A Guide to Understanding Camera Stops
To create a correct exposure, three factors come into play -- aperture, shutter speed and ISO. Collectively, they are referred to as the exposure triangle. Each setting has specific creative applications. They work together, mathematically, to create an exposure. When adjusting any one setting, you are changing the amount of light entering the sensor. To maintain the balance, another setting must be adjusted proportionately.
A stop is the doubling or halving of an exposure setting. One full stop up means doubled, one full stop down means halved. Two stops means quadrupled from your original setting (doubled then doubled again). To move one stop, you click the adjustment wheel on your camera left or right three times. You will become a master of counting to three! When you "stop down" the aperture, you move from for example, f/4 to f/5.6. To "stop up" the ISO, you would move from 100 to 200.
Each setting has its own set of numbers to memorize:
- The Aperture is the size of the opening that lets the light into the camera. The full stops are f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
- The Shutter Speed is how fast the shutter opens and closes. The full stops are 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000
- The ISO is the measure of light sensitivity. The full stops are 100, 200, 400, 800, 1600, 3200, 6400, 12,800
The photo above is recorded at a proper exposure with the following settings: f/4 1/4000 ISO 100
If I wanted the edges of the flower to be more in focus, I would adjust the aperture by stopping down one stop to f/5.6. If I did that, I would also have to adjust one of the other two settings by one stop to maintain exposure. I could either adjust the ISO to 200 or adjust the shutter speed to 1/2000.
Here's a graphic to visually show how the settings are related. Let's make up some numbers as further examples to see the relationship in action.
- A photo has a correct exposure at f/11 1/60 and ISO 100. If you stop up the ISO one stop to 200, you would have to change either the shutter speed to 1/125 or the aperture to f/16 accommodate.
- A photo has a correct exposure at f/4.0 1/250 and ISO 400. Let's say you're photographing a waterfall and you want the water to be smooth and silky, blurring motion. You adjust the shutter speed to 1/60. You now need to change the ISO to 100, or change the aperture to f/8. OR change the ISO to 200 and the aperture to f/5.6.
- A photo has a correct exposure at f/8, 1/250 and ISO 400. You are photographing a runner, using a 300mm lens. You want to isolate the runner from his surroundings. To do that the aperture needs to move two stops to f/16. To compensate for this change, you want to adjust the shutter speed, but you're not using a tripod so you can't adjust it to lower than the length of the lens, otherwise you'll get camera shake. (The closest shutter speed equivalent to a 300mm lens is 1/250). Your only choice is to adjust the ISO to 1600.
There are at least six possible combinations that will result in a proper or correct exposure for any one photo. The freedom we have as the photographer is to determine what is most important to us to convey our message, and make a creative exposure. The camera is pretty smart in auto mode. But it can't know what your intentions are when photographing a scene. Photographing in manual mode and understanding the exposure triangle will allow you to create the images you envision!
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