It's Time You Learnt About the Exposure Square

It's Time You Learnt About the Exposure Square

You've heard of the exposure triangle, which is the relationship between the three main camera settings in order to take a well-balanced exposure, but there's a fourth dimension that also has a huge impact on what your photos look like.

The exposure triangle is one of the first things we learn when starting out in photography. This is the visual cue we give to balancing three main camera settings involved with capturing a decent exposure, namely: aperture, shutter speed, and ISO. It all seems a bit daunting at first because there are lots of numbers to remember, things called f-stops, and people talk about apertures or ISO as being "fast". But soon we come to realize that it's not just these three major components that influence how we photograph and what our end results look like. There's a fourth major element that drastically changes how you shoot. You can skip to the bottom to find out the fourth element that forms this exposure square, but for now, let's recap the three main parts of the exposure triangle.

Aperture

Aperture open on lens
This lens' aperture is set to its maximum f/2.8 and you can see right through the lens

The aperture is the size of the hole in the lens. It lets light pass through and onto the film or image sensor. There's nothing particularly special about the aperture in a lens, but there are some specific characteristics to get into your head. First up, the wider a hole in a lens, the shallower the slice of focus is in front of the lens. If the hole is narrower, then the slice of focus in front of the lens will be deeper. This slice of focus is referred to as depth of field. The depth of field dictates how much of the scene in front of the lens is in focus, front-to-back.

Narrow aperture on lens
The lens is now stopped down to the narrowest aperture it can go to, notice the much darker interior of the lens and the tiny hole in the center

A lens' aperture (the hole) is measured in f-stops (hey, Fstoppers, get it?). Weirdly, the lower the f-stop number, the bigger the hole. So a wide hole (or wide aperture) might have an f-stop number of f/1.8 but a narrow hole (or narrow aperture) might have an f-stop number of f/16. A bit counterintuitive, but there we go. If it helps you remember you can use this: for a big depth of field use a big number, or for a small depth of field, use a small number.

Shutter Speed

The shutter is the little curtain that sits in front of the image sensor (or film) that opens when the shutter release button is depressed. The light then passes through the lens and onto the sensor to make an image. The number in your camera literally translates to the length of time that the shutter is open.

Imagine it like the stage curtains at the theater. They open for one second and are then quickly drawn again, that's a shutter speed of one second. Or perhaps the curtain is open for ten seconds, now you can see the actors moving and breathing as they move around the stage. The same is true with the camera. The longer the shutter is open, the more any movement is captured in the final image, and it results in a blur. So if you're capturing something fast-moving, you might want to use a faster shutter speed so that the curtains are opened only for a fraction of a second and your eyes (or the camera's image sensor) don't have time to register the movement. Essentially, the faster a shutter speed, the more like your subject is frozen in time.

Shutter closed on camera
The shutter in this camera is a black curtain that sits in front of the lens and moves out of the way when taking a photo
Shutter open on camera
The shutter is now open and the image sensor is revealed. Any light that travels through the lens now strikes the image sensor and is recorded as a photo

Let me confuse you a little, some modern cameras do not have a shutter because they use electric to scan the photosites (pixels) on the image sensor to take a photo. There are some good and bad points of using a camera like this, but ultimately all you need to know is that a camera without a shutter might still refer to the exposure length as shutter speed.

ISO Sensitivity

ISO is a (rather backward) acronym for the International Organization for Standardization. They develop and publish standards across a wide range of stuff ranging from health and wellbeing, to clean water and sanitization, and industry, innovation, and infrastructure. A digital camera has an image sensor that has photosites (read: pixels) that are sensitive to light (and to a lesser extent, heat).

The standardized chart for measuring this sensitivity is done in roughly equal tens, hundreds, and thousands. A lower number (ISO 50) is less sensitive to light than a higher number (ISO 1,000). This is perfect in theory because when it's bright and there's plenty of light you can keep the ISO low, but when it's darker you can turn it up. But it comes at a cost.

High ISO forest scene
The high ISO in this shot has resulted in discolored noise in the shadows

The higher the ISO sensitivity the more noise, or grain, appears in your photo. The extra charge running across the image sensor creates electrical interference (and sometimes heat) which is then recorded by the highly sensitive photosites on the sensor. So ideally, we keep the ISO as low as possible to make for a cleaner picture.

