# What is the “exposure triangle”?

What is the exposure triangle? How do the "sides" affect my photographs?

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This is where @mattdm shows up with his rant about exposure triangles and its better as a rectangular prism (which it really is)....photo.stackexchange.com/questions/5586/… – rfusca Jan 7 '11 at 22:53

"The Exposure Triangle" is a catchy phrase meant to encompass the three factors which affect the exposure of a photograph of a scene with a given amount of light. It's often given to new photographers as a learning aid. I'm not sure if he invented it, but it is certainly popularized by Bryan Peterson, as in his book Understanding Exposure. (The popular web site Steve's Digicams gives Peterson credit for the term, as do other authors.)

The three factors are:

• Shutter — how long you let light in
• Aperture — how large of an opening you let light through (how much at once)
• ISO — how sensitive your film or sensor is to light

Each factor is interchangeable in terms of exposure, so that a decrease or increase in one factor must be met by the same amount of change in another. (More on this below!) And each has intrinsic secondary effects on your composition — longer and shorter shutter speeds freeze or blur motion, smaller apertures give more depth of focus, and generally higher ISO causes more noise as one attempts to get more signal out of less light.

The problem with the phrase "exposure triangle" is that the relationship between these three factors does not actually share any of the properties of a triangle other than "threeness". That makes it a bad analogy, which can introduce more confusion than necessary, as new photographers attempt to reason from the information they've learned.

Let's say we have a triangle where the sides (or corners — it doesn't matter) represent a shutter speed of ¹/₆₀th, aperture of f/11, and ISO 100. We draw a triangle, and we label that "Properly Exposed for EV 13". (See the Wikipedia article on EV for where the value "13" comes from). So far, so good — we've got our triangle.

Now, what if we want to change the shutter speed to ¹⁄₃₀th, and the aperture to f/16? That should give the same exposure. But what happens to our triangle? Do we double the size of one line, and halve the other? Or is there some other relationship that's meaningful?

It doesn't take much mucking about with a pencil and paper to determine that, no, this just breaks down. If you attach any actual meaning to the dimensions of the sides or the angles of the corners, not only is there no relationship to the area or size of the triangle, many settings yield impossible triangles even though they're perfectly valid exposure settings.

An alternate approach would be to leave the triangle's geometry constant, and put labels along the sides. This is somewhat more useful for thinking about the non-exposure effects of the parameters, but fails on actually showing anything about exposure. (And worse, may cause confusion by implying a linkage between the factors where they come together at the corners — how does bokeh connect with motion blur?)

Basically, it'd be just as useful to say The Exposure Clover, or The Exposure Tricycle, or the The Exposure Set of Juggling Balls. Or the Exposure Branches of the U.S. Government, although that may be harder to draw in an intro-to-photography book. I humbly submit the following illustration for anyone's use; I'll leave the other suggestions as an exercise.

(But I'm not ending with unhelpful sarcasm, I promise. Keep reading below.)

I'm joking, but I'm also serious: there's nothing particularly useful about the triangle for explaining these factors, and in fact it might be detrimental. So, we might as well use something funny, if the only point is to be memorable and associated with the number three.

Or, we could use a better representation: The Exposure Cuboid — a rectangular box. That sounds nerdier and not nearly as catchy, but it has the advantage of actually being useful. Each dimension — width, length, height — corresponds to one of the exposure factors, and the volume of the box corresponds to the the exposure.

This works out exactly right. Each of the three settings can be adjusted independently, and a doubling of any of them doubles the overall exposure — or cutting one in half cuts the overall exposure by half. This is just the same way that changing the length of one of the sides of a box changes its volume. So, we get both a visual analogy and an accurate mathematical representation.

And it's nice from a simplicity point of view, because we can start by leaving off one of the dimensions. If one were using this as a tool to actually explain exposure, we'd start with shutter speed and aperture alone — that's a (plain old regular two-dimensional) rectangle. The Exposure Rectangle still isn't quite as catchy as triangle, but at least everyone knows what a rectangle is, too.

Take some graph paper, and mark off shutter speeds along one side. Start at, say, ¹⁄₂₅₀th at the first mark, then double that time to ¹⁄₁₂₅th at the second mark, and then ¹⁄₆₀th at the fourth mark, and ¹⁄₃₀th at the eighth, ¹⁄₁₅th at the sixteenth, and so on. Actual doubling of time is directly represented by actual doubling of space. We'll get to the concept of "stops" in a bit. (And you'll notice that the number sequence I'm using isn't exactly doubling — for whatever analog-days reason, this is the standard sequence and it isn't exactly precise. That's another chapter, really — at this point, just consider it rounded-off and close enough.)

