The Exposure Triangle
For a PDF of this article, select : The Exposure Triangle by Marc F Alter
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 1/200th of a second while 500 means your shutter will be open for 1/500th 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…
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
Keywords: Aperture, F-Stop, ISO, Light Sensitivity, marc alter photography, marc f alter, mfa images, Perfect Exposure, Shutter Speed, The Exposure Triangle
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