By Joseph Kashi, for the Redoubt Reporter
Nothing is more critical to making good photographs than proper exposure, and nothing seems more murky and arcane. Good exposure is the result of properly managing the dynamic range of the image and matching it to your camera’s capabilities and how you visualize your photo. It’s more important than the number of megapixels on the sensor.
What, you may ask, is dynamic range, and why is something that most people never heard of so important?
A camera’s dynamic range controls your ability to show detail in both dark and bright areas. Higher dynamic range allows you to show darker shadow areas with texture and detail, rather than as pitch black, while at the same time holding texture and detail in brighter highlight areas, rather than showing a blank white expanse.
The human eye’s dynamic range is astoundingly high, far more so than any digital sensor or chemically based film. As a result, one of the most basic photographic problems is to adjust the more limited dynamic range of a camera’s output so it looks natural and correct to the eye, or at least reasonably attractive and artistic.
With a higher dynamic range, a camera better deals with extremes of dark and light and gives you much more flexibility to later perfect an image using a post-processing program like Adobe PhotoShop or Lightroom. In addition to losing highlight and shadow detail, poor dynamic range also negatively affects smooth changes in color and tonality, resulting in a harsher look.
A camera’s maximum dynamic range is limited by your camera’s internal hardware. Ideally, your camera’s hardware should be capable of a dynamic range of at least 10 “stops,” which translates into an ability to hold detail over a brightness range of 1 to 1,024 (two to the 10th power).
More is definitely better. Even if you later choose to increase the contrast of a particular photo in order to force parts of an image into pure black or blank white for artistic reasons, it’s better to use a camera that has an inherently higher dynamic range so you can make any choices yourself, rather than having your equipment force them on you.
Some cameras do better than others in this area. For example, Canon’s two top compact cameras, the S90 and G11, both have an 11-stop range, the best that I’ve seen among compact cameras. The Nikon D90 and D5000 and the Pentax K-X have the best dynamic ranges in the affordable digital SLR groups. These three cameras can exceed a 13-stop range, which is excellent.
Hardware and dynamic range
A camera’s ultimate dynamic range is fixed by its sensor and processing chip. There’s no way you’ll be able to exceed that inherent dynamic range limitation without some rather more-complex High Dynamic Range (HDR) techniques, and that’s definitely a subject for another issue.
The size of an image sensor greatly impacts dynamic range. The large full-frame and APS-C sensors used in digital SLR cameras, particularly sensors that don’t exceed 10 to 12 megapixels, inherently have better dynamic ranges. On the other hand, the tiny 1/2.3-inch sensor in a 12-megapixel compact camera struggles, usually unsuccessfully, to avoid “blown” highlights that show no detail.
However, getting a dSLR camera with a large sensor is not, in itself, a guarantee that you’ll have great dynamic range.
Some cameras simply do better than others, even those equipped with the same basic sensor. For example, Pentax and Nikon dSLR cameras that use the new 12-megapixel Sony APS-C EXMOOR sensor have a better dynamic range than Sony’s own cameras using the same sensor. Pentax and Nikon processing chips simply put the same sensor to better use. Make your own comparisons with dynamic range data from reliable online camera review sites like http://www.imaging-resource.com, http://www.dcresource.com or http://www.dpreview.com.
Steps you can take
If you have a higher-end compact or digital SLR camera, some easy menu adjustments can result in a major improvement in your dynamic range.
The first is your file format. If you just use standard JPEG files, you will never have a dynamic range greater than eight stops, no matter how expensive or excellent your camera. That’s because the JPEG file format was devised in the early days of digital photography, when memory cards were expensive and had small capacities.
To save space, a JPEG image file is compressed by removing all image data in excess of eight stops (8 “bits” in computer parlance). This translates into a maximum dynamic range of one to 256 (two to the eighth power). JPEG compression also removes a certain amount of “excess” color data, which is why clouds in a JPEG image sometimes have a slight magenta colorcast — the colorcast is the result of removing some color data in the highlight areas.
RAW file formats can give you a real advantage under some circumstances. A RAW image almost always has at least 12 or 14 data bits, which translates into a maximum dynamic range of one to 4,096 (two to the 12th power) or one to 16,384.
Assuming that your camera’s sensor is capable of recording such a wide brightness range, using a RAW format allows your image file to retain all of the data the camera can capture. That’s why any exposure and contrast problems of a RAW file can later be corrected much better using PhotoShop or Lightroom. There’s simply 16 to 64 times as much information available, allowing two or three stops leeway in correcting exposure and in subduing highlights without introducing weird off-colors.
If you also need the convenience of a quick JPEG along with a detailed RAW file, then set your camera’s image recording format to RAW+JPEG. That way, you’ll save every image in both formats and have the best of both worlds. In this era of inexpensive, high-capacity memory cards, there’s no downside to choosing this image capture option. It’s just some more electrons, after all.
The second adjustment is to manually set a camera, whenever feasible, to its base ISO sensitivity. Usually, but not always, this is the lowest listed ISO speed, which also has the added advantages of lower noise and better detail. Dynamic range is best at base ISO and starts to deteriorate fairly quickly as ISO sensitivities are increased. In a few cameras, notably the newer Micro 4/3 cameras from Olympus, the true base ISO is 200. Using the less-sensitive ISO 100 setting on these cameras actually reduces dynamic range. It’s probably wise to look up your camera’s true base ISO.
The third factor is your “color space,” which is simply one of several standard ways to define how colors are recorded and saved digitally. The default for most cameras is the same sort of low-resolution sRGB color space used by most uncalibrated monitors.
This 8-bit sRGB color space contains no more data than a compressed JPEG. Setting your camera to a RAW file format is not sufficient. In order to retain more data for later use, you’ll need to set the color space of both your camera and your computer program to one that retains as much data as possible.
Most higher-end cameras, such as digital SLR cameras, include a menu item that allows you to choose a higher resolution, 16-bit color space, like AdobeRGB 1998. Choose this option whenever you use a RAW or RAW+JPEG image file format. There’s no real downside if you later process your images using PhotoShop or a similar program.
Be sure your photo program is also set to a compatible high-definition color space, like AdobeRGB 1998 or the later ProPhoto. Even though AdobeRGB 1998 is an earlier standard, I still prefer it. It’s available in nearly every high-end camera, computer program and photo printer and it just seems to work better for me.
Finally, some cameras include menu items that either boost shadow detail, decrease contrast or both. Because I post-process every image file using Adobe Lightroom, I personally prefer to turn off in-camera shadow boosting and turn contrast to a medium or low setting. I can always adjust these items more carefully on my computer later. However, if you’re shooting JPEG files or need to use an image straight out of your camera, you may find these menu options very useful when you need more dynamic range.
In case you were wondering, higher-end digital cameras have a better dynamic range than chemically based films. The best available color slide films have a dynamic range of about seven to eight stops. Color negative films, especially when slightly overexposed to preserve shadow detail, can have a dynamic range as high as nine to 10 stops. Traditional black-and-white films, when carefully developed with Zone System highlight compression procedures, can show a maximum dynamic range of 11 to 12 stops.
Local attorney Joe Kashi received his bachelor’s and master’s degrees from MIT and his law degree from Georgetown University. He has published many articles about computer technology, law practice and digital photography in national media since 1990. Many of his technology and photography articles can be accessed through his website, http://www.kashilaw.com.