Reducing Bit Depth


Today’s Question: You said that converting an image from 8-bit to 16-bit would double the file size. So wouldn’t it be a good idea to convert my finished images to 8-bit per channel in order to cut the file size in half?

Tim’s Quick Answer: While it isn’t something I prefer to do, converting a “finished” image to 8-bit per channel mode won’t create any significant problems in terms of image quality, as long as you don’t apply adjustments to the image after that conversion.

More Detail: The primary advantage of a 16-bit per channel image compared to an 8-bit per channel image is that the former can retain smooth gradations of tone and color even if relatively strong adjustments are applied. With an 8-bit per channel image there is a greater risk of a loss of smooth gradations, because there are fewer available color and tonal values available for those gradations.

Once you are completely finished optimizing a photo there is not a significant benefit to keeping the image in the 16-bit per channel mode. This assumes, of course, that you won’t be applying additional adjustments to that image. In that case, you could convert the image to 8-bit per channel mode, making sure that you first flatten the image if it contains layers, so that you are actually converting the image after all adjustments have been applied to the pixel values.

Converting the image to 8-bit per channel mode will cut the file size in half compared to 16-bit per channel mode. So this conversion can have a significant impact on your overall storage needs. That said, I prefer to take the conservative approach of retaining the 16-bit per channel mode for my master images, and also of retaining all layers for my layer-based image.

But again, if you want to reduce your storage needs, converting high-bit images to 8-bit per channel mode is an option you could consider. I would just personally favor buying a larger hard drive before converting my images to a lower bit depth.