For a PDF of this article, select : The Exposure Triangle by Marc F Alter

The Exposure Triangle

Many times, the essence of capturing a good photograph is getting the exposure right. As such, exposures can vary from image to image depending on the amount of light that is available at the time you take your picture. Exposure consists of three main elements; F/stop, Shutter Speed and ISO. These three elements are also known as The Exposure Triangle.

F/stop (also known as F-stop or Aperture) is the size of the opening in the lens’ shutter that lets in light. Technically speaking, F/stop is a ratio representing the amount of light let into the camera. Similar to a fraction, this is why the F/stop is often seen with a slash (ie; f/2.8) and why the lower the number, the larger the size of the shutter opening and the more light that is let into the lens. One characteristic is each F/stop (ie; f1.4, f2.8, f/5.6, f8.0, f/11…) doubles the amount of light coming into the camera.  Another characteristic of F-stop is the lower the F/stop, the lower will be the Depth of Field (DOF) while the higher the F/stop, the greater will be the DOF.

Shutter Speed is how fast the camera’s shutter stays open. The slower the shutter speed, the greater is the amount of light let into the camera. Conversely, the faster the shutter speed, the lower is the amount of light let into the camera. Shutter Speed is typically expressed in fractions of a second. Thus 200 means your shutter will be open for 1/200^th of a second while 500 means your shutter will be open for 1/500^th of a second. A fast Shutter Speed will help to capture an image quickly, freezing the action before you, while a slow Shutter Speed will capture an image slowly, allowing “Time” to affect the scene before you.

 ISO is the sensitivity to light for a digital camera’s sensor (or film). ISO is named after the International Standards Organization (also known as the International Organization for Standardization, IOS) and is derived from the Greek term; ISOS, meaning equal. In photography, the lower the ISO, the less sensitive the sensor (or film) will be to the light while the higher the ISO, the more sensitive the sensor (or film) will be to the light.  A higher ISO has a tendency of adding electronic noise (in digital photography) or film grain (in film photography) while a lower ISO tends to improve the quality of an image.

The Exposure Triangle therefore represents the relationship between F/stop, Shutter Speed and ISO. Each of these 3 elements work together to help you control the amount of light you let into your camera and therefore, how you expose your image when taking a picture. If you wish to let in more or less light, you can adjust any of these 3 elements or any combination of them. Which element you adjust and to what degree, will depend on your goal and preference for taking the picture.

Perfect Exposure is the right amount of light you bring into your camera with a given F/stop, Shutter Speed and ISO, which captures the brightest “lights”, the darkest “darks” and all the possible colors and tones in-between. So, what is the best combination of F/stop, Shutter Speed and ISO? It will depend on the amount of light available and what it is you are looking to achieve. Say for example, you are taking a picture of a waterfall with your ISO is set at 400 and your light meter indicates f/8.0 with a Speed of 200. This may be a “good exposure” but may not show the waterfall as best as it could (or as you would like it).

If you wish to blur the water and make it look silky smooth, you could lower the Shutter Speed but this would let in too much light. You can re-gain the correct exposure by lowering the amount of light in your exposure by increasing your F/stop and/or lowering your ISO.

On the other hand, if you wish to “freeze” the water and catch the splashes as they bounce off the rocks, you could increase the Shutter Speed but this would let in too little light. You can re-gain the correct exposure by increasing the amount of light in your Exposure by decreasing your F/stop and/or raising your ISO.

F/stop Standard Scale (Aperture Opening)

…f/1.4       f/2.8            f/5.6    f/8       f/11     f/16     f/22     f/32…

Shutter Speed (in Fractions of a Second)

…8       15        30        60        125      250      500      1000    2000…

ISO (Light Sensitivity)

…2000 1800    1600    1200    1000    800      600      400      200      125…

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More Light                                         Less Light

It is important to note that many times single change in one of these values represents a doubling or halving of the amount of light coming into the camera. Thus, if your camera suggests an exposure (see below), any of these sample combinations would work:

ISO     F-Stop     Shutter Speed (SS)

400     f/8.0         1/200               Camera recommended Exposure

200     f/5.6         1/200               Lower ISO by 1, lower F-Stop by 1 for equal light

200     f/8.0         1/125               Lower ISO by 1, lower SS by 1 for equal light

100     f/5.6         1/125               Lower ISO by 2, lower F-Stop by 1 and lower

                                                  SS by 1 for equal light

Marc’s Tips On How To Get The Perfect Exposure

1. Before going out, make sure your camera is set to your “Default Values” (this is to make sure you always know the setting of your camera when you start)
   1. Shutter Speed = 100
   2. F/stop = F/8.0
   3. ISO = 100 to 400
   4. Exposure Compensation = Off
   5. Bracketing = Off
   6. Exposure Metering Mode (EMM) = set accordingly (I mostly use Aperture Mode)
      1. Auto or Program (Professional) – If you want your Camera to do most of the thinking for you (and if you do not care about the results)
      2. Aperture – If your priority is to control your DOF
      3. Shutter – If your priority is to control your Action (Time)
      4. Manual – If you think you can control the exposure better than your Camera’s technology which cost you hundreds/thousands of dollars.
   7. Other (Your battery is charged, you have a spare battery, you have a memory card and a spare, you have lens wipes, etc).

2. Take a test shot and evaluate the results in your viewfinder
   1. Histogram / Blinkies (Highlight Alert). If you have any Blinkies blinking, there are some areas in your picture that are letting in too much light. You will need to compensate for this (lower your ISO, raise your F/stop, raise your Shutter Speed, add a Filter (ie; ND or VND), set your Exposure Compensation to under-expose, take Bracketed Shots, recompose, etc).

3. After making your adjustments, take another shot and then reevaluate, readjust and retake if necessary.

4. Unless very obviously bad, do not delete any images from your camera until after you have had a chance to evaluate your image on your computer (the camera’s viewfinder is too small, the camera’s Histogram / Blinkies are based on a compressed JPG image, etc).