Along the other axis, we'll mark apertures. Start at the first mark with something small, like f/22. Then, follow the standard sequence of aperture values from there, again making sure to double for each rather than just marking each square — so, f/16 at the second mark, f/11 at the fourth, f/8 at the eighth, f/5.6 at the sixteenth, and so on.

Now, you can directly visualize the effect of increasing or decreasing one of the factors. The exposure of the scene is the area. Any combination of aperture and shutter speed which gives a rectangle of the same area will give the same exposure. Doubling the shutter speed actually doubles the exposure, just as one would expect.

If you draw a rectangle with one corner at the origin and going out to ¹/₆₀th and f/11, with the labels as above, you get a 4×4 rectangle — area 16. (Here, the actual number 16 is just an artifact of how we decided to start our labeling and the size of the squares — the actual number is meaningless.) Change to ¹⁄₃₀th and f/16, and it's 8×2, or ¹⁄₁₂₅th and f/8 for 2×8 — either way, still the same area.

And adding in the third factor, ISO, is simply extending this to the third dimension. This is harder to draw on paper or put in a book, but once you have the idea of the rectangle down, it's not a far leap conceptually. Imagine an ISO axis extending upward out from your page, labeled similarly to the others. The exposure value becomes the volume of the cuboid, and pretty much everything works the same way.

Another convenience is that shutter-priority or aperture-priority modes can be visualized as flat rectangles within the 3D box. Turn it so whatever two values are adjustable are conveniently aligned.

But a problem with this is that the paper — or cuboid — gets big pretty quickly. And those big areas get cumbersome to work with. That's where the concept of "stops" comes in. Rather than working with linear labels and area, we can work in a logarithmic space based on powers of two. That might sound intimidating, but it simply means that we count the number of doublings, rather than using the direct numbers. Again, sounds like I'm using too many big words, but the great thing is that counting is easy. In fact, you don't even have to count — every modern camera has a meter which "weighs" these things automatically and tells you how many stops you are off. Just like a balance scale.

That loses out on the concept of "threeness" entirely, and it feels a bit unnatural to add things to darken exposure. Someone spending a lot of time thinking about which direction is up isn't really learning the important part. It also doesn't cover flash duration properly. And I sure did end up putting way too much explanatory text there. That's problematic for an introductory diagram (although I'm sure with some work it could be cleaner).

Anyway, the point is: it's catchy to say "triangle", and since there's three factors, it seems obviously true, garnering automatic nods from people who are familiar with the concepts already. And for many people it's pretty harmless. However, there's a more useful geometric explanation which can actually be used to show how the factors really relate. The cuboid may feel like jumping in the deep end, but the rectangle shouldn't be too intimidating, and going on to the third dimension is a natural progression in the lesson.

If that seems too complicated, why bother with a geometric analogy at all? I know that in the last few years it's become a common idea and Peterson's books are very popular. I know many people even find it helpful. I'm not really on a Quixotic crusade to make sure every use of "the exposure triangle" is actively stomped out — but I do recommend against introducing it to new users. Putting the three factors in a bullet-point list works just fine, and doesn't add any inadvertent confusion. Or you can stick with the tricycle.

The source files for the latest version of this article, including SVG source versions of the images, are available on my web site.

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@mattdm - you are absolutely correct regarding the representation as cuboid vs triangle. The exposure space is a 3D space, not 2D, and where a setting in one axis does not limit the setting in another axis (hence a cube and not, say, a double tetrahedron). However, the CIE color space model is also, in a way, a 3D space (RGB) represented in 2D, so there are legs to the triangle model. – ysap May 26 '11 at 21:15
@mattdm - your update made the point even clearer, and I'll iterate that you are absolutely correct in that the space that is span by 3 independent vectors is a 3-space, which is represented by a cube, and not a 2-space, representable by a triangle. – ysap May 28 '11 at 15:52
@mattdm: I remain unconvinced by your arguments and intend to simply continue teaching the Exposure Tricycle as I have been doing successfully for many years now... Everybody understands tricycles... – Jay Lance Photography May 28 '11 at 17:51
This is beyond awesome. You should be writing books about photography. – Todd Lehman Jan 8 '12 at 19:22
@mattdm: Anecdotal evidence no proof. As you well know, analogies are methods to convey simple understanding. At some point they break down, as you so masterfully demonstrate--a reason why arguing by analogy is a logical fallacy. Agreed, a triangle conveys more than just threeness, but isn't the mere fact that is one of the simplest objects to visualize with threeness apt to make it easy to remember the three aspects to exposure? On paper a bulleted list is easy enough, but how about in conversation? Do people remember simple shapes better than they do a short list of words? – Alan Jan 20 '12 at 0:03

