By Mike Wilmer
Just when film manufacturers discovered the Holy Grail--negative films that let weekend photographers produce nice photos with less than perfect exposures--along came digital photo technology and a new challenge. Simply put, when shooting with a Digital Camera
, your exposures need to be considerably more accurate than what's required when shooting film in order to achieve comparable printed results.
To understand why digital is more demanding than film where exposure is concerned, we'll delve a little into the underlying technology.
First, let's look at why film is more forgiving of bad exposure. Color film consists of multiple layers of light-sensitive material. Within those layers reside a virtual infinite number of "grains" that change when struck by light. When you expose a piece of film to light through a lens, those grains record the information used to produce a print. Even when a modern film is underexposed by several f/stops, countless grains are used to record a tremendous amount of information. Thus, a surprisingly good print can be produced from an underexposed color negative. Film is also more forgiving of overexposure, but not nearly to the same degree as it is to underexposure.
Conversely, a digital camera's sensor has only one layer of light sensitive pixels--each with the ability to capture values from 0 thru 4095 (in a typical 12-bit sensor array). That may sound like a lot of information, particularly when you multiply that by 3, 5, or 8 mega-pixels, but compared to the vastly greater number of light-sensitive grains in film, the number of "photo sites" on a sensor seems incredibly small. Worse still, when you underexpose a digital shot just by one f/stop (cutting the light reaching the sensor by half), you effectively reduce the potential number of tonal values that can be captured from 4096 to 2048. If you underexpose by another f/stop, you're down to 1024 tonal values. When comparing a print made from a negative that is underexposed by a similar amount to a digital print produced with 1024 values, the digital print can look pretty bad. True, you can make a lot of adjustments once the image is in a computer, but the one thing you cannot do is create tonal quality where none was captured in the first place.
Given a bad digital exposure can dramatically reduce the amount of tonal information you'll have to work with, it's easy to see why only a normal exposure, with the maximum amount of information possible, will produce digital prints that measure up favorably to photos made from film.
Fortunately, the auto-exposure feature of most digital cameras can handle average lighting conditions pretty well, but to get the best possible results in more difficult circumstances, here's some advice that will help you help your camera do a better job: Bias your exposures in favor of highlights
When shooting film, there are occasions when metering and exposing for the shadows is a good idea, but it's generally never a good idea to do that when shooting digital. Exposing for the shadows can produce large, white areas in your digital image that lack all highlight detail. That's because the camera's sensor was overwhelmed by the brightness of the highlights. Your best option: If your camera has the ability to lock in an exposure reading, try measuring the brightest area in the scene, lock in that exposure setting, and then recompose your shot before snapping the shutter. Beware of extreme light conditions
Your eyes can adjust to extreme light conditions where shadows are very dark and highlights are very bright, but it's impossible for a digital camera to record detail throughout a scene with an extreme range of tonal values. That's why you will see professional photographers use the softer light found in shady locations or they'll try to shoot early in the morning before the sun comes up. Soft light can solve a lot of exposure problems by keeping the range of tonal values within the range of the sensor's capability. Use flash fill or a reflector to reduce shadows
Shooting in harsh sunlight requires flash or a reflector to fill in the shadows. Here again the goal is to keep the tonal values you wish to record within the range of the sensor's ability to record them. Many point-and-shoot cameras offer the option to force the flash to fire even in bright light conditions. You can use that feature to fill shadows in harsh light conditions. Or, simply take a reflector along (any large white card will do) to bounce light into the shadows, whenever possible. Learn to use your camera's exposure controls
Spending a little extra time in your camera manual's section on exposure can pay huge dividends. With just a little practice, even weekend shooters can quickly adjust for unusual light conditions like backlighting. If your camera supports exposure compensation, learn to use those controls. And if your camera can show you a graphic representation of your exposure (called a "histogram"), learn how that can help you improve exposures by giving you real-time feedback on what your camera is recording across the full tonal spectrum, from highlight to shadow. If your camera doesn't offer exposure compensation, you may need to avoid certain light conditions entirely.
But whatever you do, don't let the modest demands of digital photography keep you from taking lots of pictures. After all, taking lots of pictures is part of the solution. Just give a little more thought to your exposures, and in no time at all, you'll be on track to getting the best possible prints from your digital camera! Click here
to join a discussion on exposure control in The Photography Forum
where you can get your questions answered. Mike Wilmer is a Brooks Institute of Photography graduate, has been a professional photographer for over 3 decades, and established The Photography Forum in 1987 and the Gadgets and Gear Forum in 2005.