So why ever shoot with a high ISO? Well, it depends on what you're shooting and how much light is available. Let's say you're trying to capture an owl in flight. Owls typically come out at twilight or in darkness, so light levels are much lower than in the middle of the day. But you still need to get that owl nice and sharp so you'll use a fast shutter speed, maybe 1/1000 sec. You've opened your aperture wide to f/2.8 but it's still too dark. So you turn up the ISO to compensate. Sure, you'll get more noise, but at least the photo will be sharp. This is the third and final part of the exposure triangle, but there's a fourth important aspect to shooting that massively impacts on your ability to get a decent picture, and can dictate the settings of these three camera settings, and that's the lens' focal length.

High ISO on back of camera
High ISO sensitivity can result in additional noise and poorer image quality

Focal Length

You see, it's all well and good dialing in the "perfect" settings to capture your subject, but the focal length of the lens changes things wildly. The focal length is the distance between the optical center of a lens and the image sensor. The longer the focal length the more "zoomed in" you are to a scene. For instance, a wide-angle lens with a short focal length may have a focal length of 24mm whereas a telephoto lens with a long focal length may be 200mm.

So how does the focal length of a lens change the three settings in the exposure triangle? Well, imagine you're standing outside with a laser pointer. You shine the laser on a nearby wall about 100 meters away. A small movement with the laser pointer shows up as a huge movement of the dot on the wall. The same is true with your camera and your subject. The longer the focal length the more exaggerated this effect is, with only minute movements at the camera-end resulting in massive changes at the subject-end. Remember that longer shutter speeds capture movement as a blur more easily and we can see why this might be a problem. You can see this in the example below where a 50mm shot appears markedly sharper than the blurrier 200mm photo.

Shooting a football match from the sidelines is tricky because the players are running around the field and often quite far away, so you'll want a long lens (long focal length) to zoom in close to the action. But if you shoot at a slow shutter speed any camera movement will result in blur, so you have to choose a shutter speed that's fast enough to freeze any movement to get a sharp photo.

This has a knock-on effect. Now you have to shoot with a fast shutter speed but your exposure is too dark, so you have to either open your aperture wider to let more light in but that changes the depth of field and you may not want to change that for a multitude of reasons. So the only other option (aside from adding your own light) is to boost the ISO sensitivity to account for the darker exposure, but then you're met with more noise in your photo. 

So as you can see the focal length of a lens has a direct relationship to the camera settings in the exposure triangle. There's even a rough law that photographers follow in order to monitor the issue between shutter speed and focal length, where a law of reciprocals falls into place in order to avoid camera shake blur. For example, a 200mm lens should have a shutter speed no slower than 1/200 sec. If you're on a 50mm lens then this is 1/50 sec and a 24mm lens this is 1/24 sec (usually rounded up to 1/25).

Lens Depth of Field

Tamron Telephoto lens
Telephoto lenses naturally generate a perceived shallower depth of field than wider lenses

Due to the physical properties of how light waves propagate and converge we find that wide-angle lenses naturally create a longer depth of field than their longer focal length counterparts such as telephotos. If we shoot a portrait at f/5.6 on a 24mm lens and compare that to f/5.6 on a 200mm lens we can easily notice the difference between the two images' perceived depth of field. The longer the focal length, the shallower the depth of field appears and as such requires us to alter our aperture to account for this. And as before, the alteration of aperture then has a knock-on effect on shutter speed and ISO sensitivity and so on, and so on.

Conclusion

It's for the above reasons that I think the exposure triangle is actually more of an exposure square. Sure, the three camera settings of aperture, shutter speed, and ISO are the key to making a good exposure, but the focal length of a lens has a huge impact on what those three settings ultimately are and even impact on the kind of kit you take when shooting. Those that photograph with huge lenses such as 500-600mm will need a monopod to steady the beast for tracking and to keep things slightly more stable so that the shutter speed doesn't have to be ridiculously fast.

In all, you can't have an exposure triangle outright, you must take into account how the focal length of a lens interferes with the settings and this shapes your ability to capture certain styles of images. So, in my opinion at least, it should be more of an exposure square, than a triangle.

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24 Comments

Tom Reichner's picture

Jason,

Thanks so much for writing an article yourself and posting it here. This is so much better than having to click on a link to watch or read something on some other website (which I simply will not do). I appreciate the time you put into original content.

I certainly agree that focal length is important, in that it often plays a major factor in what a captured image looks like. I think that most people don't think of it as part of the exposure equation because, to many of us, exposure has only to do with how bright or how dim an image is.

Other aspects of the image, such as (camera or subject) motion blur, color cast, etc., are very important, but because they deal with things other than how bright an image is, they are not considered to be part of "exposure". Perhaps this is why most folks think of exposure in terms of a triangle, and not a square. I suppose it's really just about the semantics of what is, and is not, considered to be part of the exposure of an image.

