Black and White JPEG


Today’s Question: Is there any way to optimize a JPEG for a black and white photo? Since there’s no color information, can more gray tonalities be squeezed into fewer megabytes?

Tim’s Quick Answer: While it is certainly possible to produce a black and white JPEG image, this is not something I recommend due to the relatively high risk of posterization (the loss of smooth gradations) in such an image.

More Detail: JPEG images do not support high-bit data, meaning you can only have 8-bit per channel information available for a JPEG image. For full-color images that translates to more than 16.7 million possible color values. However, for a black and white image, having only 8-bits for what is then a single channel means there are only a maximum of 256 shades of gray available for a black and white (grayscale) image.

With only 256 shades of gray available, it can be very difficult to have (or maintain) smooth gradations of tonal value. For example, it is very common to see a banded appearance in a sky rather than a smooth gradation with a black and white image in the 8-bit per channel mode.

When strong adjustments are applied to an 8-bit per channel black and white image, the loss of smooth gradations is compounded. Note that the limitations of 8-bit per channel black and white images apply even if you are working with a color original. Even with a Black & White adjustment layer in Photoshop, working on an RGB image, for example, the final image can only contain up to 256 shades of gray, even though there is more information available in the source image.

Because of these factors, I highly recommend working only in the 16-bit per channel mode for black and white images. That, in turn, means JPEG images should generally be avoided in terms of the source image you optimize for a black and white photo. Instead, only a 16-bit per channel source image should be used as the basis of a black and white interpretation of a photo. You can then certainly save the final result (after all adjustments have been applied) as a JPEG image for purposes of sharing the photo, and still retain relatively high image quality for that final output.