Photography Basics - Introduction To Exposure

By Author: Adam Coupe
Total Articles: 18
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Introduction to Exposure - Getting the right amount of light into your pictures
Film cameras control exposure by enlarging or decreasing the size of the aperture (measured in f-stops) and by having a faster or slower shutter speed. Both of these features have an effect on the amount of light that reaches the film. Digital cameras may also have the same features, or they may simulate these features by making the sensor more or less sensitive to light. But in any case, manufacturers still talk in terms of apertures and shutter speeds even if they mean the equivalent thereof.
The other factor that affected exposure in a film camera was the sensitivity of the film or its ISO rating. Film with a high ISO rating was more sensitive to light and could cope with very fast shutter speeds, but could produce a grainier image. Obviously a roll of film would have a set ISO rating and you wouldn't be able to change it between shots. Manufacturers also talk about ISO ratings when referring to the sensitivity of the sensor in a digital camera. With a digital camera you can change the ISO setting for each shot if this feature is available ...
... to you.
Depending on the sophistication of a digital camera you might find some or all of the following exposure features:
Programmed Auto Exposure
This is the fully automatic option where the camera chooses the aperture and shutter speed for you. You won't even have to think about it.
Aperture Priority Auto Exposure
In a camera with this feature the user sets the aperture and the camera chooses the shutter speed. This is good for variations in light conditions.
Shutter Priority Auto Exposure
This is the opposite of Aperture Priority, so you set the shutter speed and the camera chooses the aperture. This a good for fast moving objects.
Manual Exposure
This option, found on higher end cameras, lets you choose both the aperture and the shutter speed.
PASM
This stands for Programmed, Aperture, Shutter, Manual, so a camera with PASM does the lot.
EV Compensation (Exposure Value Compensation)
This is a feature that will let you override the camera settings to lighten or darken your image.
Exposure Bracketing
This feature ensures the camera takes a series of differently exposed shots with one press of the button. With any luck one of the shots will be perfectly exposed. Or you can take the darkest and lightest image and combine them using photo editing software to get an image with both shadow depth and highlights - this technique is known as HDR and is made straightforward by image manipulation packages such as Photoshop. HDR allows these composite images to be correctly exposed for example in an image of a landscape by taking an image where the sky is correctly exposed with a dark foreground and other images where the foreground is correctly exposed with the sky burnt out, a wide exposure range can be captured and then recombined using HDR tools to create a 'super- image'.
Scene Modes
Scene modes are so common on digital cameras that they are almost de rigueur nowadays. They usually have names like "Snow", "Portrait", and "Sport". They are really just automatic aperture and shutter settings that are designed to give optimum results in tricky lighting conditions (like bright ski slopes) or when taking action shots (like racing cars.) Some cameras have a huge number of scene modes, like about 25, so there shouldn't be a lighting situation that the manufacturer hasn't thought of.
This overview has hopefully helped to explain the basics around the tricky topic of exposure on modern digital cameras - other photography articles are available as a series.
Adam Coupe is pro commercial photographer specialising in commercial photography and architectural photography for a wide range of organisations that need to project their products, people or brand in a vibrant way see http://www.adamcoupe.com