Today’s Question: Do we need to be careful to only switch to 8-bit as a last step in editing [in Photoshop]?
Tim’s Quick Answer: In general, I recommend only converting to 8-bit per channel (rather than 16-bit) with a copy of the original image and after flattening that image.
More Detail: I strongly recommend leveraging 16-bit per channel processing for your images in Photoshop, rather than 8-bit per channel. This ensures the maximum available tonal and color values for the image. If adjustments are applied to an 8-bit per channel image there is a risk of posterization, or the loss of smooth gradations of tone and color. This is a particular concern for black and white images.
So, when processing a raw capture and opening it in Photoshop, I recommend keeping it in the 16-bit per channel mode. This is handled via the Workflow Settings in Camera Raw or the External Editing Preferences in Lightroom Classic. Save the image with all layers intact as a 16-bit per channel image as a TIFF or PSD file.
When you want to create an 8-bit per channel image, either export a copy with the desired settings, or create a duplicate within Photoshop. In the latter case you can choose Image > Duplicate Image from the menu, turning on the “Duplicate Merged Layers Only” checkbox so the duplicate will be flattened. This is very important to ensure the conversion to 8-bit per channel mode occurs after adjustments have been applied to the pixel data.
You can then convert the derivative image to 8-bit per channel mode by choosing Image > Mode > 8 Bits/Channel from the menu. You can then perform any other tasks, such as to resize to different pixel dimensions, and then saved the resulting derivative image in the desired file format.
