The exposure triangle is made up of Aperture, ISO and shutter speed. Each of these elements need to work together to create a correctly exposed image. When you are shooting in manual mode, you’ll need to control each element of the exposure triangle in order to get the exposure you want.
ISO controls your camera’s sensitivity to light. You’ll use a low ISO, like 100, in bright light situations and a high ISO, like 8400 in very low light (with no flash). It’s true that high ISO images can be noisy or grainy, but a correctly exposed image taken at at a high ISO will likely be less noisy than an underexposed image at a lower ISO. Depending on your camera’s capabilities, shooting at high ISOs can enable you to capture amazing images in very low light. The below image was taken at ISO 4000 to allow a fast enough shutter speed to keep the image in focus.
Aperture refers to the lens opening and is measured in stops of light called f/ stops. At a wider aperture (low f/ stop, like 1.4 or 1.6), your lens is opening is larger to let in more light and at a narrower aperture (high f/stop, like 11 or 22), you lens opens less to let in less light. A wide aperture, such as f/1.8, creates a narrow depth of field and blurred background (called bokeh) and a narrow aperture, such as f/11, creates a wider depth of field where more of the image will be in focus. This image was taken at f/4.5 because I wanted draw attention to the flowers and hands and leave the girl and background out of focus. I then adjusted the ISO and shutter speed to accommodate for the wide aperture.
Shutter speed is the amount of time the camera’s shutter opens to let in light and expose an image. Generally you want a higher shutter speed to freeze time and a slower shutter speed to show motion. For example, when I photograph my moving children I don’t let my shutter speed drop below 1/200 but if I were photographing a girl twirling I might slow my shutter speed to 1/80 to show some motion. Here’s an image that was taken with a shutter speed of 1/1250 to freeze motion. I set the aperture at f/2 with an ISO of 250.
As in the above examples, I begin by choosing the one element of the exposure triangle that is most important to me for creating the look I want. If I’m shooting in low light, that element will usually be ISO because I will need a greater sensitivity to light in order to achieve an in-focus image (since I mainly use natural light). But if it’s a sunny day or if I have plenty of light, I look at the conditions, environment and my subject and decide what is most important. Many times aperture is most important because I either want to isolate my subject from the background or I want everything in focus. So I choose my aperture and then adjust my ISO to allow for a shutter speed of around 200 or more. If shutter speed is my priority, I set it as high as I need and then adjust my ISO and aperture using my camera’s light meter.
If you aren’t comfortable shooting in manual mode, you can use your camera’s creative modes to achieve proper exposure while gaining control over the element of exposure most important to you. Try AV (aperture priority) or TV (shutter priority).