[Essay] Understanding the Exposure Triangle For Video

If you want to learn about videography then you first have to know about photography. After all, a video is just a series of pictures played very quickly, right? Most ideas about exposure, composition, and lighting carry over from one to the other. Central to the art of photography, and thus videography, is the concept of the Exposure Triangle. It can often be tricky for beginners to wrap their heads around it, but it is vital to understand for anyone who wants complete control over how their images turn out. 

As you might expect it’s made up of three elements which, when balanced correctly, will give you a properly exposed image. That is to say, not too dark and not too bright. These three elements correspond to three camera and lense settings.

  1. Aperture
  2. ISO
  3. Shutter Speed

In this article we will be discussing each of these elements individually, as well as how they interact together, and what the differences are between applying it to photography and videography.

The Exposure Triangle

Why a triangle? Well, as we have already mentioned aperture, ISO, and shutter speed all interact with one another and must be balanced to get a properly exposed image. Therefore, it is commonly depicted like this:

If one element of the triangle is changed then one or both of the others must be changed to compensate.

Briefly: Aperture is the size of the opening in the lense allowing light into the camera, shutter speed is the length of time the camera’s shutter stays open to let this light in, and ISO is your camera sensor’s sensitivity to that light. If you reduce the size of your aperture, letting less light into the camera, then you will need to either reduce your shutter speed (leaving it open for longer) or increase your ISO (sensitivity). Don’t worry, we’ll be diving into these concepts in more detail in a moment. First, we need to discuss another idea that is necessary to understanding the Exposure Triangle: the Stop.

The Stop

Very basically a stop is one way of measuring differences in amounts of light. If you increase the amount of light by one stop you are doubling the amount of light. Likewise, if you decrease the amount of light by one stop then you are halving the amount of light. If an image is too bright it might mean decreasing the amount of light by one or more stops. For smaller adjustments you will often see light being increased/decreased by half, third, or quarter stops. Don’t get too caught up in the mathematics of it right now. The important thing to remember is that increasing/decreasing by a full stop means doubling/halving the amount of light respectively. 

The reason this is important for understanding the Exposure Triangle is that each element of the Triangle can be thought of in one stop increments. Once you know what those increments are then you will know the exact amounts by which any of the elements must be adjusted to compensate for any change in the others. We’re going to look at those increments shortly.

Aperture

As we have already touched on, the aperture is the opening in the lens that lets light enter the camera. The bigger the aperture, the bigger the hole, the more light gets in. This also is true for the reverse. Doubling the size of the aperture doubles the amount of light getting into the camera meaning, you guessed it, an increase in light of one stop. Again, the reverse is true; halving the size of the hole halves the amount of light, a decrease of one stop.

Now comes the slightly technical part. The size of the aperture is usually expressed through something called an f-number or f-stop. Yep, another stop. While it is different to a stop of light the two are related. Increasing or decreasing the f-stop of the lense, therefore changing the size of the aperture, will increase or decrease the amount of light entering the camera.

Another wrinkle in things is the fact the smaller f-stops correspond to larger apertures. So an aperture with an f-stop of f/1.8 will be a wider aperture than one with an f-stop of f/4 and therefore it will let in more light. The reason for this is also the reason that f-numbers are frequently decimals; they are ratios. Simply put they are a ratio of the width of the aperture to the length of certain elements within the lense.

Another reason why f-stops tend not to be nice round numbers is those increments we talked about earlier. What increase in the size of the aperture (decrease in f-stop) corresponds to an increase of one stop of light? The answer involves more maths, but thankfully not the kind that most photographers will need to learn. Technically to double the size of the aperture the f-stop needs to be divided by the square root of 2, which is approximately 1.414. In reality you can simply learn common stop increments and go from there. The following are common apertures which differ by one stop of light:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11

The final thing to say about aperture is not exposure related, and that is its effect on depth of field.

In fact every element of the Exposure Triangle will have an effect on your image other than exposure, giving you even more to consider when balancing those elements. Simply put, depth of field is the area of your image that is acceptably sharp to the point of being ‘in focus’. A shallow depth of field means that less of your image will be in focus, while a deep depth of field means that more of your image will be in focus. A larger aperture (smaller f-stop) will mean a shallower depth of field, and a smaller aperture (larger f-stop) will mean a deeper depth of field.

ISO

As we’ve mentioned, ISO is the sensitivity of your camera’s sensor to light. Increasing your camera’s ISO number will increase it’s sensitivity, decreasing it’s ISO will decrease it’s sensitivity.

ISO is expressed as a number, referred to as either a value or speed(a holdover from its origins in film photography). Your camera will have a range of ISO values it can shoot at, typically something like this:

ISO 100, 200, 400, 800, 1600, 3200, 6400

Notice how the values double as you move up the range? This is because doubling or halving the ISO value represents an increase or decrease of one stop of light. Some cameras will be able to make finer adjustments to the ISO number than that, others will only be able to move up or down the scale one full stop at a time.
So what’s the catch? As we’ve mentioned each element of the exposure triangle carries some trade-off not related to exposure. The trade-off with ISO is something called noise.

The technical ‘hows’ and ‘whats’ of digital noise are a bit beyond the scope of this article. For now it is enough to know that it can give your photos a grainy, pixelated texture, particularly in darker areas. In bad cases it can appear as a scattering of discoloured pixels. 

The key thing to remember is that increasing ISO sensitivity is not actually letting any more light into your camera but it is forcing your camera to try and do more with the light that is getting in. As you increase your ISO you increase the level of noise in your image.

