Basics: sensitivity, aperture and shutter speed

Photography and videography both rely on light. Simply put, no light, no photos, no videos. You can try this by going to a room with no windows, nor skylights. Close the door behind you and try to take a photo or some video. Your device may use some sort of light or a flash, but that already should make you aware of the issue. In some cases there may be too much light, but in most cases the issue that you will encounter is having too little light.

It is possible to do use a camera without much knowledge it functions. However, the more demanding the situation is, the more you will benefit from knowing how a camera functions. This is especially the case in low light conditions, which is why I believe it is worth it to start from the beginning, or so to speak.

I will first explain the basics in photography, followed by explaining the same things using videography specific terms. I will not flood you with camera jargon. While it is beneficial to know just about everything that there is to know about how cameras work, you do not need a degree in physics to get the job done. There are, of course, also all kinds of exceptions, but I will gloss over such as it is unlikely that you will be concerned by them.

At this stage I won’t specify what devices you can use as that is beside the point as all cameras function the same way. What you need to know is that your camera is sensitive to light and your job is to control how much or little light enters your camera. There are three ways of controlling it: sensor light sensitivity, lens aperture, and shutter speed.


Instead of film, digital cameras have sensors.These sensors have a base sensitivity, typically referred to by its ISO-value. In the film days this was known as the ASA-value. The base ISO-value is typically 100. The sensor sensitivity can be adjusted. It is typically set as 100, 200, 400, 800, 1600, 3200, 6400, 12 800 and so on and so forth, so that the relevant value is doubled each time, as you can see. Think of it as amplification.

The problem with running the camera sensor on higher sensitivities is that it results in noise. The photos will have certain roughness to them. It can, of course, be remedied by applying noise reduction (NR). This can be done in the camera, using its own software, or on a computer, using the software you happen to have. Some cameras or, rather, their sensors handle noise better than others. In general, the newer cameras tend to handle noise better than the older ones, and the bigger the sensor, the better the results.

Noise used to be more of a problem, so that the best you could pull off was ISO 800 and the results were not exactly spectacular. This limited you to ISO 400, which meant that you really needed ideal lighting conditions. This is, however, no longer the case and you can now go up to ISO 6400, give or take, and, in some cases, even up to ISO 12 800, and get very good results.

Using the higher sensitivities also reduces the dynamic range (DR) of the camera sensor output. The maximum DR of a camera is typically 14-bits, but the more you increase the sensitivity, the more that DR is reduced. What does it do to your images then? Well, simply put, it reduces the shades of grey. In other words, think of going from black to white but having less distinct shades of grey in between. The reduction of DR is not ideal, but there are cases where increasing the sensitivity is your only option to get properly exposed photos. Some cameras handle this issue better than others, giving you higher DR at higher sensitivities than others.

In the world of videography sensitivity is referred to as gain. It is functionally the same. The ISO-values are just about providing you standardized values, hence the reference to ISO, short for International Organization for Standardization.


Cameras rely on lenses. They have adjustable apertures, which control how much light enters the camera body and reaches the camera sensor. Lenses have adjustable aperture blades that form a circular shape also known as a diaphragm. Closing the aperture lets in less light. This is known as stopping down. Opening the aperture lets in more light. This is known as opening up. In photography, you work with the so called f-numbers, marked by an f, indicating the ratio between the lens focal length and the aperture, which, in turn allows you to calculate the diameter of the opening itself.

For example, a lens marketed as 50mm and f/2 has the maximum opening of 25mm. If you close the aperture, stopping it down, as they say, to f/2.8, the opening is now only 17,86mm. The f-numbers are typically indicated as 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 32. The ratio between these numbers is always 1.4, so that 1 leads to 1.4, which in turn leads to 2 and so on and so forth. Note how the ratio between f-numbers is not 2. How can this be if the ratio between sensitivity values is always 2? Well, you have to keep in mind that the area that is open is an area. If we go from f/2 to f/2.8 on a 50mm lens means that the area is reduced from approximately 491 sq mm to 250 sq mm. If we go from f/2.8 to f/4, the area is further reduced from 250 sq mm to 123 sq mm. As you can see, the ratio of the area is 2. It is actually very close to it, but do not blame me for that. I did not invent this system. It is close enough.

In practice, what you need to remember is that going from one f-number to the next f-number always either doubles or halves the light entering the camera sensor. So, you can double or halve the light coming in by opening up or stopping down once, by what they call one stop. To achieve the same, you can go double or halve the ISO-value.

