In This Appendix
You may have heard about RAW capture, but you may not understand what the advantages and disadvantages of RAW shooting are. This appendix provides an overview of RAW capture as well as a brief walk-through on converting RAW image data into a final image.
If you're new to RAW conversion, a high-level overview is helpful. With JPEG images, the camera automatically processes, or edits, the image data coming off the sensor, converts the data from 14to 8-bit files, and then compresses the files in the JPEG format.
Note
JPEG is a popular file format that allows images to be viewed on any computer and to be opened in any image-editing program.
By contrast, RAW images have very little in-camera processing, and are stored as 14-bit files using Canon's proprietary file format, that has a .CR2 file extension. RAW files cannot be viewed on some computers or opened in some image-editing programs without a program that can display the CR2 proprietary file format. Canon includes a free program, Digital Photo Professional, on the EOS Digital Solution Disk that you can use to view and convert Rebel T1i/500D RAW files.
RAW capture provides significant advantages including the ability to get the best quality from the T1i/500D images. But RAW capture 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 option to explore.
RAW capture saves image data that comes off the sensor with virtually no internal camera processing or editing. As a result, you can determine how the image data is interpreted when you convert or process the RAW file. With a RAW file, the only settings that the camera applies are ISO, shutter speed, and aperture. Other camera settings have been noted but not applied in the camera, so you can adjust image brightness, white balance, contrast, and saturation when you convert the RAW image using a program such as Canon's Digital Photo Professional, or alternately, a third-party conversion program such as Adobe Camera Raw, Adobe Lightroom, or Apple Aperture.
RAW capture offers more latitude and stability in processing and editing RAW files than JPEG files offer. 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 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 may be possible to make improvements in Photoshop, but the files are not as robust and rich as RAW files.
RAW files have rich, 14-bit data depth and provide far 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.
Table B.1 illustrates the general differences in file richness between a RAW image and a JPEG image. Note that the table data assumes a 5-stop dynamic range, the difference between the lightest and darkest values in an image, for an exposure.
Note
In thinking about dynamic range, think about it this way. 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.
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, such as Adobe Photoshop, are destructive.
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 RAW image conversion, image brightness levels must be mapped so that the levels look more like what we 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.
RAW 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, as shown in Table B.1. 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 for the next f-stop, and so on.
Table B.1. Comparison of Brightness Levels
F-stop | Brightness Levels Available | |
|---|---|---|
12-bit RAW file | 8-bit JPEG file | |
First f-stop (brightest tones) | 2048 | 69 |
Second f-stop (bright tones) | 1024 | 50 |
Third f-stop (midtones) | 512 | 37 |
Fourth f-stop (dark tones) | 256 | 27 |
Fifth f-stop (darkest tones) | 128 | 20 |
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 both shadow and midtone 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 appearance of continuous gradation between levels.
The general guideline when shooting RAW capture is to bias exposure to the right so that in many, but not in all, scenes, 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, a right-biased exposure approach may just seem 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 approach is excellent for JPEG images where the tonal levels are encoded and the image is pre-edited inside the camera. However, with RAW capture, gamma encoding as well as other contrast adjustments are made during conversion with a good bit of latitude. And if some highlights are overexposed, conversion programs such as Digital Photo Professional or Adobe Camera Raw can recover varying amounts of highlight detail.
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.
Although RAW image conversion adds a step to image processing, this important step is well worth the time. To illustrate the overall process, here is a high-level task flow for converting a T1i/500D RAW image using Canon's Digital Photo Professional. The program is noticeably different from traditional image-editing programs. It focuses on image conversion tasks, including correcting or tweaking white balance, brightness, shadow areas, contrast, saturation, sharpness, noise reduction, and so on. It doesn't include some familiar image-editing tools, nor does it offer the ability to work with layers.
Be sure to install Digital Photo Professional, provided on the EOS Digital Solution Disk, before following this task sequence.
Start Digital Photo Professional, and then open a folder with RAW images in it. If no images are displayed, you can select a directory and folder from the Folder panel. RAW images are displayed with a lens icon and the word RAW in the lower left of the thumbnail, and the files are prepended with IMG_ and the file extension is .CR2.
Double-click the image you want to process. The RAW image opens with the tool palette next to an image and the RAW tab selected. In this mode, you can:
Drag the Brightness adjustment slider to the left to darken the image or to the right to lighten it. To quickly return to the original brightness setting, click the curved arrow above and to the right of the slider.
Use the white balance adjustment controls to adjust color. You can click the Eyedropper button, and then click an area that is white in the image to quickly set white balance, choose one of the preset white balance settings from the Shot Setting drop-down menu, or click the Tune button to adjust the white balance using a color wheel. Once you have the color corrected, you can click Register to save the setting, and then use it to correct other images.
Change the Picture Style by clicking the down arrow next to the currently listed Picture Style and selecting a different Picture Style from the list. The Picture Styles offered in Digital Photo Professional are the same as those offered on the menu on the T1i/500D. When you change the Picture Style in Digital Photo Professional, the image preview updates to show the change. Then you can adjust the curve, contrast, highlight, color tone, color saturation, and sharpness. Dragging the Color tone slider to the right increases the green tone and dragging it to the left increases the magenta tone. Dragging the Color saturation to the right increases the saturation and vice versa. Dragging the Sharpness slider to the right increases the sharpness and vice versa.
Adjust the black and white points on the image histogram by dragging the bars at the far left and right of the histogram toward the center.
Click the RGB image adjustment tab. Here you can apply an RGB curve and apply separate curves in each of the three color channels: Red, Green, and Blue. You can also do the following:
Click one of the tonal curve options to the right of Tone curve assist to set a classic "S" curve that lightens the midtones in the image without changing the black and white points. If you want to increase the curve, click the Tone Curve Assist button marked with a plus (+) sign one or more times to increase the curve. Alternately, you can click the linear line on the histogram, and then drag the line to set a custom tonal adjustment curve. If you want to undo the curve changes, click the curved arrow to the right of Tone curve adjustment or the curved arrow to the right of Tone curve assist.
Click the R, G, or B button next to RGB to make changes to a single color channel. Working with an individual color channel is helpful when you need to reduce an overall color cast in an image.
Drag the Brightness slider to the left to darken the image or to the right to brighten the image. The changes you make are shown on the RGB histogram as you make them.
Drag the Contrast slider to the left to decrease contrast or to the right to increase contrast.
Drag the Hue, Saturation, and Sharpness sliders to make the appropriate adjustments.
In the image preview window, choose File/Convert and Save. The Convert and Save dialog box appears. In the dialog box, you can select the file format and bit depth at which you want to save the image. Just click the down arrow next to Kind of File and choose one of the options such as TIFF 16-bit. Then you can set the Image quality setting if you are saving in JPEG or TIFF plus JPEG format, set the Output resolution, or resize the image. In addition, you can set the Output setting and resize the image.
Note
The Edit menu also enables you to save the current image's conversion settings as a recipe. Then you can apply the recipe to other images in the folder.
Click Save. Digital Photo Professional saves the image in the location and format that you choose.