The Exposure Triangle

Three variables are used to control light intensity the image sensor sensitivity (the ISO) : the lens aperture, and the shutter speed. Collectively, these make up the “Exposure Triangle.”

Exposure Triangle

ISO

In traditional photography, (film) an ASA value describes how sensitive the film was to light. The higher the ASA, the “faster” the film, or the more sensitive it was to light. Likewise, the lower the ASA, the less sensitive it was to light. So, in addition to choosing the proper color balanced film for you shooting adventure, you also needed to select the right speed. If you were shooting an outdoor sporting event in mid-afternoon, you would need a film capable of stopping action in bright sunlight. If the event is indoors, the light intensity would not only be different, so would the color. This often meant swapping rolls of film or carrying around two or more camera bodies with different films in each: one for outside shooting (Daylight film of ASA between 50 and 200) and one for inside (Tungsten film with an ASA or 200-800) Sound complicated? It was, but all that changed with the development of modern digital photography where a couple of adjustments on the fly sets your camera up for outdoor or indoor shooting at the right “film” speed. Instead of capturing individual frames on a length of film, digital cameras also capture individual image, but store them digitally on a memory card. They also allow the characteristics of those images to be set individually.

Note

ISO stands for “International Standards Organization,” which develops and publishes international standards and is headquartered in Geneva, Switzerland. ISO replaced ASA, “American Standards Association.” Kodak developed ASA in the 1940s as a scale for film speed. The higher the ASA rating, the “faster” the film could capture an image.

The same principles apply to ISO settings as in film photography – the lower the ISO value the slower the image sensor reacts to light but less noise. The higher the ISO value, the quicker the sensor reacts to light. With a digital camera, you can adjust the ISO for each frame you shoot.

Note:

When you increase the ISO, you make the sensor more sensitive to light, which also makes it more sensitive to its flaws – increased contrast and noise.

ISO settings also affect the quality of the final image. Microscopic crystals and dye layers were used to capture images on color film. Slower film had smaller crystals (grains). Faster film had larger grains, but they worked better in low-light conditions. The grain difference became noticeable as you made larger prints from the negatives. The key to success was choosing the right speed film for the type of photography.

In digital cameras, “noise” is the digital grain. Images shot at lower the ISO settings exhibit less noise. Higher ISOs are noisier. Like film, high ISOs are often used in low-light conditions, or to capture moving objects. An example is an indoor sporting event when you want to freeze the action of a basketball player the moment he/she releases the ball. Higher ISOs are also necessary for shooting subjects such as campfires, or the beach just after sunset. Keep in mind that the higher the ISO you choose the noisier shots you will get. With digital cameras, you get immediate feedback as you experiment with camera settings, something that was not possible when shooting film.

Note

You can see the noise when you enlarge an image. It shows up throughout the image; although, it is more pronounced in the darker regions. The challenge is to select the right ISO that minimizes noise while offering the best opportunity to capture the image. Since the ISO can be adjusted for every image, this is far easier to do than change rolls of film.

The following images illustrate ISO noise. The image on the left was shot using existing light. The highest ISO setting combined with a slower shutter speed captured the image without using a flash. The aperture was set wide open. The image on the right had a higher ISO setting in order to capture the energy flashes as my hand touched the glass orb. The image to the left has more pronounced noise than the image to the right. Adjusting the ISO and shutter speed made both possible.

Aperture/Depth of Field

Aperture refers to the diameter of the lens diaphragm (iris). The diaphragm is a series of “leaves” placed inside the lens. The Aperture has two functions; control the amount of light entering the camera and control the depth of field in the image.

The smaller the aperture number the larger the diaphragm opening allowing more light reaching the image sensor (f1.4 allows more light to enter the camera than f5.6). The following diagram from illustrates aperture settings:

Aperture

Depth of Field, or the amount of image area on the sensor that is in focus, is the product of “Circles of Confusion.” (Yes, that’s what they are really called.) Think about when you were a kid and used a magnifying glass to focus the sun on a spot on the ground. When you were in focus, the sun was reduced to a small circle. Now, think of your camera lens. When you focus the lens, you are creating a small dot on the image sensor; however, unlike moving the magnifying glass, the aperture setting controls the dot size. To make it even more confusing, there is an infinite number of dots striking the image sensor for each image. These dots cross each other. The larger the aperture setting, the larger the dots and the more they cross. The smaller the aperture setting, the smaller the dots and the less they cross. Photographers use this to control how much of the image is in sharp focus…all of it, or only part of it. By combining in focus and out-of-focus portions of the image area, photographers can focus the viewer on their subject.

