Exposure triangle. Part 1 Shutter speed
The exposure triangle is made of three elements (shutter speed, aperture and ISO) and is one of the fundamental principles you need to learn back to front if you want to understand how to use your camera to its full potential or improve the quality of your images. The three elements are features your camera and lens use to control the light entering your camera and it’s a balancing act of these three elements that are used to create a proper exposure.
Why do I need to know this?
If you leave your camera in the automatic preset your camera is in charge of choosing the balance between shutter speed, aperture and ISO when capturing an image without knowing what you’re trying to portray in the image, whether your camera is on a tripod or what area of the images you’re trying to expose for. This can result in the wrong shutter speed being selected when you want to freeze your subject or the wrong aperture when you want to isolate your subject using depth of field. That’s why if you want to progress as a photographer or get the best out of your DSLR to capture better memories it is vital you understand how each element affect exposure but also the look of your final image.
So what is shutter speed?
Very simply shutter speed is the time it takes for your lens to open and close for a single exposure, meaning your camera display shows you the length of time your shutter stays open and allows light into the camera to hit the sensor.
Light is everything in photography, if you have no light you can’t take photos. The more light you can let into your camera the brighter your image will be, in other words a slower shutter speed of 1/50th of a second will let more light into your camera than 1/200th of a second. That’s because at 1/50th of a second your shutter can theoretically open and close 50 times in one second but at 1/200th of a second it can open and close 200 times. That means at 50th of a second it will give you a brighter image than at 200th of a second.
How do you choose a shutter speed for a particular image?
Shutter speed not only affects the exposure of your image it also changes its overall look. That’s because when your shutter is open the sensor in your camera will pick up all the light that is entering the camera throughout the whole exposure, meaning if your subject is moving whilst the shutter is open it will result in motion blur.
Imagine that your sensor (or film) is a piece of paper and the light entering your camera is a felt-tip pen. When your shutter opens and light hits the sensor it is like the pen touching the paper, if either the paper or pen move before you close the shutter (or remove the pen from the paper) you will be left with a clean dot. But if you move the pen when in contact with the paper (whilst the shutter is open) it will draw a line following the relative motion between them until you lift the pen (close the shutter). This is what is called “motion blur” and you will commonly see that when you’re taking photos in low light situations resulting in big streaks of light across your images.
If we use our friend Bobba as an example the first exposure is taken at 1/500th of a second with Bobba standing still and the camera on a tripod. You can see he is perfectly sharp with no motion blur because neither the subject nor camera is moving.
Now if we drop Bobba into the frame and take another image with the same settings you can see there are small amounts of motion blur but you can still make out most of the detail.
If we reduce the shutter speed to 1/80th of a second and drop Bobba in again you can see there is much more motion blur due to the shutter being open longer BUT more light is getting into the camera resulting in a blown out image. To compensate for the extra light coming into the camera you need to balance this by changing either the aperture or ISO, in this case I reduced the ISO from 3200 to 800 (to find out why reducing the ISO changes the exposure check out part three of this project). With the reduced ISO but the shutter speed staying at 1/80th of a second you can see how the motion blur completely changes the image.
If we take this all the way to 1/30th of a second (and reducing the ISO to 200 to compensate for the extra light coming into the camera) you can really see how reducing the shutter speed allows more time for Bobba to fall whilst the shutter is open, hence the high levels of motion blur.
As a general rule of thumb the slower your shutter speed the more motion blur you will see, as the more time your subject has to move within your frame. However you will need to take into account how fast your subject is moving, as a fast moving object will cause more motion blur than a slow moving object at the same shutter speed. That’s because within the time the shutter opens and closes a fast moving object will travel further across your images hence more motion blur.
What about if your subject is still but you’re moving?
The same principle still applies, that’s why if you’re walking along whilst taking a quick snap shot of a building on your Smartphone you will more than likely have a blurred image, especially in low light situations. In this case if you use the pen and paper analogy it is not the pen that is moving it is the paper. That’s why you will see most professional photographers using tripods to take their images, as it keeps the camera completely still and reduces any movement of the camera. It is really important to bear this in mind when you’re hand hold when taking photos, as the movement of the camera during the exposure results in “camera shake” and a slightly blurred image. You can reduce the risk of camera shake by using something called the focal length rule which states for whatever focal length you’re at (say 200mm) you need to be at a minimum shutter speed of 1/200th of a second, or at 50mm more than 1/50th of a second etc.
So that covers the basics of shutter speed and the three main rules of thumb you need to remember:-
1. Slower shutter speed=increases light hitting your sensor (brighter image), faster shutter speed=reduces light hitting your sensor (darker image)
2. Slower shutter speed will result in motion blur and a faster shutter speed will give you sharper images
3. To avoid motion blur and/or camera shake use a tripod or the focal length rule