Exposure Triangle

• Shutter Speed

• F Stop

• ISO

Shutter Speed

Shutter speed Is the length of time light is allowed to hit the sensor. It is measured in seconds. Shutter speed is probably the easiest of the

exposure triangle sides to understand. To double the amount of light, we need to double the length of the exposure. For example, moving from a

shutter speed of 1⁄60 s to 1⁄30 s will add a stop of light because the shutter will remain open twice as long. Changing from a shutter speed of 1s to 1/8 s will decrease the exposure by three stops. Why? From 1s to 1⁄2 s is one stop. Then 1⁄2 s to 1⁄4 s is another stop. Finally, 1⁄4 s to 1⁄8 s is a further halving of the time the shutter remains open or the 3rd stop.

AperatureAperture 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.

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.