Exposure is the term that is used to describe the amount of light that is able to be captured by the camera's image sensor.
In order to take good photographs we must have "good light"or maybe a better way of thinking would be the right amount and kind of light. Generally speaking better lighting conditions will equal better quality photos.
Our goal as photographers is to be able to adjust our camera settings so that we can get to the best or ideal exposure for any given lighting condition that we face.
Understanding exposure begins with understanding the basic camera settings such as ISO, Aperture and Shutter Speed and how they relate. Understanding these important relationships will enable us to adjust our camera settings so we end up with the ideal exposure and be able to capture the best possible photo at that particular moment in time.
There are several different visual illustrations that help show the relationship between ISO, Aperture and Shutter Speed and one of them is the Exposure Triangle. Another illustration that can also be helpful is the one of "filling a bucket with water" that is explained on this page.
Understanding Exposure Through The Exposure Triangle
The Exposure Triangle is a graphic that can help us in understanding exposure. It illustrates the relationship of the three important settings that directly affect the exposure and therefore the quality and type of image we hope to capture.
As we understand how these three settings interrelate to each other we will be better able to adjust our camera settings and capture the best possible photo for any lighting condition. Adjusting the ISO, shutter speed, or aperture in relationship with each other will help you get the correct exposure while minimizing negative side effects, such as digital noise or motion blur.
Understanding Exposure---ISO, Shutter Speed, Aperture
ISO = Light Sensitivity of the Camera Sensor
ISO is a term originally used to describe the light sensitivity of camera film. It is what was referred to as "film speed".
In digital photography one of the camera settings used to control exposure is the ISO setting which lets us choose how sensitive the image sensor is in relation to the amount of available light. In low light situations we increase the ISO setting on the camera so the sensor effectively becomes more sensitive to light and allows us the opportunity to take pictures in low-light situations. The higher the ISO number means the less light is needed to capture a picture. Yet as we see in the Exposure Triangle above, a higher ISO setting also brings with it more digital noise. Therefore it is always best to use the lowest ISO setting possible that allows us to get within the ideal exposure range.
Just as using a faster film speed used to result in more "film grain" on the printed pictures, a higher ISO will result in more "noise" in the picture as well. Fortunately there are several different software solutions that help remove or "clean up" digital noise so today's modern digital cameras can capture amazingly good photos even at the higher ISO settings.
Shutter Speed = How Fast The Shutter Opens And Closes
The shutter is what controls the amount of time that light is allowed to expose the image sensor on the camera. A slower shutter speed means the sensor is exposed to light for a longer time. Therefore less available light means a slower shutter speed will likely be needed. This can be an issue because the slower shutter speed also means there is greater risk for motion blur either because the subject moves or because of camera shake.
Aperture = How Large The Lens Opening Is
The aperture is similar in function to the Iris in our eye. It is a variable opening in the lens that controls the amount of light that is allowed to impact the image sensor. The aperture setting on the camera refers to how large the iris opening in the lens is. The aperture of the lens is measured by a series of numbers known as "F-Stops".
What is important to remember about aperture settings is that the higher the number the smaller the aperture is. Therefore if you have an aperture range of F-2.8 to F-32 the largest lens opening and the most amount of light the lens can pass on to the sensor would be at F-2.8 while the smallest opening and the least light would be F-32. So on a bright day with a lot of available light you need a larger F-stop so the opening is smaller allowing the correct amount of light to hit the image sensor. On the other hand when in low light situations the lens needs to be fully open and allow as much light as possible to reach the sensor. This would be obtained by selecting a lower F-stop such as F-2.8.
Setting the correct aperture is also important because it directly affects the depth of field of the camera. A larger lens opening (smaller F-Stop) will result in a reduced depth of field which might or might not be a problem depending on what your subject is. A F-Stop of 2.8 will result in a narrow depth of field and the possibility of one part of the subject or picture being in focus while the other part is out of focus.
Understanding aperture is important to good photography because it allows us to control the available depth of field and helps us make sure the focal point of our picture is in focus.
Understanding Exposure---The Relationship Between ISO, Shutter Speed And Aperture
I created the graphic below as a way of showing how these three important settings relate to different light conditions as well as the possible negative affect they can have on overall picture quality.
Understanding how these camera settings interrelate is very important in order to be able to take the best possible picture in all situations.
I hope that you have found this explanation of the Exposure Triangle helpful in better understanding exposure as it relates to digital photography.