Posted on 2020-11-14

In last week’s blog on photography fundamentals, we covered the exposure triangle, going through Aperture, Shutter Speed, and ISO, and how each of these variables can impact an exposure. In this week’s blog, we’re going to look at how to measure your exposure and each of these three variables through stops of light. Understanding stops of light is crucial when it comes to getting that desired exposure in any given scene, as it helps you understand how to measure your settings against one another to arrive at the best outcome for you as a photographer.  

So, what is a stop of light? A single stop refers to the amount of change needed in order to either double or halve the amount of light in a given image. When you elect to double the amount of light, you are increasing the exposure by a single stop, and when you halve the amount of light you a decreasing it by a single stop. These measurements can also be understood as + 1 stop and -1 respectively. Manipulating the light in-camera is always a function of the three elements of the exposure triangle: aperture, shutter speed, or ISO. Check out last week’s article to learn the basics (insert link to last week’s article), We have to learn how to measure each of these variables in stops, as it will allow us to effectively manage each setting relative to the others.  

Starting with aperture, widening the lens will increase your exposure, while narrowing it will decrease your exposure. Now here comes the math. To darken your image by a single you must double your aperture, and to brighten your image by a single stop you must halve your current aperture. For example, if I am shooting an image at f2.8, and my result is looking to bright, I would double my aperture to f5.6 (2.8 x 2 = 5.6.). This will have effectively shifted my image -1 stop of light, as it halves the amount of light let in through the lens.  

Next up, Shutter speed, which as we know is measured in fractions of seconds. Some examples might be 1/125, 1/250, 1/500, for example. So let’s simplify the math by only focusing on that bottom number: in order to darken your image by one stop, you would want to double that bottom number, because that would mean your image is being exposed for a shorter duration, and therefore will turn out darker. So, if I were shooting at 1/250, and wanted to darken the image, I would double that denominator to 500. If I wanted to brighter the image, I would halve the denominator to 125. These changes each represent a shift of a single stop in either direction.  

ISO is perhaps the easiest to understand when it comes to stops of light, as it refers simply to your camera’s, or your film’s light sensitivity. Using ISO to brighten or darken an image by a stop simply requires you to either halve or double the ISO. If I am shooting at ISO 400 and I want to darken my image by a stop, I will simply set the ISO to 200. A lower sensitivity to light equals a darker image, and 200 ISO is half as sensitive as 400. Therefore, it is decreased by one stop. And if I wanted to brighten it, I would double that 400 to 800.  

When you understand how each of the variables is measured through stops of light, you have a common measurement that will allow you to adjust one variable and compensate for that adjustment with the other variables. Understanding this basic measurement of light is another key to improving as a photographer, and capturing the best images possible.