We saw in lesson 5 that we have three tools to control exposure: shutter speed, aperture and ISO. Of these, the easier to understand and most intuitive certainly is shutter speed, which we will talk about in this lesson.
This parameter simply refers to the amount of time during which the shutter is open and the sensor/film exposed. It is usually expressed in fractions of a second, since it will be relatively rare to need durations longer than one second. Obviously, the longer the speed, the more light can be recorded, and thus the higher the exposure. Like everything exposure related, we also talk about stops for shutter speed, which is a relative measurement unit: 1 stop of overexposure corresponds to doubling the amount of light received, so doubling the shutter speed. Of course, 1 stop of underexposure is the opposite, halving the shutter speed.
At first look, it would appear that it would be simple enough to just let the shutter open as long as you need to obtain a correct exposure, without any other consideration. However, this leads to a problem: what happens when either the subject or the camera moves during the while the shutter is open? We are of course all too familiar with the answer: motion blur. Conversely, using high shutter speeds will result in “freezing” the action, recording the exact split second where you pressed the shutter.
The game, then, is to find a shutter speed which is slow enough that you get enough light, but high enough that you don’t get motion blur. In order to achieve this, it is important to find the “handheld” limit, below which your images will be blurred. It depends on many factors:
The rule of thumb usually given is that the handheld limit is 1/focal length (in 35mm equivalent). So if you are shooting a full frame camera at 50mm, your images should be sharp at 1/50 and above, as long as the subject is static. On a DX DSLR, the same focal would require 1/75 or so (to account for the crop factor). However, this depends on so many factors that you may well find that your own limit is significantly faster or slower.
Once you have found what your handheld limit for a particular focal is, all you have to do is make sure you always use faster speeds. Whenever it isn’t possible, usually because there isn’t enough light, you will have to use a tripod.
In some cases, however, you will want to use slow shutter speeds. This usually happens in cases where you want to communicate that your subject is moving. The most common case is panning: instead of having a static environment with a blurred subject, you will try to follow the exact movements of your subject so that it is the only sharp thing. It is extremely effective when done well, but takes a lot of practice and trial and error to achieve. This is used often in automobile sports and bird photography. Here is an example from Rio 2016:
Another popular effect consists in using very slow speeds on moving water, which will result in a dreamy, surreal look. You will need a tripod and probably a neutral density filter to reduce the amount of light entering the lens. A not too extreme example would be this one:
Another more creative example is this image of NYC’s Grand Central Station:
The goal of this assignment is to determine your handheld limit. It will be quite simple: choose a well lit, static subject and put your camera in speed priority mode (if you don’t have one, you might need to play with exposure compensation and do some trial and error with the different modes to find how to access the different speeds). Put your camera at the wider end and take 3 photos at 1/focal equivalent, underexposed by 2 stops. Concretely, if you are shooting at 8mm on a camera with a crop factor of 2.5, you will be shooting at 1/20 – 2 stops, or 1/80 (it’s no big deal if you don’t have that exact speed, just pick the closest one). Now keep adding one stop of exposure and take three photos each time. It is important to not use the burst mode but pause between each shot. You are done when you reach a shutter speed of 1 second. Repeat the entire process for your longest focal length.
Now download the images on your computer and look at them in 100% magnification. The first ones should be perfectly sharp and the last ones terribly blurred. Find the speed at which you go from most of the images sharp to most of the images blurred, and take note of how many stops over or under 1/focal equivalent this is: that’s your handheld limit.
Bonus assignment: find a moving subject with a relatively predictable direction and a busy background (the easiest would be a car or a bike in the street) and try to get good panning shots. Remember that you need quite slow speeds for this to work, 1/2s is usually a good starting point.