The focus was on the poppy in the left image, with a relatively large aperture opening to throw the background out-of-focus. If the background was in sharp focus like the poppy, the flower’s impact would have been lost. In the right image, the aperture was set so that the entire image was in focus. The contrast between the feather and the sand and the desire to ensure the entire feather was in sharp focus meant using a smaller aperture opening.

Shutter Speed

While ISO sets the sensitivity of the image sensor and the aperture controls both the amount of light entering the camera and the sharpness of the image, shutter speed controls the duration of the light on the sensor itself, or the exposure.

The shutter is located in the camera body between the image sensor and the lens. There are two types a copal square (vertical) and a focal plane (horizontal). They both use two components to the shutter and both operate the same. The first component opens the shutter and the second closes. Think of it an open window traveling across the image sensor. That opening is referred to as “shutter speed.”

Note

A number of cameras do not use a focal plane or copal square shutter. However, the function of the shutter is the same, control the duration of the light striking the image sensor.

Most cameras have a built-in shutter speed range from 30 seconds to 1/4000 of a second. Higher end cameras have a broader range. Some cameras have provisions for attaching an external shutter release allowing you to shoot extremely long exposures.

Slower shutter speeds can give the illusion of movement (left image), while higher shutter speeds are best for freezing movement (right image). Both of these images were taken of the same location to illustrate how shutter speed can be used to create two different pictures of the same subject.

Summary

Determining the correct ISO/aperture/shutter speed combination for the image you want to take often takes a little experimentation. The wonderful thing about digital cameras is you get immediate feedback. So, go out and experiment. The more you learn how to use your camera controls, the better your images and the more fun you will have with photography.

It wouldn’t be a problem if we could control the sun, but that’s out of the question. To find a solution, first we need to know more about light itself. The sun isn’t the only light source. What about all those other light sources - house lamps, streetlights, campfires, or candles?

Two things happen when an object moves from cold to hot. A big chunk of iron bar gives off no heat or light. As it is heated, the chunk of iron starts glowing red, then orange, then yellow, then white, and on up to blue. The temperature change is measured in degrees Kelvin. Photographers use that same temperature scale to identify the “light temperature” of a scene being photographed. A representation of the Kelvin scale showing radiant heat (bottom numbers) and corresponding light description (top row).

Note:

This chart is not law. It is only a representation of how the color of light changes in comparison to the degree Kelvin.

In the old film days, color film was designed for daylight. Various densities of warming or cooling filters could be attached to the front of a camera lens to compensate for the color of the light falling on your subject. It took a lot of trial and error to know which filter to use and that meant developing the film before you realize that you needed a cooling filter and you used a warming filter. Digital cameras have made color correction far easier.

Color film was designed for specific light sources....daylight, tungsten lamps, etc. If the light was different than what the film was designed for, such as using tungsten film in daylight film, the color of the image would be distorted without the use of special filters designed to shift the color temperature. Film photographers either limited where they shot or carried an array of filters to compensate for incandescent lights, tungsten lamps, the different types of fluorescent lights (tubes, CFLs, etc.), mid-day sun, etc. LED light was not a problem then. Compensating for the different types of light was difficult when printing the images because not every shot on a roll of film was taken at the same location under the same lighting conditions.

The good news is today’s digital cameras, even inexpensive point-n-shoots, automatically compensate for changes in color temperature through a concept called “white balance.” White balance is a term from the film industry where a white sheet of paper was used to set the “white balance” of a video camera prior to shooting. When the white balance was set, the sheet of paper would appear white in the final image. If the white balance was not set properly, the sheet of paper would have a color overcast as would all the video. Most of the time, the color overcast was virtually impossible to remove completely.

Most digital cameras have the ability to set the white balance automatically without the need to hang a white sheet of paper in front of the lens. Setting the white balance to “Auto” is sufficient, especially if you are shooting under a variety of lighting conditions and do not intend to hang your images in an art gallery. However, experiment with the other color settings (daylight, shade, cloudy/sunset, tungsten, white fluorescent, or flash) to see how each affects the final image. For example, choose “Cloudy/Sunset” the next time you try to shoot a Lake Michigan sunset, or “Daylight” the next time you shoot outside on a sunny day.

