Monday, October 17, 2011

Elements of Exposure

It seems from my workshops this year that one of the concepts of photography people have some trouble with is exposure. So I thought I write a post on the subject.

Exposure can be controlled by:
  1. Shutter Speed
  2. Aperture
  3. ISO

Please Note:
The terms stop and f-stop can sometimes be used interchangeably in reference to exposure whereas f-stop is also used to describe the lens aperture setting. In either case we are talking exposure.

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SHUTTER SPEED
The length of time the shutter is opened, exposing the sensor to light. The sensor, or image sensor, is the electronic chip which “records” the image; it is this sensor which replaced film. The longer the shutter is open the greater the amount of light reaching and exposing the sensor.

APERTURE OR F-STOP
The iris or opening of the lens which allows light through. The wider the iris opens the more light is allowed to reach the sensor. Think of it as the iris or more specifically the pupil in your eye. When you go into a dark room your iris expands to let in more light so you can see. Inversely when the light is bright your iris contracts to reduce the light. Aperture is referred to in terms of f-stops and in today’s nomenclature a larger number f-stop say f8 is smaller when compared to f1.4, which is a larger aperture.

aperture_diagram

ISO
The sensitivity of the image sensor. With film it was referred to as film speed. Simply put the lower the ISO the less sensitive the sensor is to light, the higher the ISO the more sensitive the sensor is to light.

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Putting it all together

A stop also sometimes referred to an f-stop is half or doubling the light reaching the sensor.

In terms of shutter speed, moving from one of these values to another is one stop adjustment. So doubling or halving the shutter speed is ONE stop.

1/125 1/250 1/500 1/1000 1/2000

In terms of aperture moving from one of these values is an adjustment of one stop.

1.4    2.0    2.8    4    5.6    8    11    16    22

In this case f8 lets in half the light of f5.6 and twice the light as f11. Numbers wise doubling or halving the f-stop number is TWO stops adjustment (for example f5.6 to f11 is two stops).

In terms of ISO, moving from one of these values to another is one stop adjustment. So doubling or halving the ISO is ONE stop, as is the case for shutter speed.

100 200 400 800 1600

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Here’s how I remember it. If you double or half your shutter speed or ISO you are making a one stop adjustment. If you double or half your f-stop number, then this is a two stop adjustment.

I tried to make this as simple as possible while providing a minimal technical data. It often takes people some time to understand this or think this way. So don’t sweat it and if you have any questions please feel free to ask.