Photographic Innoventions: Mechanics of Exposure

Click for Larger Image. Copyright © Scott Thomas Photography 2007
Cinderella topiary at Epcot’s Flower and Garden Festival. Copyright © Scott Thomas Photography 2007
Nikon D70/18-200VR, 1/160s, f/11, 200 ISO, -0.3 EC, 18mm Focal Length

In digital photography, exposure is the total amount of light allowed to fall on to an image sensor. It’s really quite simple even though there’s a lot of science and engineering behind how your camera does it. I leave that to the engineers and enjoy the fruits of their labors.

In the time ahead, I will be going more in-depth into the photographic terms of exposure and how they effect our photographs. These terms are: Aperture, Shutter Speed, ISO (or Sensor Sensitivity) and Exposure Compensation.

Aperture is how wide the lens’ iris is opened. Like your eye, a lens has an iris. You can tell how wide it is by the f-stop number. Those funny numbers you see on your lens or through your viewfinder often designated with an f/ in front of them. In the photo of a topiary taken during Epcot’s Flower and Garden Festival, the aperture setting is f/11. The lower the number, the wider the aperture is and the more light is transmitted through the lens. Reciprocally, the higher the number, the less light comes through. Aperture controls the zone of focus or depth of field in your images and can be used in very creative ways.

Shutter Speed is how long the camera’s shutter stays open to expose it’s sensor to the subject being photographed. Most of the time it’s just a short fraction of a second. The photo above used a shutter speed of 1/160th of a second. The dimmer the light, the longer the shutter speed is to get a well-exposed picture. Low light and night photography can sometimes take seconds or minutes to capture a scene. For fast action like children playing or theme park rides, you’ll want to use fast shutter speeds like 1/500th or faster to capture the action. Playing with various shutter speeds is a lot of fun!

ISO is how sensitive your digital camera’s sensor is to light. You want to use the lowest ISO you can to get shootable apertures and shutter speeds. The higher or faster, as it is referred to, ISO number, the more sensitive your sensor is to light. With a lot of light available, the topiary image was taken with an ISO of 200. As day turns into night, I will increase my ISO from 200 to as high as 1600. This does increase what’s called noise (they look like little specs) into the images. Unlike film, the noise is not nearly as bad as film grain was and there are software products available, which can clean up most of the noise.

Exposure Compensation or Bias is a way for digital photographers to dial in their exposure. Most digital SLR and some advanced Point & Shoot cameras have an exposure compensation button which allows you to do slight adjustments to your cameras image sensor. Usually in one half to one third intervals, this little button can make the difference between an image with too light or too dark areas into a properly exposed gem. In this blog’s photo, I made a slight -1/3 (or -0.3) adjustment to keep from over exposing or “blowing out” the light background behind the topiary.

For an excellent book on this subject, I highly recommend Bryan Peterson’s Understanding Exposure which has been updated for digital photography.

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Scott's "Photographic Innoventions" blog focuses on intermediate to advanced photography concepts and techniques relevant for Point and Shoot and Digital SLR cameras.

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