Billy Sheahan Photography School – Class 101- Exposure Basics

Billy Sheahan Photography Class 101

Exposure Basics

From time to time I’m asked to help emerging photographers learn about their cameras. I’m always happy to do that. When I was just starting out, I was like a sponge soaking up information from pro photographers who I would meet. I haven’t forgotten their generosity.

So in that spirit, I’m offering a bit of online instruction through my blog. It’s my attempt to break down the basics of using a camera on the manual settings, which is really the only way to make sure your photographs look exactly the way you want them to.

This 101 class pertains to Exposure Basics or how your choice of ISO, Shutter Speed and Aperture all work together to create a proper exposure. Let’s begin….

Exposure Basics

Proper exposure. The Exposure Triangle:

•Film Speed (ISO or ASA) – How sensitive the film or chip is to light, such as ISO 100 or ISO 640. The lower the number, the less sensitive to light the chip setting or film is.

•Shutter Speed – The length of time the shutter stays open to let light into the camera, most commonly measured in fractions of a second, such as 1/60th of a second or 1/500th of a second.

•Aperture – How wide open or closed the lens iris is (the lens “pupil”), measured in ƒ-stops such, as ƒ2.8 or ƒ5.6. The lower the ƒ-stop number the more open or large the aperture opening.

In order to make a good photographic exposure, the proper amount of light has to enter the camera through the lens and hit the film or chip sensor. By varying the settings in the exposure triangle, you can make properly exposed image.

I’ve used many different analogies to explain the three elements of exposure, but one that seems make this fairly complicated concept a little easier to understand is to think of a sunbather. How sensitive her skin is to sunlight is similar to the ISO. She doesn’t want to burn too quickly, so she may apply sunblock to slow her sensitivity to sunlight. A high ISO setting is like someone who tans or burns quickly. Not much sun is needed to see the effects. The higher the ISO number, like ISO 800 or 1250 or 1600, the more sensitive to sunlight the film or chip is. The lower the ISO number, like 100 or 200, the less sensitive to light the chip is.

Shutter speed is like the amount of time spent in the sun. Depending on how sensitive her skin is, our sunbather may only need to be in the sun for 15 minutes to show a little color. If she stays in the sun too long, she will burn or overexpose her skin. If she only stays in the sun for a minute or two, she probably won’t get any color at all. Shutter speed is measured in fractions of a second. 1/2000th of a second is a very fast shutter speed. 1/125th or 1/60th of a second is a middle range shutter speed. Anything below 1/30th of a second is considered a slow shutter speed. Sometimes it’s even necessary to keep the shutter open for a second or longer to make a proper exposure.

Think of aperture as a larger than life venetian blind that has been set up between the sun and our sunbather. Try to ignore the fact that she’ll have bizarre striped tan lines as the result of this. Depending on how open or closed the blind slats are adjusted will dictate how much sun falls on her skin. If we close the blinds so only a little light comes through, it will take much longer for her to tan. If we open the blinds, much more sun will fall on her skin and she will tan faster. The aperture opening in a camera lens (measured in f-stops) works the same way. If the aperture is wide open, say at f2.0, more light comes into the camera. If the aperture is closed down, say, f11 or f22, less light will come through the lens.

As you can see, many variables determine whether our sunbather remains pale (underexposed), gets a tan (good exposure) or burns (overexposure). It’s exactly the same with  making a photographic exposure. Based on how sensitive the film or chip is to light, we must determine how long to leave the shutter open and how opened or closed the aperture should be to make sure the proper amount of light hits the film or chip. If our sunbather wants to stay in the sun for an hour, we may have to close down the blinds a bit so she doesn’t burn after all that time in the sun. We must be aware of the exposure triangle: Sensitivity to light, length of exposure, and how much of the light we are allowing to pass through the lens.

The art of the exposure:

Getting the proper amount of light into the camera is only the first step. Sometimes it is necessary to give priority to one of the exposure triangle elements. In a portrait, for instance, the subject is more important than the background. By adjusting the aperture, you can make an image in which the subject is in focus, but the background is slightly out of focus, giving your subject more prominence. In sports, dance or other photography where freezing the action is important, a faster shutter speed would have priority so the subject does not have unwanted motion blur. Each of the elements in the exposure triangle, ISO, shutter speed and aperture not only control how much light is needed for a good exposure but they also change the nature of the image in ways that can affect the emotional impact of a photograph.

