First off, a disclaimer: To aid in understanding, the topics discussed here are simplified, so as to be more accessible and relatable to those who are just starting on their journey to understand cameras and image creation. Each subject will, eventually, be discussed in more focused, detailed, and complicated articles.
Now that’s out of the way...
A camera’s most simple, technical purpose is to capture light, which will be processed to create an image. Many of the camera’s settings are placing points of context from which the camera relates brightness (exposure) and color (white balance). Exposure is how much light the camera’s imaging plane (sensor or film) has gathered to create an image. Looking at an image, this influences how bright or dark the image is; otherwise known as the brightness of the image.
The term ‘exposure’ means a few different things, even as it just relates to image-making. ‘Exposure’ might be used to describe undeveloped film frames, such as “How many exposures are left on your roll of film?” This use implies that the exposure is part of the evolutionary chain to creating a final image. Another way that it’s used, and the way that is the focus of this article, is the total amount of light used to create an image.
The difference between brightness and exposure is a subtle distinction. Exposure is the amount of light that a camera gathers in the process of making any given image or frame. Brightness is how that light is processed to appear in the image, through chemical development or digital processing. Ultimately, the goal of mastering exposure is to creatively control the brightness of the final image.
What is proper exposure?
Good question. Typically, you want the final image to reflect how your eyes would perceive the world. The brightness of the final image would then be sort of ‘true-to-life,’ if such a thing exists.
Depending on the project and the subject, however, it might be more appropriate to create images that are darker or brighter than you would expect.
Overexposing is when the sensor gathers too much light for a proper exposure. The overall brightness of the image is higher than it (probably) should be. This is not only potentially problematic because it doesn’t reflect the way that we usually see the world, but it also technically degrades the image. Bright parts of the image will be recorded as white, or light grey pixels, regardless of the actual color of the subject, so color information doesn’t accurately represent reality. When an image is substantially overexposed, important details might be obscured or lost in the highlights.
When trying to fix substantially overexposed images and videos in post, you can pull the exposure down, but any parts of the image that were originally white will likely stay white or a very light grey, and won’t be totally recoverable. The flip side to this is that overexposing your images means that your shadows will have less visible noise in the final image. Is this trade-off worth it? In my opinion, no. I’d much rather have color information in the highlights than clean shadows.
Purposefully and well-executed overexposed images can have an ethereal effect (especially if combined with a lovely diffusion filter). You might choose to have an overexposed final image for story and thematic reasons, such as a flashback, dream, or fantasy. One of the most common uses of overexposure in non-fiction is in wedding videos. Most people view a wedding as one of the high points in their life, and no matter how stressful or awful the actual day was, they want to be able to watch it back as if it was all a beautiful fantasy.
You might also use overexposed images to create a common perspective between your viewers and your work. Perhaps your image is meant to reflect very bright conditions, such as a sunny day or bright lights shining directly at the camera. In fiction, perhaps your character just stepped outside into the sun, so when showing their perspective (what they see and how they see it), it’s overwhelmingly bright.
In consciously creating overexposed images, it’s usually best to achieve neutral exposure first, and then push the footage in post. This will prevent excessive white clipping, and will instead have a smoother gradation in the highlights. The drawback to this method is that pushing the whole image will also make any noise more visible. So you might try just pushing the highlights, or using a curve tool to pull up the midtones and highlights, while keeping the shadows where they were. If you plan to utilize overexposed images, you should certainly test the process, using the same camera you will use in your project.
Underexposing is when the sensor doesn’t gather enough light for a proper exposure. The overall brightness of the image is lower than it should be. Just like with overexposing, underexposing technically degrades the image (although arguably not as much). Dark parts of the image might be recorded as black, or dark grey pixels. Important details to the image might be obscured or lost in the shadows.
Most modern sensors are very good at capturing shadow detail (much like the human eye). Moderately underexposed video, if recorded with a low contrast picture profile, can usually be salvaged. Increasing the exposure in post will also lift the noise floor, however, making noise and grain even more visible.
An underexposed image can reflect in many ways the inverse of overexposed images. It might be seen as a nightmare, as opposed to a dream, or a repressed memory, instead of a nostalgic flashback. Or it might simply reflect a calm, low light setting.
To avoid excess noise in your final image, you might shoot your source footage at neutral exposure, and then pull it down in post. But depending on the scene, it might make just as much sense or more to shoot underexposed in the first place.
If I had to choose between overexposed and underexposed footage, I would pick the underexposed everytime. In most cases, I don’t mind the added noise from lifting an underexposed image, but I hate losing color information in the highlights.
What affects the exposure of an image?
Let’s start where most people do when learning about exposure: The Exposure Triangle. This is by no means all you need to know about exposure, but we’ll get to that later, too.
The Exposure Triangle consists of three camera settings: Aperture, ISO, and Shutter Speed.
Simply: the aperture is the opening that allows light to pass through the lens. This is fairly easy to remember once you know that the word ‘aperture’ literally means ‘opening.’
