Bit Depth for Filters


Today’s Question: As an add-on question related to working in 16-bit per channel mode, can you offer a recommendation for how to fit in filter effects in Photoshop that generally only work on 8-bit files? Can the 16-bit file be “legitimately” converted to 8-bit for filter gallery effects and then converted back to 16-bit afterwards?

Tim’s Quick Answer: It is not recommended that you convert an image from 16-bit per channel mode to 8-bit mode and then back to 16-bit mode, as doing so may cause you to lose most of the benefits of working at a high bit depth in the first place. Instead I recommend making a copy of the image for the purposes of applying creative filters.

More Detail: There are two basic approaches you could take to applying creative filters that only support 8-bit per channel mode when your master image is in the 16-bit per channel mode.

The first approach is to simply create a copy of the master image. With that image open, first be sure to update the file on your hard drive by choosing File > Save from the menu. Then create a copy of the image by choosing Image > Duplicate from the menu. In the Duplicate Image dialog that appears, turn on the Duplicate Merged Layers Only checkbox. This will cause the duplicate image to be a flattened version of the original, so that all adjustments are applied directly to the pixel values. Click the OK button in the Duplicate Image dialog to create the duplicate.

At this point I recommend closing your original master image that has all of the layers intact, just to avoid any confusion with the derivative image you’re currently working on.

Next, convert the duplicate image to the 8-bit per channel mode by choosing Image > Mode > 8 Bits/Channel from the menu. Then choose File > Save As from the menu, and save the derivative image. In general I recommend saving this duplicate image in the same folder as the master image, with the same base filename, but with text added to the end of the filename to indicate it is a creative interpretation of the original.

You can now apply any of the creative filters you’d like, such as by choosing Filter > Filter Gallery from the menu. You may want to create one or more copies of the Background image layer as part of this process, depending on your specific intent for the image. For example, an additional layer would be necessary if you wanted to use a layer mask to apply a creative effect only in specific areas of your image.

When you are finished applying creative effects to the derivative image, be sure to choose File > Save to save the final changes to that image. When you’re finished, your original master image with all layers intact will of course still be available.