By David Saffir
One of the most under-appreciated features of newer-model DSLRs is their ability to capture images with greater bit depth. Bit depth is a way of quantifying the amount of color information in each recorded pixel. It is also a key component of high-level image quality. In this post, we’ll look first at the numbers. Then, we’ll talk about what these numbers mean in terms of dynamic range, color fidelity, and highlight/shadow detail. These are key attributes that influence the quality of the final print.
Not long ago, most cameras functioned in 8-bit mode. Then the higher-end DSLRs became capable of using 12-bit capture. Now they are up to 14-bit, which is a huge benefit to photographers. If you don’t understand bit depth, these may sound like incremental improvements. But they are actually exponential improvements.
To see what I mean, let’s start with a brief review of the numbers. It is simple but non-intuitive:
· There are three primary colors (or color channels) in each captured digital image: red, green, and blue.
· Bit depth describes the number of tone gradations (or intensities of color) provided in each pixel. Most digital images are captured and/or stored in 8-bit, 12-bit, 14-bit, or 16-bit mode.
§ An 8-bit image has 256 tone gradations in each of the three color channels;
§ A 12-bit image has 4,096 tone gradations in each color channel;
§ A 14-bit image has 16,384 tone gradations in each color channel; and
§ A 16-bit image has 65,536 tone gradation in each color channel.
Another way to look at the depth of an image file: A 16-bit image file is twice as big as an 8-bit file.
The major benefit of working with high-bit images is increased dynamic range— the range of tones and detail that the camera can record from the darkest dark to lightest light.
One software company, DXO, now provides public access to its database on camera performance. Here are some examples of the differences between 12-bit, 14-bit, and 16-bit capture:
· The Nikon D2X captures images in 12-bit mode when shooting in RAW format; its dynamic range is rated at 10.9.
· The Nikon D3 captures in 14-bit mode; its dynamic range is rated at 12.2.
Dynamic range is measured like f/stops: an increase of one step is a doubling of dynamic range. That’s a big, big difference.
There is also a noticeable upside in image editing, and in the appearance of the final print.
Figure 1 above is a screen shot of a 16-bit image being edited. Note that the histogram is smooth and even, and shows no breaks or lines.
Figure 2 above shows a screen shot of an 8-bit version of the same image. Note the white lines running vertically in this histogram. These are sometimes called drop-outs, and they show information that is lost when the image is edited . These losses can result in color distortions, posterization, color aliasing, and more.
We lose information every time we edit or manipulate an image. So, the more image information we have to start with, the more information that is available to us as we progress in our workflow.
It pays to set up your workflow to protect as much image information as you can for as long in the process as possible. Most printer drivers can handle high-bit images without difficulty. In my own workflow, I only use 8-bit images for web publishing, e-mail, and the like.
Image-editing programs such as Adobe Photoshop Lightroom are now set up to use high-bit images. And Adobe Camera RAW and Photoshop provide pretty much the same editing tools for 8-bit images and high-bit images, making workflow choices easier for the photographer. This is all good news for those of us who love creating the most detailed and beautiful prints possible.
What’s re-complicating printing workflows right now is the fact that Adobe Photoshop Lightroom uses a bigger color space—ProPhoto RGB instead of Adobe RGB. But we’ll leave a discussion of printing through Lightroom to a future post.
If you have any specific questions about bit depth, I would welcome your comments.
Posted
07-29-2009 10:58 PM
by
Eileen Fritsch