The way cameras work can be complex and confusing, so here’s a little guide to try and explain things a bit better.
Firstly, understanding how a camera works will make everything else make more sense. Essentially the two big parts of a camera are the lens and the sensor. Light enters the lens, which then focuses it onto the sensor. It’s basically the same way an eye works, where the lens focuses the light onto the retina. The biggest issue is that there is a sweet spot of the amount of light required to capture an image in detail. Too little light and the image will be too dark, too much and the opposite happens. Essentially on either side of perfect exposure there is a cliff edge of detail loss.
To control the amount of light that hits the sensor there are 3 different things:
Aperture – Works like your pupil, it’s a hole in the lens which contracts and dilates to let more or less light enter the lens. This is measured in F-stops (or T-stops for cinema lenses), with a low F stop such as an F1.8 meaning the lens is wide open, while a higher one such as an F12 means that there is a smaller opening for the light.
Shutter speed – There’s a pair of doors in front of the sensor. These can be altered to open and close for longer and shorter durations. If it’s open for a long duration more light hits the sensor, a short duration then less light hits the sensor.
ISO – This how sensitive to light the sensor is. The higher the ISO, the more sensitive and therefore the brighter the image will be.
So, each of these 3 things can control the image’s exposure in their own way. However, they also each effect the look of the photograph in a different way.
Aperture – The aperture also affects how much of the picture is in focus. A wide-open aperture will have a much shallower depth of focus compared to a more closed off aperture. So, if you want to make your subject stand out with a lovely soft blurry background, use a low F-stop.
Shutter speed – Because shutter speed determines how long the sensor is exposed to light, it also determines the level of motion blur in the picture. A slow shutter speed takes a picture over a longer time, which means anything moving in the shot during that time will leave a blurred trail where it travelled from the start of the shutter opening to when it closes. So anytime you want to photograph something moving, such as sports or wildlife, you will need to use a very high shutter speed. And if you want to create pictures of milky smooth waterfalls, have a slow shutter speed (but make sure to use a tripod!).
ISO – A high ISO increases the sensitivity of the sensor. However, it also introduces more and more visual grain, the higher the ISO is. This reduces the visual quality and detail of the image, so it is always recommended to have this set to as low as possible.
The trick is to work with these 3 elements to create the visual outcome you want. There is technically a fourth way to alter exposure, which is through the use of ND filters (Neutral Density filters). These act like sunglasses for your camera allowing you to lower the exposure without affecting the other setting. For example, if you wanted to have a nice blurred background, but while using a low F-stop the image is too bright, instead of closing the aperture off and ruining your lovely blurred background, you could put an ND filter on instead.
Exposure settings in video
These different settings mostly work in a similar way for video with one big exception, shutter speed. Because shutter speed deals with time in a similar way to the frames per second, they need to be in sync. The general rule of thumb is to have the shutter speed be twice the frame rate, so in the UK the frame rate standard is 25 fps, meaning the shutter speed will have to be 1/50, which is considered to be how we perceive things with our eyes. Having a slower shutter speed would result in a lot more motion blur, while an increased shutter speed would make the footage seem very jittery due to the lack of motion blur blending the frame transitions. To help with the lack of freedom the fixed shutter speed leaves you with, ND filters are often used in filmmaking.