Reason for 32-bit per Channel


Today’s Question: You recently addressed a question related to 8-bit versus 16-bit per channel bit depth. But I notice there is also a 32-bit per channel option in Photoshop, which doesn’t seem to get talked about very much. Is there ever a reason to use 32-bit per channel?

Tim’s Quick Answer: As far as I’m concerned, the only reason to employ the 32-bit per channel bit depth option in Photoshop would be to apply adjustments to a high dynamic range (HDR) image that has not yet had tone-mapping applied to it.

More Detail: Generally speaking, I recommend working in the 16-bit per channel mode when applying adjustments to your photos. This provides the potential of more than 281 trillion possible color values, which helps ensure images will retain smooth gradations of tone and color even with very strong adjustments.

When sharing images through digital means, such as via a digital slideshow or when sharing online, you can save a copy of the image that has been prepared for that purpose, such as to reduce the overall pixel dimensions and to convert to the 8-bit per channel bit depth. This still provides more than 16.7 million possible color values, which is in line with the estimated number of color values that can be perceived by normal human vision.

The 32-bit per channel mode provides an absolutely huge number of possible tonal and color values. In fact, the number of possible color values is so large (just under eight with 28 zeroes after it) that you couldn’t display it on most calculators without using scientific notation. That’s a lot of potential color and tonal values!

The reality is that this 32-bit mode is a “specialty” mode used for high dynamic range (HDR) images. For a normal photographic image this mode would not provide any value at all. In addition, an image saved in the 32-bit per channel mode will have a file size that is twice as large as the same image in the 16-bit per channel mode.

Even if you assemble an HDR image, you won’t generally have a 32-bit per channel image as a result. That’s because the HDR image will be converted to 16-bit per channel mode for the tone-mapping stage of creating the final image. It is possible, however, to generate an HDR image in Photoshop as a 32-bit per channel image. Very few adjustments will be available in 32-bit per channel mode, but you could then use Camera Raw to perform the tone-mapping for the image and save the final result as a 16-bit per channel image for further processing.