Shutter Speed

The last element of the exposure triangle is shutter speed. Shutter speed refers to the length of time that your camera’s shutter stays open and thus the length of time that light is allowed onto the sensor. It is typically expressed in fractions of a second, i.e.

1/25, 1/50, 1/100, 1/200, 1/400, 1/800

Once again, a doubling or halving of the number represents a change of one full stop of light. (Although most DSLRs or mirrorless cameras will give you a greater degree of control over shutter speed increments than they will over ISO increments.) However, contrary to ISO, if the shutter speed value is increased exposure goes down. So a doubling of shutter speed represents a halving of the amount of light hitting the sensor, and a halving of shutter speed represents a doubling of light. While it may be a bit jarring having just learned the opposite with ISO it makes perfect sense when you think about what shutter speed actually means. If you increase your camera’s shutter speed from 1/100 to 1/200 you are telling your shutter to only stay open for half as long, therefore letting in only half the light.

The non-exposure related trade-off with shutter speed is motion blur. At slower shutter speeds where the shutter is staying open for longer there is an increase in the amount of motion blur in your image. Likewise, if you increase your shutter speed you reduce the amount of motion blur. These two outcomes can be employed in both photography and videography for artistic effect.

Long exposure photography refers to a style of photography that employs slow shutter speeds. It can be used to blur the motion of running water or clouds to give a pleasing, dreamlike appearance to them. Best practice is to take these kinds of photos on a tripod so as to eliminate any additional motion blur that might be caused by the photographer’s hands.

On the other end of the spectrum, photos taken at high shutter speeds can be useful for ‘freezing’ fast moving subjects. Sports and wildlife photographers will often use higher shutter speeds to capture sharp photos of moving athletes or animals.

Applying the Triangle to Video

We mentioned that the effects of fast and slow shutter speeds can be applied to both photos and video. While this is true, shutter speed is actually the element of the exposure triangle that differs the most in how it is considered between photography and videography. This makes it an ideal point on which to bring home the purpose of this article.

Shutter speed is not historically a concept used in cinema. They used something called ‘shutter angle’. This term comes from the days of film cameras where a rotating shutter disc spun in front of the film as it passed through the camera. This disc would have a wedge cut out of it and the angle of that wedge would correspond to the shutter angle. The most common shutter angle is 180 degrees, meaning the shutter disc would be missing one half. The shutter disc would complete one full rotation for every frame of film, which therefore meant that each frame of film was exposed for only half the time it was behind the shutter. Therefore, at a rate of 24 frames per second (the standard frame rate for movies) a shutter angle of 180 degrees will give an equivalent shutter speed of 1/48, as each frame is being exposed for one 48th of a second.

Despite having its origins in older film cameras this formula is immensely useful to modern DSLR and mirrorless shooters whose cameras probably don’t have a shutter angle setting. To achieve a look similar to a 180 degree shutter you simply double the figure of your chosen frame rate. So if you are shooting at 30 frames per second make sure that your shutter speed is set to 1/60. Why do you want this look? Simply put; it looks natural. Whether through years of conditioning or some other psychological principle it simply looks right to us when a video employs this rule.

So what happens if you don’t use this rule? Well, filmmakers break it all the time for artistic effect. Much like in photography a slow shutter speed for video will produce more motion blur. This can lead to a surreal effect often employed in dream sequences or hallucinations on screen. Conversely, a fast shutter speed will make the image appear ‘choppy’ which is often used to add energy to gritty fight scenes, generally ones featuring handheld camera work.

The thing about both of these effects is that you can get into them pretty quickly if you stray too far from the 180 degree shutter rule. This means that it’s generally best to use this rule if you want to ensure a natural looking image, but this presents a problem in itself. 

Remember, all three elements of the exposure triangle must be balanced to ensure a properly exposed image. If you can’t change your shutter speed for fear of producing an unnatural look then you have effectively removed that element of the triangle as a means of controlling exposure. What about the other two elements? It’s generally best not to bump up your camera’s ISO too far, especially for video. Video from DSLR and mirrorless cameras is usually a lot more compressed than photos and will show noise much more readily. Aperture? Aperture can be a very effective way of controlling exposure provided you don’t have artistic needs related to depth of field. If you want to have that nice shallow depth of field look then you must keep your aperture within a certain range to preserve this, at the cost of being able to control exposure. The same is true if you want a deep field of focus.

So your shutter speed is determined by your framerate, your ISO is limited by your need for a quality image, and your aperture is limited by your artistic intent. How the hell are you supposed to control your exposure?

Thankfully, while the triangle is foundational to any understanding of exposure, it is not the only way of controlling it. There are a number of ways one can influence exposure outside of the camera and lense, lighting being the most important. In fact, besides the exposure triangle itself, learning how to properly use lights is probably the most important skill any aspiring videographer or photographer can develop. It will not only help you to properly expose your images, but it will also allow you to shape light around a subject, to achieve a certain look or atmosphere, and to spot good lighting out on the street or in nature.

Some other common methods of controlling exposure include ND (Neutral Density) filters and negative fill. ND filters sit in front of the lense like any other filter and serve to reduce the amount of light getting into the lense, almost like sunglasses for your camera. Negative fill technically comes under the heading of lighting. It’s when light is actually taken away from a scene, usually by putting something in the way of the light source so as to make certain parts of the image darker or to prevent the uncontrolled spill of light. These bear mentioning in an article about the exposure triangle because when shooting video at a high level the triangle itself is almost never enough to ensure that you get the image you want. We’ll dive into these areas in greater detail in future articles.

Well I think I’ve rambled on long enough. As potentially long-winded as this article is, I hope it gives any beginners out there a solid understanding of not just the exposure triangle but also how it differs in it’s application between photography and videography.

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