Sunday, February 5, 2012

Understanding exposure - stops


Another term that posses problems for beginners is the word stop. It is used in just about every aspect of photography.

APERTURE: "I'm overexposed. I need to close up a stop."
ISO: "My light dropped a full stop during the time I was shooting the sunset. I compensated by bumping up my ISO a stop."
SHUTTER SPEED: "The difference between freezing and blurring the action was only a few stops." 
EQUIPMENT PHYSICS: "You will loose about a stop when you add an extension tube on your lens."
STUDIO LIGHTING: "Your main light needs to be two stops brighter than your fill light."

But what is a stop, and why does it apply to so much? We'll get into that but first you should realize that it has taken photography technology a long time to get here although the principle is as old as photography.

A stop is a measurement. It measures a small sliver of light and allows a camera to mix a bunch of light slivers until you get the right amount to record an image. Each one of these slivers changes the light by a factor of about 1.4 which basically doubles the light.

Smarter people than I came up with this value but it makes understanding how light is interpreted by the camera a little bit easier. All I have to remember is that adding a stop doubles the light and dropping a stop cuts it in half. But this is still a little vague.

The chart at left is a visual representation of stops. Each step illustrates a doubling of light by a factor of 1.4 (numbers in yellow). I have also included aperture, shutter speed and ISO scales, each increasing in steps by one stop. NOTE: F-stop, shutter speed and ISO numbers are grouped together for illustration purposes only. There is no correlation between these items to each other and are used to show the doubling or halving of light at each step.

So if we look at the aperture settings, as an example (in green), you will see that each f/stop increase opens the lens opening just a little bit. Each time it opens it allows twice the amount of light as the previous setting. Open it again and you double it again. Simple.

The same principle applies to shutter speed and ISO. With shutter speed we are dealing with a time value. When you slow down the speed of the shutter you are increasing the amount of time the sensor is exposed to light. Each shutter stop allows twice as much light to fall on the sensor as the previous stop. You speed the time up and the reverse is applied.

ISO settings are the same. You increase the setting one stop and you increase the sensitivity to light. The more sensitive, the more light it records. You get the picture.

QUANTITATIVE EXPOSURE

When a camera makes an exposure (a picture) it does so in a quantitative manner. This means that is takes multiple slivers of light added together until there is enough light being collected to properly record on the sensor. This is where  your meter comes into play.

The way you attain proper exposure is by adjusting your camera's settings to allow the right amount of light in. Each element controls a different aspect of the image; aperture for depth of field, shutter speed for action and ISO for light sensitivity.

So as you are shooting keep in mind that once you set your camera for proper exposure you can make lateral adjustments  to your exposure. For example, if  you need more depth of field you close up on your aperture and open up on your shutter speed or ISO the same number of stops.

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