The Exposure Triangle: Shutter SpeedShutter speed is exactly what it sounds like—it's the speed the image is being taken at. Photography is all about light, and when the shutter is open, light is being let into the camera and a picture is being taken. If the shutter is only open for a very short period of time, not much light is being let in. When the shutter is held open for a long time, more light is let in. During the day, faster shutter speeds are no problem, but at night, if your shutter speed is too fast, your image will be dark. Any motion that occurs while the shutter is open will create blur. Sometimes, this can be used to create different effects, but most of the time, faster shutter speeds are best for moving subjects.
The Exposure Triangle: ApertureShutter speed is how long the lens is open, aperture is how wide that opening is. A f1.8 aperture, for example, is a very wide opening, while f/11 is much narrower. Since a very wide opening lets in more light, it's often used with more limited lighting.
Along with controlling the amount of light that comes into the lens, the aperture affects depth of field, or how much of the image is in focus. Smaller apertures—that f1.8 that is super wide, for example—leave smaller portions of the image in focus, making the background soft. Narrow apertures are often used for portraits and close-ups. Larger f-numbers, or wider apertures, leave more of the image in focus and are often used for landscapes.
The Exposure Triangle: ISOThe third camera setting that affects exposure is ISO, or the light sensitivity. High ISO numbers, like 3200 for example, mean the camera is more sensitive to light and the image will be brighter than if all the same settings are used except the ISO is set to 100. The trade-off to using higher ISOs is that it introduces noise to the image. Here's a visual representation of what noise is:
|ISO 100 (Cropped) ISO 12800 (Cropped)|
The Exposure Triangle: Putting it all togetherTo get the appropriate exposure, all three corners of the exposure triangle must be working together. What combination of the three you use depends on what you are shooting and the effect you are looking for. Here are a few different scenarios, what three settings were used in each case and why.
I wanted the background in this image to be soft, so I used a f2.8 aperture, the maximum allowed by the camera that I was using at the time. To freeze the motion and because there was plenty of light, I used a 1/1600 shutter speed. To balance it all out, I used an ISO of 320, since again, there was plenty of light.
This image was also taken in the middle of a sunny day, but I didn't want it to look like it. I loved how the light was filtering through the taller trees just enough to hit the only tree still holding on to its leaves. To get this effect, I exposed for the leafy tree, which left the background dark. I started with the aperture at f/4. I wanted the line of trees to be visible in the background, but not too sharp (plus the lens I was using wasn't very fast anyways). I used a 1/1250 shutter speed, again to expose for the path of sunlight in order to leave the rest of the shady forest dark. To minimize noise, and since I wanted the background to remain dark, I used ISO 200.
Inside this dimly-lit coffee shop, I used a f3.6 aperture to have some depth of field while still keeping all the little details clear. Because of the limited light, I used a 1/32 shutter speed. To exposure correctly with these two settings, a 800 ISO was used.
Exposure isn't a single element in photography, it's three. The key is to make all three elements work together to get the exposure right, while also keeping in mind elements like depth of field, motion blur and noise.