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Now we come to how ISO, aperture, and shutter speed relate to one another. They exist to balance the amount of light in the scene with the recording made by the sensor and ultimately made into an image file on the memory card. Your goal is to record the scene with as true and balanced a color and lightness of what you saw when you snapped the shutter.

To review, the ISO setting of the camera sensor is the sensitivity to light. We raise or lower it according to the light level and how we want to utilize aperture and shutter speed settings.

All Photos © 2008, George Schaub, All Rights Reserved

The aperture and shutter speed solve the exposure problem and create balance between the brightness outside and the sensitivity of the sensor inside the camera. We control them for various image effects, weighing one against the other in every frame we take. We want to create balance, or equilibrium between all the factors.

An equivalent exposure is one that relies on the same amount of light for the proper exposure but juggles the aperture and shutter speed settings to yield different image effects. This waterfall was photographed with two exposures, one at f/22 at 1⁄15 sec and one at f/8 at 1/125 sec. This is the same, or equivalent exposure, but the result is entirely different in terms of image effects. The narrower aperture resulted in a slower shutter speed to make the water appear like a ribbon, while the wider aperture caused a faster shutter speed and changed the effect.

Let’s define some terms. There’s a commonly used abbreviation for working with light and that’s EV, or Exposure Value.

EV is used in many ways, but just keep in mind that it always deals with working with light. It can be:
The combination of aperture and shutter speed that create a certain exposure.

It can be used as shorthand for a light level (how bright the scene is, expressed in aperture and shutter speed combinations at a certain ISO setting).

EV is also used to denote a change in that level, either through the actual amount of light in the scene getting brighter or dimmer, or in how you change the combination of aperture and shutter speed to allow a certain amount of light through the lens to make the exposure.

Each time you photograph you make a choice about image effects. When you work with aperture you can have a shallow or deeper depth of field, which changes the foreground/background relationship of subjects within the frame. Here the flowers and church are photographed at f/11 at 1⁄60 sec and f/4 at 1/500 sec; again, equivalent exposures yielding quite different image effects.

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For example, if double the shutter speed (say from 1/250 sec to 1/500 sec) you are changing that value by 1 EV. And if you halve or double the aperture setting (which you do by going from, say f/8 to f/11) you are changing the value by 1 EV. So, think of EV as shorthand for how we deal with changes in light levels or light recording in photography.

You’ll also encounter EV numbers in exposure compensation, flash output compensation, and in some instances in actual exposure settings.

The other terms used are AV (Aperture Value) and TV (Time Value) (or just S) for Shutter Speed or Value.

So now let’s join all these terms and see how they dance together.

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