bright, they’ll favor the right side. Of course there are scenes that are supposed to be dark or light, but a photograph of a typical scene should have a histogram with pixels that are distributed about equally from left to right (as in the example above). Use your manual to learn how to find and use the histogram on your camera, and then practice USING this essential device until you can tell at a glance whether or not you’ve achieved a good exposure.

All of our cameras are equipped with wonderful internal light meters and built-in programming designed to get exposures right the first time automatically, but sometimes the results are darker or lighter than we’d like. The quick way to correct that in our next shot is to use a feature called EXPOSURE COMPENSATION, or just EC. Check your user manual to find out where it can be found on your camera, and how to adjust it. Watch for a scale in your viewfinder that reads zero when the camera thinks the exposure is going to be good, but that can be dialed up to increase brightness, or down to darken the image. One thing to keep in mind about Exposure Compensation is that it makes its adjustments with the exact same exposure controls ; shutter, aperture and ISO–that you’ll be learning to adjust yourself soon. Also keep in mind that EC will make those exposure adjustments using only the underlying exposure control(s) that you have NOT locked in with your manual adjustments. We are a little ahead of ourselves to mention that here, but because the use of Exposure Compensation is a basic photography skill that you’ll use nearly every time you go out, I wanted to draw your attention to it before we get to the more difficult material ahead. By the way, remember that Exposure Compensation is a shortcut device that’s helpful for making quick adjustments to your exposure settings on the fly, but NOT a replacement for knowing how to make those adjustments directly.

All right, here it is: The most important information that every new photographer should know, and also some of the most difficult concepts to learn. There are THREE LIGHT MANAGEMENT CONTROLS that work together in your camera to determine the exposure of every picture you make. One is SHUTTER SPEED, one is the APERTURE of your lens, and the third is ISO (formerly called “film speed”). Each of these values are expressed in different terms, and each one also has its own peculiar SIDE EFFECT that changes how our pictures look.

The key to relating to Shutter speed, Aperture and ISO is to think in terms of whether your picture needs more or less light. From there it’s just a matter of knowing HOW each of the three provide or remove that light, what their terms of measurement are, and how to make the adjustments for each on YOUR camera. I can’t overemphasize how important this part of our program is! Until you master it, moving in the PSAM modes will remain a frustrating mystery to you. Even AFTER you’ve thoroughly memorized all of the exposure terms and concepts, don’t expect it to become intuitive to you overnight.

With that quick overview behind us, let’s take a look now at how the three exposure settings relate to each other in real life. Our cameras can do a great job of getting all three of those right without our help in the automatic settings, but the PSA & M modes give us the opportunity to take control and make some or all of the exposure decisions on our own. Let’s start out with the first one on the PSAM list, which may at first sound like we’re still back in automatic settings!