In videography aperture is typically referred to as the iris. Again, all you need to know that this is functionally the same thing as the aperture. If you end up using cinema cameras and/or cinema lenses, then you will encounter lenses that allow the opening, i.e., the iris, to be controlled smoothly, without preset clicks, and measurements indicated by their t-number, indicating how much light gets through the lens to the sensor rather than giving you an approximation based on an equation. The t-number system is not inherently better than the f-number system. What it does, however, is to offer convenience. It allows the same scene to be shot with different lenses without the hassle of having to check the exposure of the footage in post processing, to match the footage, as each lens with the same t-value provides you the same exposure, regardless of the lenses you have used.

Shutter speed

Cameras also have shutters. They act as mechanical curtains that open and close in front of the camera sensor or film, exposing it to light. The longer these curtains stay open, the more light enters the sensor or the film. The shorter these curtains stay open, the less light enters the sensor or the film. By adjusting the shutter speed, you always double or halve the exposure time. For example, if your shutter speedis set at 1 second you double the incoming light by doubling the exposure time, setting the shutter speed at 2 seconds, and you halve the incoming light by halving the exposure time, setting the shutter speed at 0.5 seconds.

The sensors can also act as a shutter by activating only for a specific time. This option is known as using an electronic shutter. The function is the same as it is in a mechanical shutter. There is just no mechanical curtain exposing the sensor to light.

What you also need to know about shutter speeds is that you need fast shutter speeds to stop motion. How fast shutter speed is needed depends on how fast what you focus is moving. In general, you need something like 1/50 second to 1/100 second shutter speed. You might be able to pull off even slower shutter speeds, especially if you are just photographing static objects. If you are inexperienced, your hands will, however, most likely shake a bit, causing blurry images at slow shutter speeds. This also affects videography. Some cameras have in-body image stabilization (IBIS) and some lenses have optical image stabilization (OIS), which counter shaky hands. They do not, however, counter other types of motion blur. You still need fast shutter speeds for that purpose.


As you can see, adjusting the sensitivity, the aperture or the shutter speed doubles or halves in amount of light that makes it way to the camera sensor. I explained them in stops, also known as full exposure values (EVs), to make more sense of them. Modern cameras do, however, allow you to adjust these in smaller increments, typically in half (1/2 EV) or one third (1/3 EV) stop increments. As lighting conditions vary quite considerably and in smaller increments, this allows you more options to work with in order to get the right exposure.

Neutral density filters

There is one additional common way of altering how much light reaches the camera sensor. Adding filters on the camera lens is a common way of altering photographs. I will not cover all the filters, for example polarizers. I will only focus on the so called neutral density (ND) filters as their sole function is to reduce the incoming light. On most days you do not need ND filters. There are three main uses that I can think of.

Firstly, you will need to rely on these filters if you want to take long exposure photos on sunny days. Why? Well, the camera sensor sensitivity has a floor, meaning that it cannot be less sensitive to light than that. That value is typically ISO 100, but some cameras go down to ISO 80 and ISO 50. On bright days closing the aperture, i.e., stopping down, cuts down the light only so much. This means that the only option that you are left with is adjusting the shutter speed, opting for a shorter exposure. However, in this case that is the exact opposite that you want to achieve. But why would you want to take long exposure photos on sunny days? Well, perhaps you want to take a photo of a river or a waterfall, creating a smooth, blurred effect. This is where ND filters save the day.

Secondly, cameras suffer what is known as diffraction. This is a bit too complex issue to explain and frankly unnecessary for you to know. All you need to know is that closing down the aperture beyond a certain threshold causes the light to diffract, i.e., disperse, in a way that is detrimental to the image quality. If you shoot with an open aperture, light enters the camera perpendicularly, i.e., hitting the camera sensor dead on. If you stop down, some of that light enters the camera perpendicularly, as you would want it to, but some if does not, entering it in a different angle and bouncing around. Simply put, your photo won’t be as sharp as it could be if stop down beyond a certain limit. The limit is typically at f/8 or f/11, but it is hard to say where things start to deteriorate as this depends on your camera sensor and there may also be other issues with your camera lenses that cause the image to degrade. I would say things take a turn for the worse at around f/16.

ND filters allow you to get the best out of your camera and your lens on sunny days, when you want be right at the limit, while using the lowest sensitivity setting. This is, of course, related to the first case as you probably want a longer exposure. This is, however, a slightly different issue. You might also be very close to the optimal settings, but just need to cut a stop or two get there, which is where the ND filters come in handy.

Thirdly, you will also need to rely on these filters if you want to shoot with an open aperture on a sunny day, while combining it with a specific shutter speed. When would you need such? In my experience, you rarely run into this issue in photography. This is, however, a common issue in videography, where too short exposure times are generally not desirable. For example, if you want to shoot video at 25 frames per second (FPS), you get the most pleasing outcome by setting the shutter speed to 1/50 second. Similarly, if you want to shoot video at 50 FPS, you get the most pleasing outcome by setting the shutter speed to 1/100 second. But why would you need to shoot at an open aperture? Why not simple stop down so that you can set the recommended shutter speed? This is connected to another thing that I need to cover.

