Today’s Question: I often apply many adjustment layers to an image, and I am wondering if that diminishes image quality. For example, I might increase contrast using Curves or Levels and then if I later decide I want more contrast, rather than tweaking the first contrast layer, I often add a second Curves or Levels layer to get the additional contrast. That way if I don’t like the additional contrast, rather than trying to get back to what I had originally, I can just delete the second layer I used to add more contrast. But is doing it this way a bad idea?
Tim’s Quick Answer: There is a minor risk of image degradation with this approach, but that risk isn’t so high that I would encourage you to avoid adding a duplicative adjustment layer.
More Detail: As noted in yesterday’s answer, using an adjustment layer rather than multiple direct adjustments to a pixel layer can help maintain greater image quality because you aren’t compounding the loss of detail from multiple adjustments. Each adjustment layer counts as only a single adjustment, no matter how many times you’ve refined the settings for the adjustment layer.
If you add more than one adjustment layer of the same type, of course, that counts as more than one adjustment. This creates the risk of a minor degradation in image quality, but as long as the adjustments aren’t extreme this would really be a very minor issue.
While I consider it a best practice to use only a single adjustment layer of a given type, there are reasons you may prefer to use more than one layer of the same type. As noted in today’s question, adding another adjustment layer rather than modifying an existing layer provides the flexibility of more easily returning to previous settings if you decide the additional adjustment layer isn’t providing the benefit you were looking for.
In addition, you may want to add multiple adjustment layers of the same type if you are applying targeted adjustments with a layer mask. You might have a Curves adjustment layer that is applying a global adjustment to the image, for example, and then add another Curves adjustment layer that is masked to only affect the sky. You might even add a third Curves adjustment layer with a mask to adjust only the foreground in the image.
So, while I consider it best to only use a single adjustment layer of a given type, there are certainly other considerations that may cause you to want to have duplicate adjustment layers. The risk of image degradation from this approach is quite modest as long as the adjustments aren’t extreme, so I wouldn’t give up other benefits just to avoid having duplicate adjustment layers.
