What are long exposures? These are photographs taken through a camera’s lens while it remains open for a significantly longer period than a traditional photograph requires. For example, when someone wants to document images of star trails or the Milky Way Galaxy, they are going to be obliged to use a long exposure to get the picture.
It isn’t as easy as setting up a camera to keep its shutter open for a few minutes or even an hour, however. The photographer will also need to consider the effects of the ISO and the aperture (f/stop) that the camera is set to as well. This is because the three factors are heavily connected. When one of these settings is adjusted, it is going to affect the others. Usually if one is reduced the others must be increased.
What is f/stop then? It is the amount of light that the camera’s lens allows through to the sensor (or film). The lower the f/stop number, then the larger the aperture or opening of the lens. This also affects what is known as the depth of field, and this makes anything in the foreground or background blurry, while the main subject is in focus.
So, when we have the opening of the aperture very large, such as f/2.8, we will then need to adjust the exposure to a shorter amount to get the best results? Basically, that is going to be true.
Let’s take an example – a photographer hoping to record a good image of the night time sky. They will begin by setting up a tripod, connecting the camera and mounting a cable shutter release. This is all necessary for the best results because any sort of wiggle to the camera can cause blur and even just hitting the shutter release with the finger can ruin the entire exposure. The next thing to do is take a test shot using settings that are good for the scene, but not for the final result. For instance, someone in low light can crank the ISO as high as 3200 and the aperture as low as f/2 to get a brief exposure and instant results. This image will give a somewhat accurate representation of the final shot, but will not be of very good quality.
Using these settings, the photographer can then perform some simple math to determine the appropriate settings for the best results. This is where ISO comes into the equation. ISO was once known as “film speed” but now means the amount of sensitivity the camera sensor has to light. Optimally it should be at the lowest setting possible – 100. If we adjust the other settings to match this, we would have “halve and double” the other settings to determine the right ones. For this photo the ISO is halved five times to go from 3200 to 100. This means the shutter speed is doubled five times to accommodate the change – going from 2 to 64 seconds. If we wish to change the f/stop more maths is necessary to increase exposure time.