The aperture is made larger or smaller by overlapping metallic blades. These are aptly called aperture blades.
The aperture is most commonly measured in what are called f-stops, or f-numbers. Some examples of f-stops are f/2.8 and f/16. The f-stops are ratios of how wide the aperture is, relative to the focal length of the lens.
An f-stop like f/1.4 is a wide aperture, and has a relatively large opening for the light to pass through. And so more light comes through the lens, creating a brighter image. On the other hand, an f-stop like f/16 is a very small aperture and doesn’t let in very much light, creating a relatively darker image. Logically, people think that the larger f-number would be the larger opening, but the opposite is true, and many people struggle with this idea. The reasoning for this is that the f-number is a denominator (more on that elsewhere).
Many people will write the aperture like f1.4, but the proper way, if you feel like being proper, is to write it like f/1.4. The reason for this is, once again, because the f-number is the denominator of a fraction.
In a dialog, people might refer to the measurement as either the aperture (such as “Where’s your aperture?") or the f-stop (“What f-stop are you at?”). I personally don’t find that one way is any more right than the other, but I’m sure someone would disagree with me. Bear in mind that this no doubt differs by region, culture, and language.
In my experience, if one is to indicate that they are changing their aperture, they would say that they are ‘opening up’ (or even ‘stopping up’) to a wider aperture, or ‘closing to’ or ‘stopping down’ to a smaller aperture. For instance, if I’m changing my aperture from f/2.8 to f/2, I might say something like “I’m opening up to f-two.” If I’m changing from f/4 to f/5.6, I might say “I’m stopping down to f-five-six."
Camera sensors and photographic film are, believe it or not, sensitive to light. ISO is the standard measurement of how light sensitive the imaging surface is. The higher the ISO value, the more sensitive to light it is. (Modern CMOS sensors complicate this simplification quite a bit, but more on that elsewhere.) So by increasing the ISO value, you will increase the brightness of your image.
Most modern cameras have an ISO range between 200 and 12,800. You might write your ISO value as ‘200 ISO,’ ‘ISO200,’ or ‘ISO 200.’
There are noticeable, and sometimes severe, effects to increasing your ISO value, however. While increasing your ISO value does create a brighter image, it also increases the visibility of noise and grain. Depending on your camera and desired aesthetic, this might not be a bad thing, but certainly be aware of your camera’s noise characteristics.
If you don’t know a camera’s noise thresholds and characteristics, it’s best to be safe, and keep the ISO values as low as reasonably possible. Modern technologies are quickly making cameras much more capable of shooting at higher ISO values with acceptable levels of noise, however.
The term ‘ISO’ is actually an acronym for the International Organization of Standardization, which created the measurement as a means of consistency across photographic film stock. Before the adoption of ISO, there were many other measurements for film sensitivity, but ISO became the standard we still use today.
‘Gain’ is a term reserved for video cameras, but means the same thing as ISO. It’s just a measurement of the sensitivity of the sensor. Gain, like audio, is measured in decibels; so your gain setting might read 0dB, +6dB, or even -3dB. 0dB is a camera’s native gain setting, so dynamic range and signal-to-noise ratio (SNR) will be at their highest. Gain settings above zero will be brighter, but will have more visible noise (sacrificing overall dynamic range). Gain settings below zero will be cleaner, especially in the shadows, but will sacrifice dynamic range in the highlights.
If your camera measures in Gain instead of ISO, your best bet is to shoot at 0dB.
Light has passed through the aperture mechanism, reached the sensor, and we’ve established the ISO; What now? We determine the length of time that the sensor reads the light, called the ‘shutter speed.’ Shutter speed is measured in fractions of a second, whole seconds, minutes, or even hours, in the case of long exposure photography. For video, however, shutter speed is measured in fractions of a second.
So, in creating a single frame, the sensor might read the light for 1/60th of a second. In the grand scheme of the universe, 1/60th of a second is an infinitely small amount of time, but in the context of cameras, it’s average, if not a relatively long exposure time. Light moves pretty fast, and a lot of photons can hit the sensor in just 1/60th of a second.
During exposure, the sensor is constantly being bombarded by photons of light. The rate of light that reaches the sensor, however, determines the shutter speed necessary to achieve a proper (appropriate) exposure. You could think of this like a bucket and a hose. If the water coming out of the hose (the light) is of low pressure (low light), it will take a long time (shutter speed) to fill the bucket (achieve proper exposure). If the water is coming out full blast (bright light), then the bucket will be filled in no time at all. The shutter speed is then the amount of time needed to fill the bucket.
BUT WAIT, THERE’S MORE.
The ‘exposure triangle’ have come to be known as such because they are the three settings that you always have full control over when using your camera, but there are other things that greatly affect the exposure of your image that you might have less control over.
This one is the obvious one that no one ever really thinks of. How bright is the setting that you are pointing the camera at? The exact same aperture, ISO, and shutter speed in two different settings will give two different exposures. Why? Because there are different light levels in different scenes.
