Digital Focus: All About Exposure Control

Learn about shutter speed and f/stop settings. Plus: Are digital photos fakes?

Dave Johnson

Feature: Digital Photo Basics--Exposure Control

In many ways, digital cameras aren't all that different from traditional film cameras. To capture an image they still need to expose a light-sensitive material to light; it just so happens that the material is a hunk of silicon, not a reel of thin plastic coated with a chemical soup. After reading a recent flurry of e-mail messages from readers who want to understand the essentials of topics like exposure and image resolution, I thought we should start the new year by covering the digital photography basics. This week: everything you ever wanted to know about exposure.

Balancing Speed and Size

It's probably no surprise that all digital cameras expose photos in more or less the same way: by opening the lens for a brief period of time to admit light that registers as an image on the camera's CCD or CMOS sensor. But there are two factors that you or your camera can control to perfect the exposure: the duration of the exposure (the shutter speed) and the size of the lens opening (the aperture).

Here's the complicated bit. The size of the lens opening at any given moment is called the f/stop. F/stops are represented with numbers following the symbol "f/," like f/2, f/4, and f/11. For mathematical reasons, the bigger the number, the smaller the opening, and each "whole" f/stop increment admits either twice or half the light of the nearest f/stop. So an aperture set to f/8, for instance, admits half the light of f/5.6. But f/8 admits twice as much light as f/11.

It stands to reason, then, that a long exposure with a large aperture (say, f/5.6) will admit more light than the same shutter speed with a small aperture (like f/11). When your camera is set to its automatic exposure mode, it figures out how much light the scene should get for optimum exposure, and then sets the shutter speed and aperture accordingly. Most digital cameras will try to pick the best combo--a shutter speed that's fast enough to minimize camera shake, for instance, with an aperture to match.

There are many aperture/shutter speed pairs that will give you the same overall exposure. If an aperture of f/5.6 and a shutter speed of 1/125 second is just right for a given scene, then making the aperture smaller (f/8) and shutter speed slower (1/60 second) will give you the same results; or conversely, a larger aperture of f/4 and faster shutter speed of 1/250 second would take the same picture.

Get Creative

Because f/stop and shutter speed settings can be changed independently, you can take control of your camera to get creative.

Suppose you're at a racetrack taking pictures of cars whizzing by, and you'd like to introduce some motion blur. If your camera wants to shoot a scene with f/4 and 1/250, you can change the settings to f/8 and 1/60. You'll be admitting the same amount of light, but you're exchanging aperture size for exposure duration. When you take the shot, the longer exposure will blur the cars.

Or what if you're taking a portrait? You'll probably want to blur the background by using a relatively open aperture. A very small aperture opening--like f/11 or higher--creates a sharp image with a deep depth of focus. That means much of the foreground and the background will be in focus. A wide-open aperture, like f/2 or f/4, gives you a shallow depth of focus. The subject will be sharp, but most of the background will be fuzzy.

Dave's Favorites: Dazzle 6-in-1 Reader

Perhaps you're like me and have a bunch of portable devices. If so, you may have a hard time getting data from all those devices to the PC and back again. One easy solution is to use the USB cables that came with your various gadgets. But if you have one or two digital cameras, an MP3 player, a digital camcorder, a PDA, or even a digital voice recorder, the cables can get out of hand. Personally, I hate cables. I tend to lose them, anyway--actually, I think my cats drag them off. It's a lot easier to simply pop the memory card out of your device and insert it into your PC, where you can drag and drop files as needed.

But therein lies the rub: Most computers don't have built-in slots for memory cards. Even if they did, which kind of card would the slot support? It's not enough that we have CompactFlash and SmartMedia; we also have two or three kinds of Memory Stick, Secure Digital, MultiMediaCard, and more. If every device in your house potentially uses a different kind of memory card, is there an easy way to transfer data?

Yep. Go get Dazzle's 6-in-1 Reader, a small card reader that sits on your desktop and handles every major type of memory card on the market, including CompactFlash, Microdrive, SmartMedia, Secure Digital, MultiMediaCard, and Memory Stick. It doesn't handle the brand-new XD memory, but there are only a handful of cameras that use that format.

The best part is that Dazzle's reader doesn't need drivers--just plug it into your PC's USB port and it automatically configures itself. That makes it easy to install and even easier to move between computers or to take on a trip.

When I last checked, Dazzle's card reader cost as little as $36 on PCWorld.com's Product Finder.

Q&A: Are Digital Photos Phony?

I'm new to digital photography, but have been interested in photography for many years. I'm not comfortable with the idea of people doctoring a digital image to achieve a spectacular picture--it is not the real scene. Your recent Hot Pic of the Month (the mask photo) is a fake. The great photographers in the past were honored because they were in the right place at the right time. They didn't make the photo in the dark room.

--Jack Henderson, Waretown, New Jersey

You raise a great point, Jack. Is digital photography less valid because images can be--and often are--doctored in some way?

Not by a long shot. Consider traditional photography. Most photographers--certainly the great ones--who shoot with film are not simply "in the right place at the right time." They edit their photos in the dark room, sometimes rather extensively. Dodging and burning, which selectively changes the exposure of certain parts of the photo, is but a single example of how traditional photographers capture the images they do. They also crop their photos to deliver the best composition, and in some cases even physically manipulate the scene itself before pressing the shutter release. More than once I've read about a famous photographer who moved some branches in a landscape to frame the shot perfectly.

Of course, there's the film itself. There are dozens of kinds on the market. Why? Because various types of film have specific characteristics that photographers want for different situations. Some produce vivid, in-your-face colors. Others capture more subdued hues. Which type of film produces "real" photos? Neither, because no camera ever captures the same scene that your eyes see. Likewise, the lens has a dramatic effect on the final photo. A telephoto lens creates an artificial compression effect, pushing the foreground and background close together. A wide-angle lens used for the same scene would capture something dramatically different. Is one image more real than the other? Nope, they're just different artistic interpretations of the same scene.

Certainly, few things done by traditional photographers are quite as explicit as adding eyes to a mask, but when you acknowledge that photography is an art form--not an antiseptic method of representing reality--you'll see it's little more than an evolution of the traditional photo techniques that have been used for over a hundred years.

Hot Pic of the Week

Get published, get famous! Each week, we select our favorite reader-submitted photo based on creativity, originality and technique. Every month, the best of the weekly winners gets a prize valued at between $15 and $50.

Here's how to enter: Send us your photograph in JPEG format, at a resolution no higher than 640 by 480 pixels. Entries at higher resolutions will be immediately disqualified. If necessary, use an image editing program to reduce the file size of your image before e-mailing it to us. Include the title of your photo along with a short description and how you photographed it. Don't forget to send your name, e-mail address, and postal address. Before entering please read the full description of the contest rules and regulations.

This week's Hot Pic: "The Net," by Paul Garbe, Glen Carbon, Illinois

Paul writes: "My youngest son was playing in a soccer goal while his older brother was practicing with his team. I was lucky enough to have my camera ready when my youngest tried to push his way through the net.

"I used my Sony DSC P50 digital camera. I didn't have a chance to check if it was in focus, or to take another picture--as soon as I snapped the image, he was off playing somewhere else."

We want your feedback! Send your comments, questions, and suggestions about the newsletter itself to comments@bydavejohnson.com. If you have a question that you'd like to see answered in the weekly Q&A, send it to question@bydavejohnson.com.

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