Today’s Question: I understand that a higher bit depth is a good thing for an image, but why do you say it is especially important when working in the ProPhoto RGB color space or when working with a black and white image?
Tim’s Quick Answer: Working in 16-bit mode is important with the ProPhoto RGB color space because of the very large size of this color space. It is important with black and white images because such a small number of shades of gray are available with 8-bit per channel mode.
More Detail: The bit depth is particularly important for black and white (grayscale) images because there are a relatively small number of shades of gray available for monochromatic images to begin with. The 8-bit per channel mode only provides a maximum of 256 shades of gray ranging from black to white, while 16-bit per channel mode provides up 65,536 shades of gray. The result is that with black and white images it is extremely likely to see posterization (a loss of smooth gradations) when adjustments are applied to a black and white image in the 8-bit per channel mode.
The numbers are a bit more dramatic for color images, though the numbers can also be a little misleading. An 8-bit per channel RGB image can contain up to almost 16.8 million colors (16,777,216 to be exact). A 16-bit per channel image can contain over 281 trillion colors (281,474,976,710,656 for those keeping score). While there’s a huge difference between those numbers, in general 8-bit per channel mode works perfectly well for color images. However, with the ProPhoto RGB color space I recommend being sure to work in the 16-bit per channel mode.
The reason 16-bit per channel mode is more important with the ProPhoto RGB color space is that this color space is particularly large. That means it covers an extreme range of color values, to the point that it actually includes colors that are well beyond the visible spectrum.
Due to the large size of the ProPhoto RGB color space, with the 8-bit per channel mode the distance between neighboring colors is relatively large. That can lead to a relatively high risk of posterization, or the loss of smooth gradations of colors, when strong adjustments are applied.
I consider it a good practice to always work in the 16-bit per channel mode when editing images. But as outlined above, that is especially true when it comes to working with images in the ProPhoto RGB color space or when working with monochrome images.