The exposure triangle refers to the three major settings that effect exposure:

1. ISO
2. Aperture
3. Shutter Speed

Digital Photography School has an excellent write up about this, but I will summarize as best as I can.

As the name suggests, each element effects image exposure, or how bright/dark the image is. Pick a side of the triangle, increase its value (aperture is special--a larger aperture corresponds to a smaller F-number), while holding the other sides constant, and the image will get lighter.

So for example you have an ISO of 100, Shutter Speed of 1/250s, and Aperture of F2.8, simply increasing the ISO to 200 will double the amount of light in the image. Likewise increasing the shutter speed to 1/125s, while holding other values constant, will again double the amount of available light in the image.

Now while, each of these three settings give you an independent control (the sides are not explicitly linked) of how much light goes into your image, each has a secondary use, often used for the artistic purpose of the photo.

As shown in the image:

ISO controls how sensitive the sensor/film is, which in turn effects how much noise/grain is in the image. For films, you could control the amount of grain in the image, usually for artistic effect. In modern digital camera's, ISO controls the amount of sensor noise comes from the camera sensor. While film grain was desirable in some situations, sensor noise is almost never desirable. So often photographers will pick the absolute lowest ISO setting they can get away with maintaining enough light to effectively shoot their images. The lower the ISO value, the less sensitive the sensore is, the more light required, and the less noise that is expected.

Aperture controls how large the lens opening is, allowing more or less light to come in, while also controlling the depth of field. The depth of field refers to the distance behind and in front of the focal point that is in focus. A shallow depth of field will "blur" out the background. This effect is often used for portraits to put more emphasis on the subject. The larger the aperture, the more light gets used, the shallower the depth of field.

Shutter speed controls how long the camera's shutters are open, again allow more or less light to come in, while controlling motion. A fast shutter speed effectively stops motion. This is useful for action shots, like sports. A slow shutter speed lets motion blur, many times used for artistic effect, such as blurring flowing water.

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+1 Superb! Love the image and how it relates secondary effects of each setting. – jrista Jan 7 '11 at 23:06
+1 Excellent response, should be in a text book! – John Cavan Jan 8 '11 at 0:20
"A longer shutter speed"? I think you mean a slower speed or a longer exposure – RedGrittyBrick Jan 8 '11 at 12:46
Yes, my terminology is confusing (though I think it's still correct (a 10 minute wait is longer than a 5 minute wait). – Alan Jan 8 '11 at 17:47
I like the text and explanation, but I'm really not convinced by the image. Can you explain how the triangle is a useful visualization? What does it add over, for example, listing all three values independently (as you do in the text)? – mattdm May 26 '11 at 18:45

I've found various versions of "exposure triangles" in the web, and also here on this site, but if I see it well, they all just show a qualitative relation between ISO, shutter speed and aperture to the related "secondary effects" noise, motion blur and defocus blur/diffraction. But the most important parameter LIGHT simply is disregarded. Of course it is true, 4 parameters are too many for a triangle, but here is my try:

Imagine you print this colored large triangle on a thick cardboard:

Then you print these 2 images, preferrably on transparent film, and cut out all the triangles (they are just examples):

Now imagine you measured the light of the scene and you found e.g. sunshine with EV15. Place the transparent EV15 triangle on the cardboard diagram and align it with the edges for ISO, shutter speed and aperture. Wherever you place it (as long as camera and lens allow such settings) you will get the same exposure. So this is actually a quantitative exposure triangle, that can tell you the exact relation between the 3 (or 4) parameters of exposure. But with the little colored triangles it also shows the qualitative differences in noise, motion blur and defocus blur/diffraction, just like all the other exposure triangles do.

What do you think? Can this be helpful? And please excuse my english, I'm not used to writing in english about photography in a forum. Should I replace the wording anywhere?

