Aperture and shutter speed are two parts of the exposure triangle. ISO is the third. While all three work together to create the desired exposure and effect, ISO is easier to isolate and explain separately. There is really no way to discuss shutter speed and aperture without each other.
When you think about choosing the proper shutter speed, you are thinking about gaining the proper exposure while selecting a shutter speed that is: fast enough to stop action, slow enough to blur action, or going to balance the aperture (f-stop) you choose.
Shutter speeds double (approximately) as you move from a slower shutter speed (1/30) to a fast shutter speed (1/2000). As the speed of shutter doubles, the amount of light let in is cut in half. This halving, or doubling of light if you are moving to slower shutter speeds, is called a stop of light. To compensate for the lesser amount of light you can change the ISO, but we have already talked about how higher ISO can degrade your photo. The other option for creating the proper exposure is to change the aperture.
When deciding on the proper aperture, you must be aware of proper exposure and deciding how much of the photo you would like in sharp focus. Depth-of-field is the term used to indicate how much of the photo is in focus. A shallow depth-of-field indicates that only a small portion image is in focus while a deep depth-of-field indicates that nearly all of the photo is in focus. A shallow depth-of-field is achieved by selecting a larger aperture (smaller number such as f 2.8) while a deep depth-of-field is achieved by selecting a smaller aperture (larger number such as f 16).
Similar to shutter speed, as you move along the aperture scale, light is either halved or doubled depending on whether you are selecting a larger (smaller number) or smaller (larger number) aperture. Each halving or doubling of light is called a stop of light. So if you reduce the exposure by selecting a faster shutter speed, then you will need to compensate by choosing a larger aperture that will let in more light.
This is probably getting a little confusing and, if you are anything like me, you are ready for a visual. Let’s start with a reminder about some common aperture’s and common shutter speeds.
Now let’s take a look at a diagram showing some equivalent exposures. Each change in shutter speed represents a stop of light and each change in aperture represents a stop of light. By changing one to compensate for the other, you can reach the same exposure using a several different pairs of settings. ISO also enters this equation, but for the sake of clarity we will assume that you have your ISO set at 100.
The numbers on your camera may not be exactly the same as the shutter speeds and f-stops listed above. Many cameras have numbers between these standard settings.
You might be thinking that, if there are so many equivalent exposures, why does it matter which one you choose? Here are a few case studies that can help you relate the technical details to some actual photographs.
shutter speed: 1/13 second aperture: f-22
shutter speed: 1/200 second aperture: f-5
In the top photo, my shutter speed was too slow to freeze the action of my daughter riding by on her bicycle and resulted in a blurry image. Additionally, it was a bright sunny day so the slow shutter speed would allow too much light to enter the camera. To compensate, I had to change my aperture to f-22 to restrict more light and create the proper exposure. In the bottom photo I chose a shutter speed fast enough to freeze the motion of the bicycle. The faster shutter speed required a larger aperture to allow enough light for a proper exposure. By changing aperture to compensate for shutter speed, I have two properly exposed photos whose settings are different. I had to change the shutter speed by four stops of light (1/13 to 1/200th) and compensated by changing the aperture by four stops of light (f-22 to f-5). In this example, I did not want a blurry photo so I found a new combination of settings to achieve the desired effect and proper exposure.
The photos below illustrate an example of using blur intentionally.
shutter speed: 1/2000 second aperture: f-4.5
The photo above was taken at a fast shutter speed resulting in the water being frozen mid-stream. The relatively wide aperture compensated for the fast shutter speed to create a proper exposure.
shutter speed: 1/2 second aperture: f-22
In the photo above I wanted the water to be blurred to create a silky effect and the illusion of motion. To achieve this I had to leave the shutter open longer. It was bright outside resulting in the need for a small aperture to reduce the light enough to prevent the photo being overexposed (too bright). I had to use a tripod to keep the camera still during such a long exposure. Neutral density filters can be used to help block more light, but I didn’t have on at the time.
Notice the difference in the texture of the water in the two photos. Understanding the use of equivalent exposures allowed me to to create the blurred effect of the water while still maintaining the proper exposure.
The previous two examples have focused on using shutter speed to achieve the desired effect and using aperture to compensate. The next example reverses the roles.
shutter speed: 1/125 second aperture: f-9
In the example above, I was interested in the mangled juniper remains in the foreground as well as “The Titan” rock formation in the background. I wanted both to be in focus and wasn’t too concerned about the additional formations farther in the background. I chose an aperture of f-9 to make sure the two desired items were in sharp focus.
In the example below, I really wanted to focus on the cairn. It was the end of a long uphill climb and that pile of rocks marked the point where we could turn around and start walking back down hill. So even though the scenery behind the cairn was beautiful, it was not the focus of my photo. I chose a larger aperture to create shallow depth-of-field and had to use a faster shutter speed to prevent overexposure.
shutter speed: 1/640 second aperture: f-4