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Wednesday, March 7, 2007

Digital Exposure Basics:

Understanding Shutter Speeds And ƒ-stops. Here are some tips to help you get consistently good exposure

digital exposure basicsSlow shutter speeds blur action, as did a 1/15th sec. shutter speed when I photographed this horse and rider, also at the Double JJ. Slow shutter speeds also convey a sense of motion or speed. Too slow a shutter speed, however, can result in just a blurry picture. That said, here’s a photography joke: One blurry picture is a mistake. Twenty blurry pictures is a style.
Tech Info: Canon 1D Mark II, Canon 100-400 IS lens @ 400mm, ISO 400, 1/15th sec. @ ƒ/5.6.

Understanding ƒ-stops
The ƒ-stop (or aperture or lens opening) you choose is important, too. Large openings/ƒ-stops (small numbers like ƒ/2.8 and ƒ/3.5) let more light into a camera and onto the image sensor than small openings/ƒ-stops (large numbers like ƒ/16 and ƒ/22). Using the room analogy, a large window shade opening lets in a large amount of light; a small window shade opening lets in less light.

Small ƒ-stops offer more depth of field (the part of the scene that’s in focus behind and in front of the actual focus point) than wide apertures. So, when getting an entire scene in focus is important, as it was when I was photographing these Buddha statues at the Temple of the Moon in Bangkok, Thailand, choosing a small ƒ-stop is the way to go.
Tech Info: Canon 1Ds Mark II, Canon 16-35mm lens @ 16mm, ISO 400, 1/125th sec. @ ƒ/16.

Wide ƒ-stops, which offer less depth of field than small ƒ-stops, are ideal when you want to selectively blur foreground and background elements in a scene, as I did when photographing this jaguar at the Ft. Worth Zoo in Ft. Worth, Texas. Here the animal is very sharp, but the surrounding leaves are quite blurred.
Tech Info: Canon 1Ds Mark II, Canon 70-200mm IS lens @ 200mm, ISO 800, 1/250th sec. @ f/4.5.

Find The Right Focal Length
The focal length of the lens also affects the depth of field. Check out these two pictures. They were both taken with the lens set at ƒ/8. From the same shooting position, telephoto lenses have less depth of field than wide-angle lenses set at the same ƒ-stop—and vice versa. So, the lens you choose also affects the depth of field.

Here’s the deal: by choosing the appropriate lens and by adjusting the shutter speed and ƒ-stop, you can either blur or freeze the action and change the depth of field in the same lighting conditions. How cool is that!




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