Grain and Noise – Using faster film or a higher ISO setting on a digital camera can also affect the look of your photograph. Faster film that is more sensitive to light is usually more grainy than slower film. In digital cameras, the same rule applies. Higher ISO means more noise in your image. Sometimes grain or noise can be perfectly acceptable or even desired. But other times grain or noise can be distracting and make your image less pleasing.

Motion Blur – When we make a still image, we are capturing a split second of a moment in time. Sometimes the subject is moving. Sports, dancing or other subject matter with movement might require more effort to freeze the action. This can be done by making the moment in time a tremendously small sliver of a second. Photographing a runner with a slow shutter speed like 1/30th of a second will probably result in blurry arms and legs because even in that relatively small time that the shutter is open, his arms and legs will have moved from the time the shutter opened until it closed. By using a fast shutter speed, such as 1/500th or 1/1000th of a second, the shutter will open and close so fast, there will be virtually no arm and leg movement in the minuscule amount of time the shutter is open. Sometimes a little motion blur is desirable to illustrate speed or activity. Shutter speed will determine how much or how little there is. Incidentally, unwanted motion blur can also be caused by camera shake by the photographer at the time of exposure. We’ll talk more about that later.

Depth of Field – Adjusting the aperture affects the depth of field of an image. Depth of Field is defined as the distance between the nearest and farthest objects in an image that are in focus. If I am sitting at one end of a long dinner table and take a picture towards the far end of the table, will my wine glass be in focus? How about the flowers in the center of the table? How about my dinner companion sitting at the other end? It depends on the depth of field. More importantly, what do I want to be in focus?

Adjusting the aperture will determine whether one or more of those choices will be in sharp focus. An open aperture, say ƒ2.8 will have a smaller depth of field, enough to only keep the wine glass or the flowers or my hungry companion in focus depending on what you choose to focus on. The depth of field may only be an inch or two at a very wide open aperture setting of ƒ1.4. If you choose to make the flowers the main subject of the photograph, maybe only a few of the flower petals are in focus while the wine glass in the foreground and my increasingly ravenous friend in the background are out of focus.

You might choose to focus on her face at ƒ2.8 and the flowers and my wine glass would then be out of focus. However, if you close down the aperture to ƒ22, increasing the depth of field to several feet, it might be possible to get the wine glass, the flowers in the center of the table and my starving companion in focus all at the same time.

Whether you’re shooting with a long zoom lens or a normal or wide lens will also affect your depth of field, but we’ll also talk more about that in the 102 class. We should eat before she becomes too angry.

Metering the photograph:

There are a lot of things to consider when choosing your exposure settings before making an image. Until you’ve done it for a while, it’s almost impossible to look at a situation and simply guess what the shutter speed and aperture should be set at to get enough light into the camera. That’s where the light meter comes into play. The light meter looks at the lighting conditions and helps you determine, based on your ISO (film speed or chip sensitivity), what your shutter speed and aperture should be.

Most modern cameras have light meters built into them. But you may find yourself using an older camera or in a situation where you need to use a separate hand-held light meter. They both basically do the same thing. They measure light. Let’s look at two different photographic situations, the portrait and the dancer, and compare how we would meter each of them.

The Portrait:

In this image we will make a photograph of a subject sitting in a chair. The most important part of the image is her face, so we want to make sure that is the first thing your eye goes to when you look at it. Everything else in the photograph is of less importance. She is not moving, so shutter speed is not critical here. But since we want to direct your attention to her face, what is in focus is the most important thing. So the aperture setting should get priority in this instance. We want the depth of field to be small. In other words, her face is in focus, but the background should get a little soft, or be out of focus. So we’ll use a very wide open aperture like ƒ2.8. She’s sitting near a window, so we have a good amount of light coming in to light the scene. The camera has been switched on and set to ISO 100, a good setting for a clean image with no grain or noise.

Jillian Ann ©2009

Jillian Ann ©2009

Set the aperture to ƒ2.8 and look through the viewfinder and compose the shot. On most cameras, if you depress the shutter release button half way, you’ll wake up the camera if it is in sleep mode (to conserve batter power) and the light meter in the view finder will activate to tell you whether your settings will let enough light into the camera to make a proper exposure.

Here’s where the exposure triangle comes into play. We’ve already chosen two of the three settings. The ISO (sensitivity) is set to 100. The aperture (lens iris) is set pretty much wide open to ƒ2.8 because we want only a small area to be in focus – her face. We want a small depth of field.