There are some folks that do not even consider ISO sensitivity to be part of the exposure equation, because, to their way of thinking, it really doesn't have anything to do with how much light the recording medium was exposed to. I have a hard time disagreeing with this, because ISO technically does not affect the exposure itself, if one is a semantic purist.

Technically, exposure only involves how much light the recording medium was exposed to, and therefore the only things that actually affect exposure itself are shutter speed and the lens' light transmission (which involves more than just the aperture). To be consistent and precise, exposure should be spoken of in terms of T stops instead of F stops.

Alexander Petrenko's picture

Let’s make it pentagon by adding IS and IBIS.

stuartcarver's picture

Let’s not.

Gary Pardy's picture

Well, that was cute. Clickbait achieved.

Uddhav Sabal's picture

More important than the opportunity for Costmar and Persson, however, is the health and safety of all the players, coaches, and staff heading to Edmonton for the World Junior tournament. https://worldjuniors2021ca.com/

Alex Reiff's picture

I've been kicking around an idea for a while of developing a new, more comprehensive exposure shape. I was thinking of adding filters, flash, and ambient light, but focal length isn't something I would have considered personally.

Colton Stark's picture

If you shoot large format or even some medium format cameras, you have to consider light fall off when your bellows extends past a certain point.

Malcolm Wright's picture

Nice article.
I would personally still stick with the exposure triangle, but add the caveat that every different focal length lens, camera body, processor, sensor, Ibis, lens IS, anti- aliasing filter or lack of, plus added lens filter(s), results in a unique exposure triangle for that particular configuration.
Just as variable weather or light conditons do.

Dave F's picture

Exposure is only about light, not focus or freezing motion. The clarity of the image has nothing to do with exposure. A sharp image and a blurry one can have the same amount of light. The fact that you change settings to account for the clarity of your image is valid, but not when defining exposure. Period.

By that logic, the size of your tripod affects exposure too. Let's make it an exposure pentagon! And what about the surface you're standing on? Maybe it's a floating dock vs solid ground. Better make it a hexagon. Wind speed? Lordy, it's an exposure heptagon.

Sorry, no.

John Peltier's picture

Don’t forget about white balance, polarizing filters, high-key vs low-key styling...sweet Jesus, this is going to have a lot of sides...

Mike Shwarts's picture

Well that is a stretch. What if I am shooting a stationary subject and my camera and long focal length lens is on a tripod? Then I can get away with a much slower shutter speed. Your example of needing a fast shutter speed for a long lens or a particular aperture to get desired DoF are just the starting point. It is from that starting point that you then decide on the choice of other two factors (sides of the triangle) to get desired exposure. You have not found a new side to make a triangle into a square. You have only demonstrated that to use the triangle to get desired exposure, you must start with one of the sides before proceeding to the other two sides.

Dan Jefferies's picture

I couldn't figure out how to say it. Good job.

Khuất Nguyên Vũ's picture

When I read the title, I thought the fourth one is Flash Power.

Alexander Petrenko's picture

For photographers using artificial light that’s true.

Ryan Swank's picture

This is dumb. Your subject does not affect exposure which is essentially what you're saying when it comes to the "impact" of focal length on exposure. Capturing action without blur impacts shutter speed. Longer focal length impacts handheld camera shake. None of that really impacts the exposure. I'll never consider focal length as an input on exposure. The focal length is just connected to shutter speed depending on my subject and artistic intent and whether I'm handheld or on a tripod. Additionally, what depth of field has to do with ev is beyond me. Again, depending on artistic intent, wanting deep or shallow dof determines your apeture and thus your other 2 variables for the ev. This is just silly.

Wolfgang Post's picture

If one wants to stay in Geometry, and for the sake of argument let's keep it an exposure triangle, then I would suggest drawing a circle whos circumference touches the corners of the triangle and dump all these other points along it. Focal length, subject movements etc. are affecting the shape of the triangle, but they are not part of it at all.

Stu Eddins's picture

While I see where the author was going with this for me it's a stretch. In any given situation the hardware is the constant, aperture/shutter/ISO are the variables used to achieve exposure. Change lenses and you have a new constant. I'm not aware of any time that I've thought to myself 'you know, if I shoot this with a longer lens I'll get the exposure I want'. I'm all in with lenses changing composition and exposure requirements, but it's not a really workable model to add focal length and expand from the exposure triangle.

