Digital Exposure Basics

Shutter Speed Basics
Let’s begin with shutter speed. Shutter speed is the time the shutter, which lets light onto the digital image sensor, remains open—or the time the image sensor is activated. If you’re new to photography, look at it this way. If you’re in a room and open and lower a window shade, you’re basically doing the same thing a shutter does: letting light expose an area. The longer you leave the shade up, the longer the room is illuminated.

Knowing which speed to choose is the key to getting a desired effect. Sometimes it’s an educated guess. Sometimes it’s just plain luck. But most of the time, it should be the result of knowing how different shutter speeds affect subject movement.

Fast shutter speeds “freeze” action, as did a 1/500th second shutter speed when I photographed this horse and rider at the Double JJ Ranch in Rothbury, Mich. The faster the shutter speed, the better chance you have of freezing the action. But there’s more. Telephoto lenses (above 100mm), exaggerate handheld camera shake, as binoculars exaggerate hand movement. So, you need a fast shutter speed to freeze your movement, too. A tripod, other camera support or an Image Stabilization lens can also help reduce camera shake. Tech Info: Canon 1D Mark II, Canon 100-400 IS lens @ 400mm, ISO 400, 1/500th sec. @ ƒ/5.6.

Slow 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!

Keep in mind that when using a digital SLR, the ƒ-stop at which you view a scene in the viewfinder can be and is usually different than the ƒ-stop at which you view and take a picture. That’s because one of the cool things about a digital SLR is that you look through the viewfinder at the widest opening of the lens, which lets in the maximum amount of light so you can easily see the scene. When you press down the shutter release button, the lens stops down to the selected ƒ-stop. Some cameras offer depth-of-field preview buttons, which when pressed, stop down the lens to the selected aperture to show a preview of what’s in focus and what’s not. Tech Info: Canon 1Ds Mark II, Canon 70-200mm IS lens @ 200mm, ISO 400, 1/250th sec. @ ƒ/8 (musician); Canon 1Ds Mark II, Canon 17-40mm lens @ 20mm, ISO 200, 1/125th sec. @ƒ/8 (monks).
Summing up, take your camera off the P (Program) mode and take control of your images.

Rick Sammon has published 27 books. Visit www.ricksammon.com for more information, and meet up with Rick at one of his PCPhoto/Outdoor Photographer workshops.