With amazing cameras like the Sony a7S achieving an unheard of dynamic range of more than 15 stops—trumping the specifications of even the most sought-after, high-end filmmaking cameras like the ARRI Alexa—it might be time to take a look at what’s happening under the hood with color information. Regardless as to the system, dynamic range is the ratio of the brightest measurable point in an image in comparison to the darkest color capable of being reproduced or captured. These endpoints in the scale of luminance then become the "true" white and black points within which the rest of the dynamic range falls as tonality— the subtly graduating shades between colors determined by bit depth.
Myth: Color Information Is Black-&-White
There are differences when discussing dynamic range in regards to the device, however. With a printer, the maximum black is going to be the deepest saturated black capable of being produced by the inkset, while the white is based on the absolute white of the paper. For photography and image capture devices like camcorders, dynamic range is actually the same as the range of luminance—the intensity of light—that’s capable of being captured from the noise floor to the maximum light intensity capable of being measured by the sensor.
When discussing dynamic range and the color fidelity of the human eye, however, there’s no true white or black in our everyday world, so it’s instead a measurement of the contrast ratio between darkest shadows and brightest whites. Additionally, the organic construction of the eye allows it to adjust to different lighting conditions via chemical means and structural changes in a manner far superior to cameras. This gives the human eye adaptation to wildly shifting situations that can cover a range of roughly 30 ƒ-stops from dark to light. In more common scenarios with unchanging light, on the other hand, the human retina can only see approximately 6.5 to 12 ƒ-stops of detail from shadow to highlights at once, which is handily beat by most current camera systems.
While dynamic range is defined by the contrast ratio of luminance sans color information, bit depth is related specially to the shades and colors found between the dynamic range. The higher the bit depth, the more gradation possible between tonalities. If using an older camera with 8-bit analog-to-digital conversion, for example, image posterization or banding will be introduced as transitions between color shades that can be as pronounced as the bands of alternating color in a rainbow. Bit depth uses "bits" of information to describe these colors by binary as 0s and 1s, or combinations of these two possible tonal representations. So, at two to the eighth power, an 8-bit file is capable of 256 different combinations of intensity values, which multiplies out to a possible 16,777,216 different shades of coloring, because there are three channels of color information to be mixed: red, blue and green.
When you have a 16-bit system at 65,536 tones per color channel, this works out to an incredible 281 trillion tones—overkill for the human eye that’s only capable of discerning approximately 10 million shades. But capture at the highest possible bit depth is still important for image enhancement because images with more fidelity can be manipulated with less aberrance than what you’ll find with a lower-bit-rate file, which is why Photoshop offers a 16-bit mode for editing of imagery. If the numbers are confusing, simply try to remember that higher bit depths will provide more detailed steps throughout the dynamic range of a device, while the dynamic range itself is the entire walkway comprised of these steps.