By Charlotte Lowrie
If you want to get the highest quality images from your camera, then choosing to capture RAW images is the way to get them. In addition, you have the opportunity to determine how the image data from the camera is interpreted as you convert, or process, the RAW images. While RAW capture offers significant advantages, it isn’t for everyone. If you prefer images that are ready to print straight out of the camera, then JPEG capture is the best option. However, if you enjoy working with images on the computer and having creative control over the quality and appearance of the image, then RAW is the best option.
Learning about RAW Capture
One way to understand RAW capture is by comparing it to JPEG capture, which most photographers are familiar with already. When you shoot JPEG images, the camera edits or processes the images before storing them on the CF card. This processing includes converting images from 14-bit files to 8-bit files, setting the color, saturation, and contrast, and generally giving you a file that is finished. Very often, JPEG images can be printed with no editing. But in other cases, you may encounter images where you want more control over how the image is rendered — for example, you may want to recover blown highlights, to tone down high-contrast images, or to correct the color of an image.
Of course, you can edit JPEG images in an editing program and make some corrections, but the amount of latitude for editing is limited. With JPEG images, large amounts of image data are discarded when the images are converted to 8-bit mode in the camera, and then the image data is further reduced when JPEG algorithms compress image files to reduce the size. As a result, the image leaves little, if any, wiggle room to correct the tonal range, white balance, contrast, and saturation during image editing. Ultimately, this means that if the highlights in an image are overexposed, or blown, then they’re blown for good.
If the shadows are blocked up (meaning they lack detail), then they will likely stay blocked up. It is possible to make improvements in Photoshop, but the edits make the final image susceptible to posterization or banding that occurs when the tonal range is stretched and gaps appear between tonal levels. This stretching makes the tonal range on the histogram look like a comb.
By contrast, RAW capture allows you to work with the data that comes off the image sensor with virtually no internal camera processing. The only camera settings that the camera applies to a RAW image are ISO, shutter speed, and aperture. And because many of the key camera settings have been noted but not applied in the camera, you have the opportunity to make changes to settings, including image brightness, white balance, contrast, and saturation, when you convert the RAW image data into a final image using a conversion program such as Canon’s Digital Photo Professional, Adobe Camera Raw, Adobe Lightroom, or Apple Aperture.
An important characteristic of RAW capture is that it offers more latitude and stability in editing converted RAW files than JPEG files offer. RAW images have rich data depth and provide significantly more image data to work with during conversion and subsequent image editing. In addition, RAW files are more forgiving if you need to recover overexposed highlight detail during conversion of the RAW file. These differences in data richness translate directly to editing leeway. And maximum editing leeway is important because after the image is converted, all the edits you make in an editing program are destructive. Another advantage of RAW conversion is that as conversion programs improve, you can go back to older RAW image files and convert them again using the improved features and capabilities of the conversion program.
Proper exposure is important with any image, and it is no less so with RAW images. With RAW images, proper exposure provides a file that captures rich tonal data that withstands conversion and editing well. For example, during conversion, image brightness levels must be mapped so that the levels look more like what you see with our eyes — a process called gamma encoding. In addition, you will also likely adjust the contrast and midtones and move the endpoints on the histogram. For an image to withstand these conversions and changes, a correctly exposed and data-rich file is critical.
Proper exposure is also critical, considering that digital capture devotes the lion’s share of tonal levels to highlights while devoting far fewer levels to shadows. In fact, half of all the tonal levels in the image are assigned to the first f-stop of brightness. Half of the rest of the tonal levels account for the second f-stop, and half into the next f-stop, and so on.
With digital cameras, dynamic range depends on the sensor. The brightest f-stop is a function of the brightest highlight in the scene that the sensor can capture, or the point at which the sensor element is saturated with photons. The darkest tone is determined by the point at which the noise in the system is greater than the comparatively weak signal generated by the photons hitting the sensor element.
Clearly, capturing the first f-stop of image data is critical because fully half of the image data is devoted to that f-stop. If an image is underexposed, not only is important image data sacrificed, but the file is also more likely to have digital noise in the shadow areas.
Underexposure also means that during image conversion, the fewer captured levels must be stretched across the entire tonal range. Stretching tonal levels creates gaps between levels that reduce the continuous gradation between levels.
The general guideline when shooting RAW capture is to expose with a bias to the right so that the highlight pixels just touch the right side of the histogram. Thus, when tonal mapping is applied during conversion, the file has significantly more bits that can be redistributed to the midtones and darker tones where the human eye is most sensitive to changes.
If you’ve always shot JPEG capture, the exposing-to-the-right approach may seem just wrong. When shooting JPEG images, the guideline is to expose so that the highlights are not blown out because if detail is not captured in the highlights, it’s gone for good. This guideline is good for JPEG images where the tonal levels are encoded and the image is essentially pre-edited inside the camera. However, with RAW capture, gamma encoding and other contrast adjustments are made during conversion with a good bit of latitude. And if highlights are overexposed, conversion programs such as Adobe Camera Raw or Digital Photo Professional can recover varying amounts of highlight detail.
In summary, RAW capture produces files with the most image data that the camera can deliver, and you get a great deal of creative control over how the RAW data is converted into a final image. Most important, you get strong, data and color-rich files that withstand image editing and can be used to create lovely prints.
However, if you decide to shoot RAW images, you also sign on for another step in the process from capturing images to getting finished images, and that step is RAW conversion. With RAW capture, the overall workflow is to capture the images, convert the RAW data in a RAW-conversion program, edit images in an image-editing program, and then print them. You may decide that you want to shoot in RAW+JPEG so that you have JPEGs that require no conversion, but you have the option to convert exceptional or problem images from the RAW files with more creative control and latitude.