Where’d my Saturation Go? The Secret Life of Sharpening

A couple days back Tronam made a perceptive comment on my JPEG export saturation loss post, noting that he’d noticed saturation loss being caused by sharpening, but wasn’t quite sure why that was happening. I immediately smacked my forehead, because I’d known about the ways sharpening could sap saturation, but hadn’t thought about it when writing that previous article. So, today, we’ll dive into it.

There are three key points. First, sharpening can (but won’t always) have an effect on saturation. Second, that loss isn’t always avoidable. Finally, if you’re working with images in Lightroom, it’s possible (although unlikely) that you won’t notice that saturation loss until you actually export the image.

First, just to give you a sense of the problem, I’ve taken an image with lots of high-frequency detail and exported from Lightroom with default input sharpening, then with no input sharpening, and finally with “too much” input sharpening. By input sharpening I’m referring to the sharpening settings in the Detail pain of the Develop module in Lightroom. (Note that all three of the images got additional output sharpening in the form of “normal” sharpening for the screen in the export dialog.)

I don’t see a lot of difference in color between the unsharpened and default-sharpened versions of this image. (Because monitors vary and because I’m writing this from a laptop, you may be able to.) Either way, it’s certainly possible for a default level of sharpening to have a palpable effect on image saturation, and if you’ll look at the oversharpened version, you’ll at least be able to see the effect I’m talking about.

Is this a bug in Lightroom? Absolutely not. Is it specific to JPEG export? Nope, not that either. The effect we’re seeing is an inherent side-effect of sharpening. Sharpening works by lightening and darkening pixels near light/dark edges. It’s often said that “Lightroom only works on the luminance channel”, and yeah, that statement is true, but the idea that this has no effect on saturation is false. How can that be?

If you look at the gradient below you’ll see what I mean. I’ve taken a saturated red image and replaced the luminance channel with a left-to-right gradient from 0 to 1. Note that the most saturated looking colors are in the midtones, and that even modest variations in luminance have a big effect on saturation.

In short, there’s nothing you can do about it. If you lighten or darken a saturated color you’re going to get stuck with a loss of saturation, lighten and/or darken enough pixels in an image, as can happen when you sharpen, and you’ll eventually sense that loss in your image.

But noticeable where?

When I first made the three test images, I just went to the image in Lightroom, created a couple of virtual copies, set their sharpening parameters and then exported. When I did so, the saturation loss in each image was not reflected in the Lightroom’s image display because I was zoomed all the way out (to “Fit”). I would have seen that saturation loss if I had zoomed in (creating the 1:1 preview) , or if I had glanced at the small display in the Detail sub-panel. This isn’t normally an issue because you’ll almost always want to be adjusting your sharpening sliders while looking at the image at 1:1, but it can be a bit of a surprise if you bang out an image without adjusting the default input sharpening at all.

How strong this effect is going to be is going to depend a lot on your image and how you’re sharpening–for many images the default sharpening settings are just fine. Know that there’s no magic formula for sharpening well without saturation loss. Instead, pay close attention to how your images are effected by saturation, the loss will be greater on areas that get a lot of sharpening, less so in smooth areas of the image. Finally, work your sharpening controls with an eye to saturation. It’s tricky, but it can save you from a great deal of disappointment in the end.