Today’s Question: Why is 8-bit a bigger problem for black and white images compared to color?
Tim’s Quick Answer: Put simply, for a black and white image there are very few shades of gray available in 8-bit per channel mode compared to a color image. This leads to a very high risk of posterization (a loss of smooth gradations of tone) especially with relatively strong adjustments.
More Detail: The bit depth for an image refers to the total number of tonal or color values available. A higher bit depth translates to more tonal or color values available, which helps ensure smoother gradations of tone and color in the image, even with relatively strong adjustments.
When working in 16-bit per channel mode there are a tremendous number of tonal or color values available. For a black and white image, that provides a total of 65,536 shades of gray from black all the way to white. For a color image the number is over 261 trillion color and tonal values.
For a black and white image there is only one channel rather than the three channels for an RGB image, which greatly impacts the number of tonal values available. In 8-bit per channel mode a color image can consist of up to almost 16.8 million color and tonal values. That’s nowhere near the huge number for a 16-bit per channel color image, but it is enough to provide smooth gradations unless extreme adjustments are applied.
For black and white images in 8-bit per channel mode there are only 256 shades of gray available. This creates a very real challenge for avoiding posterization even with relatively moderate adjustments. A very common example is a loss of smooth gradation in the sky, where banding can result even with minor adjustments. This takes on the appearance of something like a black and white rainbow, with bands of different shades of gray rather than smooth gradations across the sky.
So, while I always recommend working with images in the 16-bit per channel mode to maximize image quality especially in the context of avoiding posterization, this is especially important for black and white images because they are so much more limited in terms of the number of tonal values available.

