Be Aware of Your Camera's Dynamic Range

There is a great deal written (some accurate, some not) about the dynamic range of digital cameras and how this range differs from that of film cameras. I find it interesting and informative to read the varying (sometimes wildly varying) opinions about it, but I usually end up more confused by the end of an hour of reading than I was at the start--largely because the experts tend to disagree pretty emphatically (and if you read some of the online bulletin board exchanges, pretty rudely, too) about just how digital cameras stack up against film cameras in dynamic range.

No one argues about how the dynamic range is actually defined: in very simple terms it's the range between the darkest shadows and the brightest highlights in which the camera can record detail. And it's pretty much accepted that a digital camera's contrast range is narrower than film's (particularly color-negative film); in other words, most experts agree that a good quality color negative film, properly exposed, can record a wider range of tones than a digital camera (also, only when properly exposed). The actual dynamic range of a particular digital camera depends on a lot of factors, including the size and design of your camera's digital sensor. Your camera manual will provide some insight into those particulars. Generally, the larger the sensor your camera has, the wider its dynamic range; which is why a full-frame digital camera--one that has a sensor that's the same size as a frame of 35mm film--usually has a broader dynamic range than a camera with a smaller sensor. (In fact, overall, all aspects of image quality get better with a larger sensor.)

The important thing to remember is that your camera does indeed have a limit to its ability to record contrast and if you try to exceed that range, something is going to give. Not maybe, definitely--either the highlights or the shadows (or both) are going to get lost. How your camera reacts to a very contrasty scene (and which end of the scale it dumps) depends almost entirely on how you expose things. If a scene has very bright highlights and very dark shadows, you can usually (not always) expose to capture one end of the range knowing that you are willingly sacrificing the other. In other words, if you really want to record detail in bright highlights (and normally you should--there are exceptions), then you will have to cut the exposure to bring highlight detail into range. Doing this, however, will cause the shadows to lose all detail. On the other hand, if you set exposure for the shadows, you will gain detail there, but lose most highlight detail. The contrast range never changes for a particular scene at a particular moment, but by altering the exposure, you shift which part of that range you're going to record with detail.

There are times, of course, when the scene will wildly exceed your camera's contrast range and even your ability to only record a portion of that range. In the scene here (shot in the Chincoteague National Wildlife Refuge in Virginia), for example, the presence of specular highlights made it impossible for me to hold the very bright highlights on the water. If I had exposed for them (and theoretically that might be possible), the rest of the scene simply would have gone black or near black. Blehh! Instead, I simply chose to expose the best I could for the grasses and the silhouette of the goose in the foreground and let those super-bright highlights just blow out. Yes, I could have shifted the exposure down a bit, perhaps underexposing by two or more stops from what I shot it at, but then the light airy feeling of the brilliant morning sun would be lost. Detail or not, I liked the way the highlights turned to a wash of specular highlights. Also, since I shot this in RAW format, I could easily have corrected the exposure to a fair degree in editing but chose not to do that.

Two things to keep in mind about dynamic range: One is that camera makers are improving it and stretching it all the time--in a few years, the range is probably going to be extraordinary. Two, in many situations you can use a technique called high-dynamic-range imaging to vastly extend the dynamic range of digital images. I'll talk about the latter technique in a future tip. In the meantime, if you want to read more about it, Ferrell McCollough has written a great book on the topic called Complete Guide to High Dynamic Range Digital Photography (A Lark Photography Book).

Understanding the Three Types of Light Meters

Many digital cameras (including virtually all DSLR cameras) offer a choice of three light-metering options, typically including: matrix (also called evaluative metering), center-weighted and spot metering. Understanding how these metering types differ from one another and knowing when to use each is an important part of getting a good exposure. In today's tip I'll discuss matrix metering which is, by far, the most frequently used metering method (and with good reason).

Matrix meters work by dividing the frame up into a complex pattern grid that covers the entire frame. The camera then uses a series of algorithms to compare the various brightness regions of the frame and arrive at a good exposure. The way that these algorithms are programmed is nothing less than astonishing in their sophistication and complexity. The cameras have been programmed with the results of hundreds of thousands of potential exposure situations and when you point your camera at a scene, the camera's micro-computer compares the scene you're shooting to all of those exposures to find the one that most closely matches your subject. Sounds like science fiction, I know, but it's all true--and it happens in the blink of an eye. Many matrix meters also take into account not only the colors of your subject but, using information from the lens and focusing system, determine which parts of the scene are closest to the camera and are likely the main subject. Wheww!

What the camera is looking for as it analyzes a scene are familiar landmarks of shape, color and tonality (brightness). In a really simple scene, for example, if it sees a tall vertical subject of average tone in front of a bright background in the upper half of the frame and a slightly darker foreground below the subject, it makes an educated guess (a very educated guess, since it was probably a team of supergeeks from MIT that programmed it) that what you're shooting is a person standing on a green lawn in front of a bright blue sky (and wearing a red shirt). The camera no doubt also knows that your subject just got back from a week in Miami and has a nice tan. And in most cases, it would be correct. It then rummages around in its memory banks to find what it "thinks" is the best exposure for that subject.

The amazing thing about matrix metering though, is that it can handle far more complex scenes than that. In my shot of Notre Dame above, for example, the meter has determined that what I'm shooting is a bright yellow subject (it probably even knows it's a piece of architecture based on the shape pattern of the subject) against a blue sky. But even that is a mild challenge for a matrix meter. I commonly trust matrix metering when shooting subjects like a bright blue dragonfly sitting on a dark green leaf in front of a black pond with sunlight reflecting off of the water. And the exposures are (as the Brits say), spot on.

Matrix meters are extraordinarily accurate in a huge range of lighting situations, both complex and simple, and that's why they are the default metering systems of most cameras. In the next few tips I'll talk about the other types of metering methods and why you might choose them over the genius of matrix metering.

Include a Dark Foreground in Landscapes

I like landscape photos that really pop off the page, that have some element of drama that forces you take notice of them. One simple way to do that is to frame your scenes, when possible, with a dark foreground. Because the eye naturally seeks lighter areas or areas with more color, using a dark foreground helps to point the viewer to your main subject. It's easy to find dark foregrounds just by exploring a scene a bit until you find an area that is not as brilliantly lit as your main subject. In this scene I used an area of marsh grass along the riverbank that was in shadow and contrasted it with the brightly lit boats. By exposing for the brighter areas the foreground went even darker and made the frame a much bolder element. Also, I was careful to include the dark pilings on the left side of the frame to tie the foreground and middleground areas together.

Photo notes: Photographed with a Nikon D90 with a 24-120mm Nikkor zoom at 120mm (180mm in 35mm terms). Exposure was 1/125 at f/5.6. White balance: cloudy weather.