A photo is created when light passes through the lens and hits the photosensitive surface, which could be film or an electronic image sensor, such as a CCD or a CMOS chip in the case of digital photography. When taking a photo, there are three main elements that will control the exposure, or amount light that will fall on the sensor. These three elements are Shutter Speed, Aperature and ISO.
SHUTTER SPEED
Your shutter speed setting determines how long the shutter will remain open when you take a photo. A longer opening will allow more light to pass through the lens, where a shorter opening will be less light.
The important thing to know about shutter speed is it controls the movement in your photography. A very fast shutter speed will stop action, where a lower setting will blur your subject. Depending on the type of photo you are taking you may want a blur if you are shooting a waterfall and you want the soft look of water moving, then you might use a shutter speed of 1 sec or less. But if you are taking photos at a tennis match, you'd likely want to stop the action and have a nice clear shot of the player in the air with their raquet connecting with the ball, and in this case you might shoot it at 1/500 or higher.
If you find your photo is too dark and you are shooting at 1/500 at an indoor arena, and you really want to stop action on the ice, yet not compromise the exposure. You now have to make adjustments using the the other two elements to get the balance you require. The image below demonstrates how increasing shutter speed will ultimately reduce the light on your subject, and under expose your image.
APERATURE
Your aperature setting determines how long the shutter will remain open when you take a photo. A longer opening will allow more light to pass through the lens, where a shorter opening will be less light.
The aperature is similar to the human eye, when we are in a dark room our pupil gets larger to let in more light, and the opposite happens when we are in bright sun. Imagine shooting outside on a really sunny day - would you want to let in a lot of light or reduce the light getting into your lens? The rule of thumb is f/16 or higher on a bright sunny day. On the other hand, if you were in a dim room, there wasn't enough light to take a photo, then lowering the aperature to f/1.4 might solve your problem.
Another amazing feature about aperature is it allows you to get creative by changing depth of field (DOF). The image below shows you how much or little of the image is in focus. A shallow depth of field shown in f/1.4 shows a very small area in focus and a wider depth of field in f/16 give a very clear photo of the entire tape measure. Typically in photography, a wider depth of field would be used to shoot landscapes, and the lower f/stops are reserved for portraits to get the nice blurred backgrounds behind your subject.
ISO
The last element that affects light in photography is the ISO and this controls the sensitivity to light on your camera's image sensor. The higher the ISO number, the more sensitive the sensor becomes to allow more light and the opposite of the lower numbers.
When selecting an ISO setting, it is important to know that there is a risk in using a very high ISO. The optimal ISO is 100 to produce a nice crisp photo. As you go higher in your settings you will start to add digital noise or grain to the image. Although cameras are getting better at producing better quality photos at higher ISO's, I safely keep within 100-800 ISO, and rarely ever push my ISO over 1000. Removing the grain in Adobe Lightroom can help improve the photo, but will never be as good as if the photo had been taken at a lower ISO.
IN CONCLUSION
When planning your photos, there are three technical details to be considered that will ensure your photos are properly exposed.
Aperture controls how large the lens opening will be.
Shutter speed controls how long the aperture will be open.
ISO controls the camera sensor’s sensitivity to light.
In dim conditions, you want to let as much light in as possible. This means setting a large aperture (small number), a slow shutter speed, and a higher ISO. An example might be f2.8, 30 seconds, ISO 5000.
In bright conditions, you want to reduce how much light gets into your camera. This means setting a small aperture (large number), a fast shutter speed, and a lower ISO. An example might be f22, 1/2000th of a second, ISO 100.
But always keep in mind there are other factors that affect the creative outcome of your photos. Aperture controls the depth of field (focus). Shutter speed affects whether the motion is frozen or blurred. High ISOs introduce grain into the image.
When shooting in manual mode, there are three ways to control exposure, and you don't have to rely on one setting controlling your light. Use an optimal combination of all three settings and you will quickly be in control of your camera and instinctively know what you need to get the best shot, on the fly.
This video goes into great detail in explaining how the 3 pillars of photography work together to produce great photos!
The Exposure Triangle - image source
Here are some great websites to learn about digital photography and download useful cheat sheets and diagrams:
- Digital Photography School - Expert Photography - Photography Life - Capture the Atlas - SLR Lounge - The Exposure Triangle