Typically, digital cameras offer a number of color corrections in addition to a manual white balance control (AWB). Together, they help the camera see "white" regardless of the light source. In most cases, these settings are:

The two images of the dancer statue were both shot with subdued daylight filtering in and large tungsten lamp chandeliers. The difference is the one on the left had the white balance set to “Auto,” and the one on the right had it set to “Tungsten.” Even though the sun was the dominant light, by compensating for the tungsten lamps the overall image is closer to the true color.

Today, the logging boom is only remembered in pictures and the Bed and Breakfasts and art galleries of “Logger Barron’s Row,” the magnificent mansions they built around the turn of the last century. Small manufacturing operations, tourist related businesses, and the largest charter fishing fleet in Western Michigan have replaced the sounds of chain saws and saw mills. Downtown Ludington is lined with brick-front turn of the century commercial buildings, now filled with coffee shops, small specialty stores, art galleries, craft shops, restaurants, craft breweries, and pubs.

An excellent city beach and the walk out the North Breakwater to the Ludington Light draws thousands of visitors each summer. Ludington State Park with public access to Hamlin Lake and the incredible fall colors bring even more visitors to the area. Nearly all of them have some type of camera, from simple smart phones to professional DLSRs….all taking pictures of the Ludington Light, the beaches, the rivers or the forests. Many of those images with Lake Michigan in the background have the inland sea running uphill or downhill.

NEWSFLASH….LAKE MICHIGAN IS ONE OF THE WORLD’S LARGEST BODIES OF FRESH WATER. IT MAY HAVE INCREDIBLE WAVES, ESPECIALLY DURING THE WINTER STORMS, BUT IT DOES NOT RUN UPHILL OR DOWNHILL.

The good news is there is a simple way to keep the lake level, or any horizon level for that matter. We all know how to play tic-tac-toe. You draw four lines on a piece of paper and put Xs or Ox in them. Next time you take a picture of Lake Michigan, draw an imaginary tic-tac-toe grid in the viewfinder. Use the lines to help find that elusive level horizon. Then, when you show off your pictures of that fabulous vacation, instead of “nice picture,” you’ll get a “WOW, that’s really beautiful.”

Notice that the horizon in the first image is lower on the right, making it look like the ship is sailing uphill. By using the tic-tac-toe grid, I leveled the horizon creating a better image. Also notice also that in the upper right image, the Ludington Light is tilted clockwise. Do not use the light as a reference to level your horizon. In the late 1960s the light’s foundation shifted. It has since been stabilized so there is no danger of it falling over, but since that time, the Light has a slight tilt to the north. Depending on where you take your picture, you may or may not see the tilt. Know that it is there.

When you have a lake to help level the horizon, use vertical elements. For example, I used the bins of Asparagus as my reference in the left image and the “SELF SERVE” shelf as my reference in the right image. The result is the image on the right is more realistic. At least the honey and Maple syrup bottoms won’t slide off the shelf.

So, next time you get ready to shoot that fantastic picture, take a second to make sure the horizon is level.

I lost count on how many times I’m approached by individuals who look at my camera/lens setup and say “you must be able to take great pictures with that camera.” Honestly, the quality of the camera and size of the lens really has little to do with the quality of the image. The camera and lens are just tools.

Back in the early 1980s, I worked at two photographic labs while putting myself through the University of Alaska Fairbanks. One was in the Elmer. E. Rasmuson Library. It was directed by Richard Veazy, a great guy who taught me a lot about photography.

A fellow “student” employee and I used to comment that we could take better pictures if only we had better cameras. “A camera is only a tool,” replied Richard emphatically. “Is an expensive wrench better than a less expensive wrench of the same size? The important thing is knowing how to use that tool.”

While our comments were meant in jest, Rich was right. A camera is only a tool a photographer uses to create art, much like a painter uses a paintbrush, a sculpture a chisel, or a glass artist a furnace and blowpipe. It is the skill of the artist (or craftsperson) using those tools that create the art. Your present camera may limit the subject matter, but buying a more expensive camera or bigger lens won’t make you a better photographer. Learning what makes a good photograph will help you use the tools you have to make a better photograph.

There is another benefit to learning what goes into making a good photograph. It will help you decided if you need better equipment and help guide your purchase.

This is only the beginning. Later blog entries will discuss everything from how and where to sign your prints, to demystifying the exposure triangle, and tricks to keep that horizon straight.

PS: This is my first blog entry for my website, so it is as much of an experiment in discovering how it works as it is the beginning of a conversation with new and experienced photographers.