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Working with 8, 16, and 32 Bit Images
It's all about the numbers, and that's a fact. The number of colors available for displaying or printing each pixel in an image is called bit depth also known as pixel depth or color depth. A higher bit depth means more available colors and more accurate color representation in an image. A bit depth setting of 2-bit displays 4 colors; 4-bit displays 16 colors; 8-bit displays 256 colors; 16-bit displays 32,768 colors; and 24-bit and 32-bit, both of which display 16.7 million colors. Normal digital images have 8-bits of data per channel. For example, an RGB image with 8-bit channels is capable of producing 16.7 million colors (a 24-bit RGB image: 8 bits x 3 channels) possible colors per pixel. While that may seem like a lot of color information, when it comes to color correction and adjustment, it isn't.In response to needing more control, Photoshop supports 16-bit and now 32-bitknown as High Dynamic Range (HDR ) (
)images. High Dynamic Range works with images in 32-bits-per-channel, extended dynamic range. It's all about dynamic range. Dynamic Range is the ability of a channel to capture information from black to white, dark and bright areas of an image. An 8-bit channel image has a dynamic range of 250:1 (per channel), similar to the dynamic range of printed paper or a computer display. A16-bit channel image has a dynamic range of 65,000:1, and a 32-bit channel image has a dynamic range of over 200,000:1. The greater dynamic range translates into better control over an image, when making fine color, and contrast adjustments using Levels and Curves (shown below). Working with HDR images is very similar to using camera RAW files and applying exposure changes after the fact. Photographers can capture the full dynamic range of a scene with multiple exposures and merge the files into a single image.
Changing Bits Per Channel
The ability of working with 32-bit images is new to this version of Photoshop, so you have a limited use of adjustments and filters. However, when you convert an image from 32-bits into 16-bits, all of your adjustments are available even though you lose some filters.Therefore, when color or contrast adjusting an image, first convert a standard 8-bit image to 16-bits, and do all the corrections. This helps prevent loss of color information, and banding between light and dark shades. Once all the color/contrast adjustments have been made, you can (if necessary) convert the image back to 8-bit. It's that simple. When you convert a 16-bit image to 32-bits, you need to use the Flatten Image command on the Layers menu on the image first. You can change an image's bits by displaying the image, clicking the Image menu, pointing to Mode, and then clicking 8 Bits/Channels, 16 Bits/Channels, or 32 Bits/Channels.When you convert a 32-bit image to 8- or 16-bits per channel, Photoshop opens the HDR Conversion dialog box to let you make exposure and contrast corrections, so the image retains the dynamic range you want. The Exposure And Gamma option lets you manually adjust brightness and contrast. Drag the Exposure slider to adjust the gain and drag the Gamma slide to adjust the contrast. The Highlight Compression option automatically adjusts highlight values to fit within the range for 8- or 16-bit images. The Equalize Histogram option automatically preserves image contrast. The Local Adaption option adjusts the tonality (local brightness regions) in the image. Drag the Radius slider to specify the size of the local brightness regions and then drag the Threshold slider to specify the distance between tonal values before they are not included in the brightness region. If you want to reuse these settings in the future, you can save them, and then load them again as needed.
Viewing 32-Bit Images
The dynamic range of HDR images exceeds the display capabilities of standard monitors. When you view a 32-bit HDR image, the highlights and shadows may look dark or washed out. To correct the problem, Photoshop allows you to adjust 32-bit preview options, so 32-bit images display properly on your monitor. The preview options are stored in the image file, so each file retains its own settings. To set preview options, open a 32-bit HDR image, click the View menu, and then click 32-Bit Preview Options. In the 32-bit Preview Options dialog box, select the preview settings (described earlier in this topic) you want, and then click OK.
