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The Exposure Triangle: Understanding How Aperture, Shutter Speed, and ISO Work Together

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This article is shared from our friend Alex Cooke at Fstoppers!  The original article can be found here: The Exposure Triangle

Don’t worry, this isn’t a geometry lesson. The exposure triangle is a common way of associating the three variables that determine the exposure of a photograph: aperture, shutter speed, and ISO. One must balance all three of these to achieve a desired result, an adjustment of one requiring adjustments of at least one of the others. They do not only affect exposure, but are also the largest determiners of the global appearance of an image; thus, their mastery is absolutely crucial both for technique and composition.

Side 1: Aperture

Aperture is a measure of how open or closed the lens’ iris is. A wider aperture (or lower f-number) means more light will be let in by the lens, simply because the opening is larger. A narrower aperture (or higher f-number) allows less light to reach the sensor.

You might wonder why we would ever want less light to reach the sensor. The answer the majority of the time is that we want a larger depth of field. Depth of field is a byproduct of aperture. Narrower apertures (higher f-numbers) give a greater depth of field, allowing more of a scene to be in focus (think landscapes). Wider apertures (lower f-numbers) create a narrow depth of field, which can help isolate a subject and is one of the greatest compositional tools at your disposal (think portraiture).

You should also note that most lenses are their sharpest around f/5.6 or f/8. However, many photographers are willing to trade some sharpness for the subject-isolating effects of a wider aperture.

Side 2: Shutter Speed

Shutter speed is a measure of how long the shutter remains open and thus, how long the sensor is exposed to light. Faster shutter speeds give the sensor less time to collect light and thus, result in a lower exposure. Slower shutter speeds allow more time for the sensor to collect light and result in a higher exposure.

In this case, the reason we might want to use a higher shutter speed is to stop motion, whether that be camera shake or a subject that is moving, allowing us to maintain sharpness. Remember, as long as the shutter is open, the camera is essentially recording the position of elements in the frame; if one of those elements moves, the result will often be undesired blurriness.

Many photographers will argue that this is the most important aspect, saying if your shutter speed isn’t fast enough to give you a sharp image, nothing else will save the image. In general, I agree.

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The use of a long shutter can create very pleasing effects as well. 6s, f/16 at 17mm and ISO 100

Side 3: ISO

Back when film ruled the land, there wasn’t the kind of flexibility in this third side that we have now. You might say the exposure triangle was a two-sided polygon (the geometry of that is another discussion, but I promised this wasn’t a geometry lesson). One could control the sensitivity to light of the film they used, but once the roll was in the camera, there was no changing it (unless you had a swappable back, but these were generally restricted to the medium format world). Nowadays, we can control the sensitivity of the digital sensor on the fly, though technically, we’re not controlling the sensitivity; this actually controls a post-image gain applied to the signal, but for all intents and purposes, you can think of this as sensitivity.

Increasing the ISO essentially allows you to work with less light. As always, though, there’s a tradeoff: increased ISOs result in increased noise and less detail. Noise is the result of random fluctuations in an electrical signal. At lower ISOs, the magnitude of the image signal is large relative to the noise (signal to noise ratio), meaning the noise generally remains unobtrusive. When working at higher ISOs, the image signal is generally close in magnitude to that of the noise and thus, noise enters the image.

Think of the image signal and noise as a pit of balls. If my image signal is 1,000 balls, I won’t notice if noise adds 4 or 5 balls to the bunch. If my image signal is small though, say 10 balls, it will be very noticeable if noise adds 5 balls to the bunch. When I amplify that signal through the use of a high ISO, the relatively high level of noise will be amplified considerably too. When high ISO makes the 10 balls into 1,000 balls again, the noise suddenly becomes 500 balls.

So, why ever use a high ISO? Often, when working in lower light, you will find yourself at a point where you are using the widest possible aperture and the slowest shutter speed you can to stop action. At this point, your only choice is to increase the ISO. The lens cannot physically open itself any wider and as discussed above, sacrificing sharpness for a slower shutter speed is rarely advisable. I would rather have a grainy image that shows a well-defined subject than a smoother image with a subject lost in blur.