Best regards and thanks in advance, Matthias

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Don't worry about or apologize for your English; it's fine! – mattdm Sep 13 '11 at 21:04
I really like this; it's a clever way to think about it. – Evan Krall Sep 14 '11 at 4:12
I just was reminded, that the transparent triangles should read LV rather than EV. Sorry, that was my bad translation. So please think "LV", when you look at them. I will update the images later. – Matthias Sep 14 '11 at 8:26
Oh, it is even worse. The triangles have to be labelled with Bv (brightness value) according to APEX, and from the numbers I have to subtract 5. – Matthias Sep 14 '11 at 13:19
This is a little complicated to read, but very clever. I hope you get around to updating the images. – mattdm Sep 22 '12 at 20:23

While the other answers explain the concept of exposure triangle quite well, exposure triangle has the downside that it gives impression you only have three variables to control exposure - shutter speed, aperture and ISO sensitivity. Actually there is fourth variable that works similarly in the system - illumination of the scene.

By doubling illumination, any of the other three variables can be reduced by one stop; and by halving illumination, any of the other variables needs to be raised one stop. It also works in opposite direction - e.g. increasing ISO by one stop allows halving illumination.

Relationship between shutter time and flash exposure is a bit quirky since flash illumination is not linear in time:

• flash at faster than sync speed will illuminate only part of scene
• flash at slower than sync speed will illuminate just as much as on sync speed

Illumination can be increased by

• using an additional light source
• increasing power of artificial light source
• removing an obstructing object between light source and scene
• bringing a light source closer
• focusing light to a narrower area
• placing light source under angle where it reflects more (usually closer to on-axis)

and reduced by doing the opposite -

• removing a light source
• reducing power of artificial light source
• adding an obstructing object between light source and scene
• placing a light source further
• spreading light over wider area
• placing a light source under angle where it reflects less (usually further from on-axis or behind scene)

Some light sources are not under photographer's control, but their behavior can be somewhat anticipated (such as sunlight or streetlights).

Photography means "writing with light". Exposure triangle gives the impression that you may not bring your own pen.

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 I touched on this briefly at the end of my answer, but this is a really nice explanation. Thanks. – mattdm May 30 '11 at 12:36

Given all the excellent answers so far, I'd just add that whoever came out with this metaphor, probably had a bad (but catchy) selection of words, and what he meant to say is "the exposure tripod" - i.e., "the three legs of exposure". He did not intend to imply a relationship (dependency) between the 3 variables, but merely to name the parameters affecting the exposure.

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Yeah, I thought about that too. There's the issue with confusion with actual tripods, which may or may not be problem. But worse, tripod implies that the three "legs" need to be adjusted with each other to get a "level" exposure — another broken analogy, since what needs to be balanced is these factors with the lighting of the scene. (I'm actually in-progress with drawing a balance scale to illustrate the last part of my answer, about stops.) – mattdm May 28 '11 at 16:28
But there is a relationship between the three variables: exposure. If you're trying to take a photo exposed to a certain value, you only have two degrees of freedom -- if you change one variable, you must change one or both of the others. – Evan Krall May 28 '11 at 18:12
@Evan Krall - what I meant is that the three can be changed independently. I was aiming at the semantics rather than at the model. in a 2D space span by 3 vectors, a change in one will cause a change in at least one other. Anyway, the exact mathematical representation is a cuboid, as @mattdm suggests. It is important to realize that a constant exposure in this space is not a plane cut in a the cube, but a hyperboloid. Any point on a given hyperboloid will yield the same average brightness of the image. Jumping to a higher hyperbole means a brighter image and vice-versa. – ysap May 28 '11 at 21:22

I've seen several illustrations and comments as to whether they do or not convey the relationship between the three parameters: ISO, shutter speed and aperture. Please allow me to point you to the illustration on my article on exposure triangle. Once you have seen the illustration, think of it the exposure triangle this way:

If you pull one of the angles of the triangle further from the center (thus increasing its value) then it will automatically, in turn pull one or both of the other angles (thus decreasing its/their value). While the exact math is not necessarily respected, the conceptual visualization is there.

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 Basically what you're doing here is projecting the 3d cuboid into a 2d space (where your blue lines are the x, y, and z axes), and then drawing a triangle connecting the different factors. If you draw the 3d cuboid instead, you have my answer. And I still think looking at the entire shape is more useful than an arbitrary triangular slice — doing that really seems like more of a stretch made just to preserve the triangle phrase than an actually useful tool. – mattdm Sep 22 '12 at 20:18