Determining the appropriate color balance can be far more difficult. Kodak makes a set of color filters that can help send you in the proper direction. The Kodak Color Print Viewing Filter Kit consists of six cards, one for each color filter (blue, cyan, green, yellow, red, and magenta). Each card contains three filters of increasing strength—10 units, 20 units, and 40 units of filtration for each color. You can view your print through any of these filters to see if it’s improved by adding or subtracting filtration. You can also combine two filters to see if that improves the image—say, 10 units of yellow and 20 units of magenta.
If you are making prints from a transparency, hold the card containing the filters to your eye with the black side toward you; it will tell you which filters to add or subtract to achieve an improved color balance. If you hold the card with the white side toward you, it will tell you the filtration change if you are printing color negatives. The filtration changes can be rather bold, so you must decide whether you need intermediate changes. Sometimes when you have a problem, you can’t figure out how to correct it. The filter kit is invaluable in helping you decide which direction to go. Any good camera shop should have this kit, or you can order it if the store doesn’t carry the kit.
Chapter 11. The Digital Zone System
THE INFORMATION ON FILM AND DARKROOM PROCEDURES in the preceding chapters is directed at giving you, the photographer, control and predictability over the final image. Although digital logic differs from that of classical (i.e., traditional) photography, the goal is the same in the digital realm: predictable, controlled results.
The discussion that follows begins with a summary of how the photosites and related filters (collectively, the sensor) inside the camera work; turns to digital exposure, referred to as “capture”, and how to optimize it; and then discusses techniques to use multiple captures when the brightness range of the scene exceeds that of the sensor. This will explain how to work with virtually any brightness range to produce the best possible print quality. The digital sensor’s range falls far short of that of negative film (both color and black-and-white), but with multiple captures can equal, or perhaps even exceed, that range under ideal conditions when the light remains fixed and nothing in the scene is moving. So, just as the making of a silver print cannot be separated from the properties of lightsensitive materials and their related developers, digital photography cannot be separated from the processing of the information produced by the camera’s sensor and the refinement of the image in the software.
I was setting up my 4x5 film camera on a particularly wonderful Inca wall during a steady morning rain. Turning to get my film holder from my backpack, I saw several llamas standing behind me, apparently enjoying what I was doing. I quickly grabbed my digital camera, and made a series of captures as they roamed around, and eventually all walked past me in the direction I had originally been intending to photograph. This was one of the first in the series, where you can almost see their sense of surprise and curiosity. There are no lawn mowers on the Inca terraces at Machu Picchu, instead the llamas keep the grass trimmed.
Figure 11-1. Wall and Llamas, Machu Picchu
At the heart of optimum digital capture is the largely unprocessed data referred to as the “RAW” file. The RAW file consists of sensor data together with ISO, exposure, and other information (referred to as metadata), that is saved to the camera’s memory card and from which a photograph can be rendered. By its nature, a RAW file is not an image and cannot be viewed directly. Rather, it is information from which an editable image can be formed by a computer using software referred to as a RAW converter. (The RAW file is analogous to exposed film—the latent image—prior to chemical development. It’s there, but it has no life yet and you can’t see it until it’s developed). The output from the RAW converter is saved in a commonly recognized image format such as Tagged Image File Format (TIFF) or Photoshop Document (PSD) (analogous at this point to the developed negative, ready for printing), and the image file can later be printed or opened and refined in an image editing program such as Adobe Lightroom (Lightroom), Adobe Photoshop (Photoshop), or Apple Aperture.
In brief, the sequence from the memory card to the print is as follows:
Import the RAW data from the memory card into a RAW conversion program.
Use the RAW conversion software (which may be a standalone program or may be integrated into your image editing program) to set values for global adjustments to the image (including color balance, overall contrast, the correction of aberrations, and initial sharpening); apply the adjustments and process the RAW data into an array of picture elements known as pixels with each pixel being composed of three color channels, one each for red, green and blue information; and then save the resulting file in a common image format such as TIFF or PSD.
In your image editing program (which may or may not be the same as your RAW conversion software), refine the image, perhaps a TIFF or PSD file, by adjusting such things as contrast, brightness, color balance, and sharpening; burning and dodging selected areas; resampling (changing the number of pixels per inch) and resizing the image for printing; and doing any number of other modifications available in your image editing program. All this is done to prepare your image for printing.
Because there are a number of excellent books on the use of specific RAW conversion and image editing programs, our goal here is to place the digital process in context, explain some fundamentals, and give you the tools to optimize your captures for the best possible prints.
Basics of Digital Capture
Let’s start with the sensor output, showing how it relates to classical film processes. In the film world, the result of an exposure is the latent image recorded on film. In the digital world, the result of an exposure is electrical output from the many photosites on the camera’s sensor. Although the specifics vary among manufacturers and among camera models, all digital sensors are composed of a number of photosites arranged in a geometric pattern known as an “array”. Each photosite provides an electrical signal in proportion to the amount of light striking it. For the overwhelming majority of sensors, photosites are capable of measuring brightness but not color. To record the information required to produce a full color image, a single color filter is placed over each photosite, arranged in what is referred to as a color filter array (CFA), often consisting of a repeating sequence of one red, two green, and one blue filter known as a Bayer pattern. (Other arrays and color filter patterns are in the works, so we can expect CFA patterns to evolve). An illustration of an array and its CFA are shown in Figure 11-2.
At the conclusion of an exposure, the electrical signals from all of the photosites are transmitted to the camera’s internal processor, where the signals are converted to binary digital data and are either given minimal processing and written to the memory card as a RAW file or they are fully processed in the camera and written to the memory card as a fully processed JPEG (Joint Photographic Experts Group) file or a TIFF file.
Figure 11-2. Diagram of a sensor and color filter array
Let’s look into optimizing the image quality from the point of capture. Most cameras allow the photographer to select RAW, TIFF, or JPEG image quality. The choice is simple: For the highest quality, set your camera to save your captures in RAW format using the highest bit setting available. As noted, with the camera set to write to the memory card in RAW, the camera does little if any processing of the digital information and simply records the sensor information and related metadata to the memory card. In contrast, if you set your camera to write JPEG or TIFF files, the camera processes the sensor output into three grayscale images, each of which is referred to as a channel (one for each of the three primary color filters), applies a contrast curve, adjusts the color balance, makes other modifications based on the camera settings, performs edge sharpening, and saves the fully processed image to the memory card. In the case of JPEGs, the image is limited to 8 bits per color channel and is compressed in such a way that some of the image information is irretrievably lost. Any post-capture change to a JPEG or TIFF file will result in a loss of data. With a RAW file, before doing any editing of the image, the original capture information should be saved and kept available as a starting point should you wish to try a different rendering in the future (similar to keeping your original, untouched negative).