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To go RAW or not to go RAW? For many digital photographers who now have the option of RAW capture, that is a pressing question. In his new book about digital imaging, Photoshop CS3 Workflow (published by Sybex, an imprint of Wiley Publishing, Inc.), author and digital-imaging guru Tim Grey devotes an entire chapter to the subject.
"RAW capture offers many benefits for the digital photographer, and it provides the best opportunity for maximum image quality and detail," Grey writes in the introduction to his chapter on RAW Conversion. "Of course, the advantages of RAW capture come at a price: the step of RAW conversion added to the workflow. Still, when you need optimal quality in your images, the slight reduction in overall workflow efficiency is well worth the benefits you can achieve with RAW capture."
In the following excerpt from Photoshop CS3 Workflow -- available through the Wiley Publishing Website -- Grey discusses the benefits and drawbacks of RAW capture, as well as the detailed process of converting RAW image files. -- Jack Crager
Benefits of RAW
Photographers take a lot of steps to ensure the absolute best quality in their images. Using a tripod, employing mirror lockup, selecting lenses with image stabilization or similar technology, and using the lowest ISO setting possible on a digital camera are all examples of such steps. Capturing in the RAW mode offered by many digital cameras is yet another way to help ensure the best image quality.
The quality benefits of RAW capture derive from several factors. Each of these represents a potential benefit to the photographer as well as the quality of the final image.
White Balance
Digital cameras introduce the need for photographers to deal with color temperature and white balance. Film photographers have to choose film appropriately balanced for the lighting conditions to be used. However, temperature is a much more significant issue with digital color. The camera needs to compensate for the color of the light, which is measured as a color temperature in degrees Kelvin (see image number 2 in the accompanying slide show). In a general sense, you can think of this as a way to make a white sheet of paper look neutral no matter what type of lighting is illuminating it. Whether the light source is daylight, fluorescent, incandescent, or flash can have a considerable effect on the color of the light as seen by the camera.
Most digital cameras do an exceptional job of automatically determining the appropriate compensation for the lighting conditions under which you are photographing. For that reason, the Auto setting for white balance can generally be trusted to produce accurate results. With RAW capture, you can trust the Auto setting completely because even if the camera does a poor job, you can correct the image in the RAW conversion with absolutely no penalty in image quality. In fact, adjusting the white balance settings during RAW conversion produces exactly the same effect as if you had used the white balance preset in the camera that matches the color temperature you use during conversion.
Exposure Errors
Because RAW captures are recording the amount of light that reaches the imaging sensor during exposure and aren't being converted immediately to a standard image file format, they offer additional exposure latitude above a JPEG capture. The best digital single lens reflex (SLR) cameras offer about six stops of exposure latitude. RAW capture adds about three stops of potential latitude. However, it is important to realize that the final image file will still be able to contain only about six stops of latitude. RAW simply offers the ability to extract more detail from the image and make adjustments to recover from exposure errors.
In general, it is possible to compensate for an exposure error of about one stop overexposed or two stops underexposed with RAW capture (see slide show, image 3). That doesn't mean you'll always avoid blown highlights if you overexposed by one stop. It also doesn't mean quality will be maintained at the same level as though the exposure were correct at the start. For example, if the exposure is dark and needs to be brightened considerably in the RAW conversion, you'll still have a higher risk of noise and posterization in the darker shadow areas of the image.
Think of this capability as a way to recover from some exposure mistakes, within reason, as well as a way to fine-tune the exposure after the fact. It won't necessarily save the day with captures that have bad exposures, but it can help make mediocre exposures look exceptional.
High-Bit Data
The final major advantage of RAW capture is that high-bit data (typically 10- or 12-bit data stored in a 16-bit file) is preserved in the capture and can be preserved in the conversion process. This means that more discrete tonal and color values are being recorded and retained in the final image. This is a twofold benefit.
For one thing, the higher bit-depth used for the in-camera analog-to-digital conversion results in more information, so that smooth gradations of tone and color can be maintained. In short, more detail is retained within the image because there are more available values for each pixel. Second, by converting the RAW capture to a high-bit image file, you maintain the benefit of being able to make significant adjustments to the image without risking posterization (slide show, image 4).
For images captured with proper exposure, the high-bit advantage is a minor one. However, whenever you need to make significant adjustments to achieve your vision for an image, starting with high-bit data will help you maintain maximum quality throughout the process to the final print.
Higher Image Quality
When you capture in RAW, no image compression is applied, so you are retaining the highest image quality possible. With JPEG, which is the most common alternative to RAW, image compression is always applied to reduce the file size, even when the highest quality option is selected. As a result, there is a risk of artifacts (random pixel variations) in the image. Also, because JPEG compression divides the image into blocks of pixels, in some cases a grid pattern may be visible within the image. Capturing in RAW completely eliminates the risk of such artifacts. The price is a larger file size, but the benefits can be significant in some cases in terms of image quality.
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