That leaves only the third side of the exposure triangle, the shutter speed. Every camera and light meter are different, but the light meter’s job is the same. It will tell you whether your need to adjust one of the exposure settings because you either have too little light coming through the lens, too much light, or if it’s just right. Looking at the light meter, it will tell us whether we need to increase the shutter speed to reduce the time of exposure if there is too much light, or perhaps to slow the shutter speed down because there is not enough light and we need more time for the light to come into the lens and properly expose the film or chip.

By adjusting the shutter speed on the camera one way or the other, you will see the light meter’s little indicator move back and forth. You want to get it to rest as close to zero as possible. Not too much light which would cause the indicator to move towards +2, or too little light which would show a reading on the other end of the scale near -2. We want to get the light meter to read closest to 0 as possible. Right in the middle.

Once you have determined the proper shutter speed using the light meter, set the focus on her eyes. You usually want her eyes to be the sharpest focus point. Eyes are the windows to the soul as they say. Compose the image. And click.

Nice job.

The above portrait was made of Jillian Ann in my Chicago studio.

The Dancer:

In this image we will make a photograph of a dancer in mid-jump. We want to make sure that she is not a blur, that she is effortlessly floating in mid-air. Because we want to freeze a very small moment in time, shutter speed, not depth of field is the most important part of this photograph. So unlike the portrait we just took, shutter speed gets priority.

For subjects with lots of movement, a shutter speed of 1/500th of a second or faster is usually necessary to freeze the action. We will be shooting this in a studio, and we have big lights to make sure we have enough light to create a good exposure.

Michelle Lee jumps ©2002

Michelle Lee jumps ©2002

Once again, because we want a good clean image, the ISO on the camera is set to 100 so we do not get any grain or noise. And since we want to freeze the action, we’ve already set the shutter speed to 1/500th. That takes care of two of the three exposure triangle settings. Now all we have to do is find the right aperture opening in the lens to make sure we have enough light coming into the lens.

Look through the viewfinder and depress the shutter release button halfway. The light meter will come on and instead of changing the shutter speed to get the meter indicator to zero, we will change the aperture. If there is not enough light (indicator near -2), open up the aperture to a lower number to let more light into the lens, like ƒ4 or ƒ2.8 or even ƒ2.0 if your lens opens up that far. If your light meter says you are overexposed (indicator near +2), close down the aperture to let less light into the lens to ƒ8 or ƒ11 or ƒ22 until your meter indicator says zero. Just right.

One, two, three… jump! Click! Oooh, you’re good at this. She looks like she’s completely stationary, floating in the air.

The above photograph was made of the talented Michelle Lee in Lois Greenfield’s New York City studio. Lois is a dear friend and she lets me use her studio from time to time.

The Shutter Speed/Lens Focal Length Rule:

Now back to that unwanted camera shake motion blur thing I mentioned a little earlier. When you make a photograph, chances are you are hand holding the camera. In other words, you’re not using a tripod or other means of steadying the camera. At some of the slower shutter speeds, depending on how steady your hands are, you may introduce an unwanted shake of the camera while the shutter is open that may seem imperceptible to you but can cause your image to look blurry even though your lens was properly focused. The longer the lens you use, the more telephoto the focal length, the more a slight shake is magnified and can really blur your image.

There is a good rule of thumb to use on shutter speeds when shooting stills. Try to keep the shutter speed number at least as fast as the focal length of the lens. In other words, if you’re using a 50mm lens and you’re shooting hand held, your shutter speed should be 1/50th of a second or greater (probably 1/60th of a second). For a 200mm lens, shutter speed should be at least 1/200th of a second or greater. That is usually fast enough to prevent any hand held shake from making your pictures soft. You’ll have to experiment to see how steady you are. There was a time when I could hold my camera steady down to 1/8th of a second if I was leaning against something and I hadn’t had much coffee that day. Your mileage may vary.

Using a monopod can also be a great way to help steady your camera if you don’t want to carry around a tripod. Easy to carry and not cumbersome, it will make a difference in how steady you can shoot and allow you to lower your shutter speed below the rule of thumb above, unless you are shooting something moving quickly, which will blur at any shutter speed below 1/250th of a second or so.

Congratulations! You have now learned how to use the exposure triangle to not only make a technically correct exposure, but you’ve modified certain elements of the triangle to give you a certain type of image. It’s not a snapshot anymore, it’s a true photograph. It’s art, baby!

Up next time… Billy Sheahan Photography School 102 – Choosing a DSLR (Digital Single Lens Reflex) Camera and Lenses.

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