In fact, some would argue that with ISO invariance it is no longer a significant part of the exposure equation. But that's a whole 'nother argument.

Stuart Brereton's picture

I hope you didn't get paid for writing this crap.

Tom Reichner's picture

That's not a very nice thing to say, nor is it very insightful.

Jason did make some good points in this article. But he seems to have categorized his points in an odd way, because "exposure" only refers to the amount of light that the medium is exposed to, and he seems to take the liberty of making the term a much broader one.

Many of the things that are part of an image have nothing to do with exposure. Exposure ONLY refers to the amount of light that the recording medium was given at the time of capture. Therefore, and "exposure square" is not really a term that respects the actual definition of the term, "exposure". We really do need to keep the semantics here very literal, and not take liberties with terminology.

Pierre Dasnoy's picture

I didn't come for 3 months, and because of this article, I won't come back for another 3 months or more. Thanks for reminding me the low level of this clickbait website.

John Peltier's picture

It’s become the BuzzFeed of photography sites.

William Hutton's picture

As written this article contains misinformation and should be retracted. Understanding exposure is important because exposure determines signal levels and the signal is the information content source used to render an image.

When the luminous flux incident on unit area of a surface (illuminance) is constant, exposure depends on the shutter time and lens aperture. Exposure occurs only when the shutter is open because each photosite can only generate photoelectrons when the shutter is open.

When the illuminance generates enough photoelectrons to exceed a photosite's ability to store the resulting electrical charge, that photosite is overexposed. When the electrical charge is below the photosite's maximum linear response level, that photosite is underexposed. Underexposure decreases signal levels which reduces information content. Unless the illuminance is spatially uniform, some sensor photosites must be underexposed. The goal is to minimize their underexposure while avoiding overexposure of photosites for the brightest regions of interest.

At constant illuminance the irreducible number parameters required to describe for exposure is two. There is no exposure triangle. And an exposure rectangle is absurd.

What about camera ISO setting? Any change in camera ISO setting affects image brightness. Any change in camera ISO setting occurs after the shutter closes. How can exposure increase when the shutter is closed?

Camera ISO setting changes light meter response in order to estimate two exposure parameters (shutter time and aperture) and an image brightness parameter (analog signal gain, in-camera raw-data integer multiplication - or both). Together these three parameters produce an aesthetically appropriate rendered image brightness.

Often camera and, or subject motion along with depth-of-field requirements require the shutter time and, or aperture to be set such that the scenes brightest regions are underexposed. The result is an image with insufficient brightness. Camera ISO settings, other than the sensor's native value, affects image brightness after the shutter closes. But increasing image brightness after the shutter closes can not increase exposure. What the author describes as an "exposure triangle" is an image brightness triangle.

Put another way, the information content can not increase when the camera ISO setting increases. This is important. When camera ISO setting is above a the minimum value required to address subject motion and, or depth of field requirements, data information content is needlessly compromised. This is an indirect effect since the light meter estimate for optimum image brightness leads to a shorter than necessary shutter times and or a narrower than necessary apertures.

Use the lowest practical camera ISO setting above sensor's native sensitivity to optimize exposure.

What about changes in illuminance due to changes in focal length? These are no different than any number of circumstances where scene illuminance changes while the shutter time and, or aperture remain constant. Examples would be changing lens hoods, using lens filters (especially rectangular ND gradient filters that are moved over the lens), adding on and off-camera supplemental lighting, using another lens with identical focal length and aperture that has a higher T factor, etc . In all these cases the shutter time and, or lens aperture are adjusted to optimize exposure. The number of polygon sides to describe all the possible sources of scene illuminance changes is much larger than a square.

John Peltier's picture

Coulda shortened it to just the first and last sentence, but thanks for the important background for those who want it :)

Dick Rehnquist's picture

This article is unbelievably bad and almost entirely wrong. The explanation of aperture is one of the worst things I have ever read, especially this part:

"A lens' aperture (the hole) is measured in f-stops (hey, Fstoppers, get it?). Weirdly, the lower the f-stop number, the bigger the hole. So a wide hole (or wide aperture) might have an f-stop number of f/1.8 but a narrow hole (or narrow aperture) might have an f-stop number of f/16. A bit counterintuitive, but there we go."

F-stops are only are "weird" and "counterintuitive" if you don't understand them, and the writer perpetuates this misunderstanding by making no attempt to explain what the "f" stands for or why a larger aperture (sometimes) has a smaller f-number. It's shameful that this garbage was filed under "education".