APERTURE & SHUTTER SPEED
THEIR RELATIONSHIP

by Florence W Deems

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Aperture is the name of the lens opening. F/stop refers to the lens opening setting (designation on the lens). Most people use these terms interchangeably. The aperture is made of blades around a central opening. These blades can open wide or be closed down until only a small opening is left. The size of the opening is written as f/5.6 or f/8, or so on. Below is a diagram that shows 3 apertures with their f/stop designations:

In larger cameras the shutter opens and closes at the rear of the camera body right in front of the film or the digital sensor. Sometimes, especially with smaller fixed lens cameras, the shutter is built into the lens instead. Wherever it's installed, it's the shutter that stays closed until you click on the shutter release button. Then it opens for a certain amount of time to allow light to reach the film or sensor, and then closes.

The aperture or f/stop and the shutter work together to determine the exposure - which means the amount of light that is allowed to fall on either the film or the sensor. If we have an understanding of this relationship, then we have better control over our exposures and thus produce images with best exposure.

This relationship is reciprocal. This means that if the aperture or f/stop is wide open (lots of light), then the shutter speed must be very brief (very little light). As we close down the opening in the lens (smaller amount of light), then we must slow down the speed of the shutter (more light), so that the amount of light that enters the camera is the same as in the former setting.

An example would be: The camera's light meter says the aperture is f/8 and the shutter speed is 1/250th of a second. But you want to open the aperture one "stop" to f/5.6 (or twice as much light). So this means that since the lens is admitting more light, then the shutter must close by one "stop" faster. So the shutter speed must be 1/500th of a second (or half as much light).

A "stop" means a certain amount of light. If a lens is open to its widest, say f/4, but you want to reduce the amount of light that goes through the lens by half, then this is considered one full stop. The lens' new aperture will be f/5.6. Or if the lens is set for f/5.6 and you want to let in twice as much light, then you would open the aperture again one full stop, to f/4.

In a similar manner, if the shutter speed is 1/125th of a second and you want to cut the light by half, this means cutting the light by one full stop - so the setting would be 1/250th. But if at 1/250th, you want to double the amount of light, then you'd slow down the shutter speed by one full stop to 1/125th.

For apertures, to determine the f/stop, you multiply the widest opening by 1.4. This concerns the actual area of the opening, rather than its diameter - a little bit of geometry that we don't have to figure out. Just know that the factor is x1.4.

Fortunately for us, figuring new shutter speed settings is simply multiplying or dividing - by 2. And fortunately for us, all cameras have automatic settings so that when we decide on a particular aperture, also called Aperture Priority, the camera chooses the correspondingly "correct" shutter speed. And vice versa - for Shutter Priority, the camera then chooses the "correct" aperture.

But there may come a time when we want to override the camera's automatic settings. Some point and shoots, most "bridge" or non-interchangeable lens cameras, and all the DSLRs (interchangeable lens cameras) have a Manual setting. This means that we choose both the aperture and the shutter speed. So this is when we really need to know the relationship between the aperture and shutter speed.

Below are two charts. On the left chart, beside a gray scale that shows the progression from not very much light to a lot of light, are the f/stops listed in one stop settings from smallest to largest. On the right chart, the gray scale is reversed, from a lot of light to not much light, along with the shutter speeds in one stop settings from slowest to fastest. I flipped the gray scale to show the reciprocal relationship between apertures and shutter speeds.

Chart below: the most-used apertures will be the 6 openings from f/16 to f/2.8; while the most used shutter speeds probably will be the 6 speeds from 1/30th to 1/1000th seconds. F/stops on the first row; shutter speeds on the 2nd row. So the relationships will look like this:

To compare the relationship between aperture, shutter speed and ISO, here's the info: ISO.

For more info on the relationship of ISO, shutter speed and aperture, please read this Digital Photography School's article.

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