If you are shooting in true documentary fashion, there might be little to nothing that you can do about the luminance of the scene. In these scenarios, you have to use the other exposure tools to achieve as close to ‘proper’ exposure as you can.
In most filmmaking settings, however, you’ll have the ability to alter the scene luminance to at least some degree. You can turn lights on or off. You can open or close the blinds. You can turn the light up, or move it closer. You can move your subject toward or away from light sources. The list goes on, but keep in mind that you can usually control many aspects of the scene.
Filters are a very common tool in photography and cinematography. They are made of glass or plastic, and are meant to filter the light before it reaches the sensor.
Filters can come in many different sizes, and a few different shapes. They might be screw-on filters, which are circular filters that screw-on to the filter ring on the very front of the lens, filtering the light before it enters the lens; or internal filters, which drop into place in front of the sensor. There are also larger square and rectangular filters that might be used in a matte box or proprietary filter holder.
Filters can serve many different purposes, but the filters that primarily affect exposure are neutral-density (ND) filters.
Neutral-density (ND) filters are simply meant to filter out light without otherwise affecting the image. ‘Neutral’ implies that it’s not doing anything substantial to the image, and ‘density’ implies a certain amount of opacity that blocks light. You can think of these like sunglasses for your camera. They are helpful for eliminating excessive amounts of light; to prevent overexposure. These are most helpful when you can’t or don’t want to further close your aperture, shoot with a higher shutter speed, or lower your ISO.
While not usually a significant consideration for exposure, the final factor when choosing exposure is the processing that will happen to your image. Most cameras have a handful of different processing styles that are usually called ‘picture profiles,’ which affect the way that contrast and color are handled, among other things. Most picture profiles don’t drastically affect the way that the overall brightness is affected, but there are differences to consider.
If you are going to extensively process your images in post-production, you might learn that it’s optimal to underexpose or overexpose your image to most easily achieve the look you desire. Depending on the project and use, it’s usually best to figure this out during a testing period before actual production.
THE PATH OF LIGHT
As someone who for years taught the Exposure Triangle, I recommend a rejection of it, to instead embrace a more exhaustive chain of stages that more wholly affect the overall exposure of any given image: The Path of Light.
COOL. BUT HOW DO I MONITOR MY EXPOSURE?
Great question. There are several different ways in which photographers and cinematographers monitor the exposure of their images, but the most common ways are the Histogram, Zebra Stripes, and Waveforms. For now, though, let’s just cover the first two.
A histogram is a box or rectangle that will show you hills and valleys along a 2-dimensional axis. The far left side of this box represents what is being clipped as black pixels. Just to the right of that is what you would consider the ‘Shadows’ region. In the middle of the graph is the ‘Midtones’ region. To the right of that, you’ll find the ‘Highlights’ region. Finally, on the far right, you’ll find what is being clipped as white pixels.
The hills, valleys, and spikes represent a number of pixels in that luminance range. So, if your histogram shows you a big hill on the left side of the graph, it means your image has a lot of pixels that are in the shadows, and so on.
You can find many examples of what a ‘proper’ or ‘perfect’ histogram look like, but frankly, those people are full of shit. There is no perfect histogram, but it does tell you information that should help you figure out whether your exposure reflects what you think it should be.
The histogram of a scene with low dynamic range (so there aren’t any bright lights or deep shadows) will likely have one large hill, which you should place somewhere in the midtones.
The histogram of a scene with high dynamic range (bright lights and dark shadows) might not have any hills or peaks in the middle, but might have a lot going on at each end, representing the bright pixels of the lights in the highlights, and the dark pixels of the shadows in the, uh, shadows.
As a whole, histograms are pretty unique, and any given scene will give you a different pattern of peaks and valleys. Learning to understand what those peaks mean in relation to the final exposure of your image is all you can really do, so that you’ll understand what your image would look like, and make any necessary adjustments.
Many video-oriented cameras offer a tool called ‘Zebra Stripes,’ which can show you which pixels on your image are of a certain luminance by overlaying alternating, diagonal black and white bars.
For instance, if you were to turn the zebra stripes tool on at 70% luminance, the camera’s screen would then indicate which parts of your image are at 70% luminance, if any.
Some zebra stripe settings operate in a way that it shows that specific luminance, plus or minus 5%, so in the previous example, the zebra stripe tool would display over everything in the image between 65% and 75%.
Other zebra stripe functions work by placing the overlay on everything at the designated luminance and above. So if my zebra stripes were set to 70%, then the tool would put zebra stripes over everything in my image that has a luminance of 70% and higher.
Exposure might seem like a basic thing that you just have to figure out how to do, but it’s actually more important than that. Not only will you not be taken seriously as a professional if you can’t create technically exposed images, but there’s also the point of creative control. The overall idea of consciously deciding and purposefully achieving appropriate exposure for whatever image you’re making means that you have creative control over it, and you can use that to help communicate your message and meaning to the viewer.