Every camera has an exposure level indicator. It's usually on your viewfinder as -3..2..1..0..1..2..+3 with a small indicator below that changes based on the lighting of your scene. A 0 on the expose level indicator means you have the correct exposure for your shot. The numbers to the right and left of the 0 are known as stops. The right being X stops over exposed and the left being X stops under exposed. So what exactly is a stop?
Every time you take a photo there are 3 different settings at your control: Shutter Speed, Aperture, and ISO. Each setting must be properly set for you to achieve a correct exposure. A stop lets you compare these settings to each other. Every stop is either halving or doubling the light of the previous stop. In other words: if your exposure level indicator showed 0, but you changed your settings for the shot and it now shows -1, you now have half the light you had before hitting the sensor. Let me give you an example...
If I know I want my ISO set to 100, but by changing my shutter speed I halved my exposure by one stop (0 to -1), I now have to lower my aperture to let in more light to get back to normal exposure.
Since a full stop is always half or double the amount of light, let's look closer at Shutter Speed, Aperture, and ISO to see how these effect the amount of stops added or removed from your exposure.
Shutter speed's can typically be increased/decreased by 1/3rd of a stop. In other words, you would have to increase/decrease your shutter speed 3 times to go up or down one full stop (1/2 to 1 or 1/60 to 1/125). Just note: a faster shutter lets less light enter the camera. Changing your shutter speed to a achieve natural exposure is ideal when you want to keep your ISO were it is and have a specific depth of field (aperture) in mind. Also, if you didn't already know, when using a slow shutter speed it's best to use a tripod.
ISO is the easiest to understand since it typically doubles or halves when you change it. The higher the ISO, the more sensitive your sensor is to light. Changing your ISO to achieve a natural exposure is ideal in low light situations or when you know the exact shutter speed and aperture you want to use.
Aperture is represented by f-stops. Just note: the lower the number, the wider the aperture. The wider the aperture, the more light that hits the sensor. There are two ways to understand stops when it comes to your camera's aperture. The simple way of looking at it is by knowing most camera's apertures can typically be increased/decreased by 1/3rd of a stop (like your shutter speed). In other words, you would have to increase/decrease your aperture 3 times to go up or down one full stop. The more complicated way of looking at it is by the f-stop numbers themselves. As confusing as it sounds, changing from f/4 to f/5.6 is actually a decrease of 1 full stop. When it comes to calculating a one full stop change in aperture you multiply or divide your current f-stop by the square root of 2 (roughly 1.41). That's why 4 * 1.41 = 5.6. Just note: To decrease the amount of light by 1 stop, you multiply by the square root of 2. To increase the amount of light by 1 stop, you divide by the square root of 2. I will explain this further in an upcoming post. In the mean time, learn to accept it since it's not going anywhere by the looks of it! Changing your aperture to achieve a natural exposure is ideal when you want to keep your ISO were it is and aren't too concerned about the change in depth of field, but need a specific shutter speed.
Knowing how your Shutter Speed, Aperture, and ISO effect your exposure lets you know how to deal with every situation. For example: if you are trying to take a picture of anything sports related and notice your shots are coming out blurry, you will want to increase your shutter speed. Now your options are to either lower your aperture or raise your ISO to bring your exposure back to normal. Good luck and Happy Shooting!
If you have any questions, comments, or additional information please let me know below!