I’ve never liked the Exposure Triangle because it doesn’t reflect a good enough correlation between aperture, shutter speed, and ISO to one-another. For example, if shutter speed goes up, and you make that side longer, the other two sides of the triangle don’t react in a way that is indicative of real exposure balancing.
In this video, I introduce what I’m calling, the Exposure PIE. The exposure pie has three slices — aperture, shutter speed, and ISO. The more light you let in with the aperture, the bigger the pie slice for aperture becomes. The more light you let in with the shutter, the bigger the shutter speed pie slice becomes and finally, the higher the ISO, the bigger the ISO pie slice becomes. For proper exposure, when one slice changes size, you’ll need one or both of the other slices to change size in an opposite way. For example, if the slice of pie for aperture becomes smaller, that means you’re using a smaller aperture, letting less light in, to get proper exposure, you’ll need the pie slice for shutter speed, and/or ISO to get bigger to fill out the pie. For the shutter speed slice to get bigger, you’ll need to use a slower shutter speed. For the ISO slice to get bigger, you’ll need a higher ISO.
Let’s start out with Shutter Speed. The pie above has one slice showing that represents a shutter speed of 1/500th of a second.
If we want to let more light in, we could use a slower shutter speed. The pie above has a bigger slice for 1/250th of second because at that slower shutter speed, more light is getting in through to the sensor or, to the film.
It’s similar for aperture. At F8.0, the pie above is showing one slice that reflects an F8.0 aperture.
What if we use a larger aperture — when I say larger, I’m not referring to the number — I’m talking about the physical size of the lens diaphragm. A bigger aperture lets more light in hence, the pie slice is bigger. An aperture of 5.6 is bigger, and lets more light in than an aperture of F8.0. Hence the pie slice for F5.6 is larger.
ISO is the gain applied to the signal from the sensor, Above, we see a slice of pie that represents an ISO of 100.
Higher ISO, bigger slice of pie. ISO of 200 is indicated by a larger slice of pie than an ISO of 100.
All three attributes affect exposure and to get proper exposure, we need the pie filled in completely with all three slices. Above, we see a slice for aperture, F4.0 and a slice for ISO, 200. To get a proper exposure, the missing area that is gray, must be completely filled in by the shutter speed slice.
For this scene, a shutter speed of 1/60th of a second, coupled with an ISO of 200 and an f-stop of 4.0, completely fills in the pie and we have proper exposure.
But, what if we change any of the three attributes?
Here, we dialed in bigger aperture of F2.8. More light is coming in through the aperture so its slice, got larger. Since it’s slice is larger, we need one or both of the other two slices to get smaller so everything fits without overlap. In this example, we used a faster shutter speed. The faster shutter speed of 1/125th of a second let in less light than the 1/60th of second shutter speed. Less light coming through means a smaller pie slice for shutter speed and, we have a balanced the exposure!
Let us bring it to an extreme and really open the aperture!
Here, we open the aperture wide to 1.4 letting in even more light then the previous example’s aperture of 2.8. More light coming in the aperture means bigger aperture pie slice and that means, one or both of the other pie slices must get smaller. In this example they both did. 1/500th of a second shutter speed is letting less light in than the previous example of 1/125th of a second and, the ISO of 100 is applying less gain to the sensor than the previous example of ISO 200. So, both the shutter speed and ISO slices are smaller. Making those adjustments to aperture, ISO, and shutter speed mean that all three slices fit the perfectly with no gaps or overlap hence, a perfect exposure.
One last example, let’s go to the other extreme and stop the aperture down to F16:
Less light is coming in at F16 hence, smaller aperture pie slice (red). The other two slices must compensate for that so we get a good exposure. ISO of 500 gives us a much larger slice of blue, ISO pie and a shutter speed of 1/60th of second, takes care of the rest.
Quantitatively, all of the values will change as the light in the scene changes.
Qualitatively, I think the Exposure Pie is a better representation of the relationship between Aperture, Shutter Speed, and ISO as they relate to one-another.
In the video, I reference an article on the website PetaPixel. You can read it by CLICKING HERE.
Exposure Triangle used in video thumbnail:
WClarke and Samsara [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
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