The lens aperture not only allows you to control how much light enters the camera, but, unlike the sensitivity and the shutter speed settings, it controls the depth of field (DOF), which is the distance between the closest and the farthest object that appear sharp in photo or in a sequence of video. This is also unnecessary to explain in detail. All you need to know is that the more open your aperture is, the shallower the depth of field tends to be, depending on other factors, such as the lens focal length, distance to the subject, i.e., what it is that you focus on, and the more closed your aperture is, the deeper the depth of field tends to be. You’ll want a shallow depth of field to separate what you want to be in focus from what you do not want to be focus. You probably want a shallow depth of field when focusing on a person, for example in portrait photography. Conversely, you probably want a deep depth of field when focusing on the whole scene, for example in landscape photography. Telephoto lenses tend to be best suited for the former, whereas wide angle lenses tend to be best suited for the latter.

To make more sense of the benefit of using ND filters when shooting video, you may want to isolate what you focus on, typically a person, more specifically his or her face, from everything else. If you shoot video on a sunny day, you want that 1/50 or 1/100 shutter speed, but now you also want that open aperture to isolate what you focus on from everything else. You probably already operate at the lowest sensitivity setting, which means that you need set a shorter exposure time or adjust the aperture or limit the incoming light in other ways. This is also where ND filters save the day.

This is also relevant to those cases where you have an abundance of light, simply too much of it, but want to pan on what you are focusing on. For example, if you are taking photos of a football player on the move, but want to create partial motion blur, so that most of his or her body looks like it is in motion, while the face and the upper body is in focus, you will need ND filters. Simply put, you will need these filters if you want to isolate the player from the background.


Knowing how sensitivity, aperture and shutter speed affects exposure allows you to control your camera. Most cameras digital single-lens reflex cameras (DSLRs) and mirrorless interchangeable-lens cameras (MILCs) have multiple modes. You typically begin by setting the ISO-value. The auto mode does all the work for you, but it may not give you the settings you want. For example, you may wish to have deeper or shallower depth of field than what your camera provides in the auto mode. The semi-auto or priority modes make you control either aperture or the shutter speed, while calculating the other for you on the basis the ISO-value that you have set. These modes are handy if all you want is to make sure that your depth of field is this deep or shallow or that you either get or do not get any motion blur. There is less fiddling with the settings. Some cameras also allow you to control both at the same time, like in the manual mode, but calculate the ISO-value for you, either from the full range of sensitivities that the camera can pull off or from a range that the photographer finds useful for this purpose. This can also be fine tuned to prioritize lower or higher sensitivities. The manual mode makes you do all the work, setting the ISO-value, followed by controlling the exposure with the aperture and shutter speed settings. There is typically a +/- indicator which shows whether you are over- or under-exposing the photo.

Cameras also allow you to measure and suggest the correct exposure in different ways. Typically, you have settings that base the suggested correct exposure on the whole scene, what the sensor is provided, prioritizing the central part of the scene or just the very center of the scene. This won’t affect your photos, as such. It only helps you to get good results. For example, you may have a brightly lit room, but you are focusing on a very dark object. To make sure that you can see the details on that very dark object, you need to set your exposure according to that object, not on the otherwise brightly lit room. You would, of course, get to the results you want by simply changing the settings until you get the results you wanted, but you get to those results faster if you use the camera settings to your advantage.

The auto and priority modes can also be fine tuned by setting exposure compensation (EV compensation). This simply means that you set your camera to over or under-exposure by this or that much, for example by 1/3, 1/2 or 1 EV. Cameras may struggle in certain lighting conditions, resulting in over or under-exposed photos. Setting the EV compensation allows you to override the system, altering the calculations, to provide you the correct exposures.


Being able to control your camera not might not be needed in most cases, but it is certainly useful in demanding lighting conditions. Taking photos and/or video in low light conditions is particularly challenging. It would be tempting to simply rely on your camera, hoping that it can pull off whatever it is that you are trying to achieve. However, it is only likely that you will need to make a lot of compromises with the settings, especially if you are hoping to take photos handheld, which is why knowing what affects what will help you tremendously.

I did not cover any specifics, for example which cameras and lenses you might want to consider, nor which accessories you might also want to consider. There is no right or wrong way to go about it. The cameras produced by well known manufacturers are all good enough for the job, as long as you know what you are doing. Some are, of course, more suitable to demanding low light conditions than others, which is especially the case if you want do video, but I will write another post to cover these aspects.