The term bit depth, also referred to as color depth, identifies the amount of color information that is associated with each pixel. Bit depth describes the number of brightness levels, or shades, that are available to describe the color of each pixel. Greater bit depth means more shades resulting in higher color fidelity, smoother and more subtle tonal gradations, and files that will withstand substantial editing without visible degradation. Let me caution, however, not to confuse bit depth with the number of pixels; pixel count is analogous to film resolution, whereas bit depth is the measure of the number of discrete tonalities each pixel can represent.
Although the JPEG and TIFF logic of the camera producing a completely processed photograph is convenient, JPEG and TIFF images contain less information than do RAW images, and the information will be progressively lost as successive edits are made. However, there are two things worth considering about JPEGS. First, the screen display on the back of your camera is a JPEG image. So, in fact, the camera always creates a JPEG image in buffer memory. However, that file is not permanently saved (unless you specifically request it saved with or without a RAW file). Second, if you’re a sports photographer using JPEG, you can make your image, download it to your laptop via a wireless link, and have it to the newspaper in seconds. (Soon—if not already—you may be able to go directly from camera to newsroom, making things even faster). It’s hard to beat that speed. Furthermore, if you want snapshots of your kids to send to relatives, or vacation shots to send to friends or to post on the Internet, JPEGs will do the job nicely, and will do it instantly. Again, that’s hard to beat. Many cameras can be set to save both the RAW file and a JPEG from the same capture at the expense of some in-camera processing time and the use of additional memory. For this book, we’re really talking about personal expression, not quick-and-dirty (or really, to be fair, quick and clean) uses of the photographic process, so from here on, we’ll confine the discussion to the higher quality RAW files.
Presently, most digital single lens reflex (DSLR) cameras record RAW information as either 12- or 14-bit data. With 12-bit data, each pixel can represent more than 4,000 levels of brightness (212) in each of the 3 color channels. Fourteen-bit data can represent more than 16,000 levels of brightness (214). By comparison, 8-bit data, such as JPEG, can represent only 256 levels per channel (28). To take full advantage of your camera, set your camera to record RAW files with maximum bit depth and to either not compress or use lossless compression.
To preserve all of your photographic information from capture to print, be sure to set both your RAW converter and your image processing software to work and save in a large gamut color space with a bit depth of 16 bits per channel. For example, if you are using Adobe Camera Raw (ACR) included with Photoshop, from the Image Menu select Mode and choose 16 Bits/Channel and from the Edit Menu select Color Settings and choose Adobe RGB or ProPhoto RGB from the color space drop down menu. If you are using Lightroom, which incorporates the same conversion technology as ACR, under the Lightroom Menu select Preferences, choose the External Editing tab, set the color space to Adobe RGB or ProPhoto RGB and set it to 16 Bits/Component.
Differences in the levels of information (i.e., 8 bits, or 16 bits) are significant, particularly in shadow areas. This is so because more information produces smoother tonal transitions and because changes to the image resulting from RAW conversion or post-conversion editing, including changes in white balance (to be discussed below), contrast, and brightness levels always result in the loss of data. If enough data is lost in processing, the resulting photograph will be noisy (i.e., it will exhibit undesirable grain, texture, or random color data that has no meaning), and will possibly present the abrupt changes in what should be smooth transitions (such as the sky toward the horizon) known as posterization.
Today’s hardware and software almost universally support 16-bit processing. However, if your camera or software is limited to 8 bits, you can nevertheless produce fine quality photographs, but it becomes all the more important to capture as much shadow information as possible by giving the maximum exposure you can without blocking the highlights. (More on this later. Stay tuned.)
The Sensor’s Useful Brightness Range
Each sensor has its inherent brightness response range (referred to as the dynamic range). The range varies from one camera model to the next and may very with ISO setting. Just as with film, the digital sensor requires a minimum level of light to register shadow values (equivalent to the exposure threshold for film). All brightness levels in the photographic subject that fall below the sensor’s response threshold will be depicted as black. (This is similar to the film negative having areas exposed below threshold, having no density, and therefore supplying no information for the print. For transparency film, it’s simply getting unexposed black areas). In addition, at low brightness levels (equivalent to exposures slightly above the film threshold), random electrical signals generated by the sensor and related circuitry will constitute a significant portion of the information and can be expected to appear as noise. At the highlight end of the sensor’s range, above a maximum brightness level, the sensor will “clip” the highlights, that is, it will not differentiate additional brightness (equivalent to a pronounced shoulder on film) and all brightness beyond the dynamic range will appear as blank white. Clipping is analogous to overexposing transparency film to the point where the highlights are rendered as clear film base.
In the early days of DSLR cameras, the dynamic range was similar to that of most outdoor color transparency film, a range of approximately 5 f-stops. Current DSLRs have a dynamic range of as many as 10 f-stops or more using RAW capture, the camera’s base ISO, and no image adjustment, thus equaling, or perhaps even exceeding Fujichrome 64T transparency film, but falling far short of the useful range of color or black-and-white negative film. The useful dynamic range may vary with the ISO setting and can be expanded to some extent in both the shadows and highlights using the shadow and highlight adjustments in the RAW converter. Determining the dynamic range of your camera is discussed in the next section.
The brightness information from each photosite is converted by the camera’s circuitry from analog output to digital data, which quantifies brightness in a geometric progression. That is, the output from a photosite that receives an exposure of up to one f-stop above the threshold will contain one bit of binary data representing one of two possible responses—black or the first brightness level of red, green, or blue (depending on the color filter over the photosite) that is lighter than pure black. The output for the same photosite that receives up to 2 f-stops above the threshold will contain 2 bits of data representing any one of the next 4 lighter shades of its color. The output for the same photosite that receives up to 3 f-stops of exposure above the threshold will contain 3 bits of data representing any of the next 8 lighter shades of its color. The progression continues so that a photosite that receives an exposure within the last f-stop of the dynamic range will record any one of several thousands of shades of its color from very light to white.