Unexpected Bit Depth

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Today’s Question: Based on your discussion about bit depth, my JPEGs, as shot in the camera, show up as 24 bit depth when looked at in my Windows [operating system] folder. I’m not sure if that information is available on Photoshop or Lightroom, but I am confused by this bit depth. How do I change that number to a 16 bit as you recommend, or change the number in general?

Tim’s Quick Answer: The reference to “24-bit” is actually the same thing as “8-bit per channel”. The operating system is simply describing the total bit depth rather than the per-channel bit depth.

More Detail: Bit depth refers to the total number of potential tonal and color values in an image. In digital photography we generally refer to the per channel bit depth, such as 8-bits per channel or 16-bits per channel.

In other contexts the total number of bits is used instead. This is often the case with film scanners, for example. With an RGB image you have three channels (red, green, and blue). So, if the image is 8-bit per channel there is a total of 24 bits (eight bits multiplied by three channels). For a 16-bit per channel image the total would be 48 bits (sixteen bits multiplied by three channels).

It is worth noting, by the way, that JPEG images can only be in the 8-bit per channel mode. Furthermore, if you have an 8-bit per channel image I don’t recommend converting it to 16-bits per channel. Doing so would double the base file size of the image with no real benefit in terms of image quality or flexibility in optimizing the image.