Adobe added a dehaze filter to Lightroom CC 2015. It's designed to cut through the glare caused by haze or mist, but it is also very useful for an off-label use: finding dust spots.
Dust on the sensor is an annoying fact of life if you're changing lenses on the go. Even if you stick to only changing lenses indoors (which isn't practical much of the time), keeping the camera pointed down, and doing it quickly, chances are excellent that you're going to get some dust or dirt on the sensor. You can make the consequences less visible by using a wide aperture, but a certain percentage of your photos are likely to have dust spots on them.
A lot of the time they're very easy to spot, but sometimes they're more subtle. But even if they're essentially invisible on screen, they might become more visible in prints or if an end user decides to do some further post-processing of their own. And there's the professional pride in producing images that aren't marred by dust spots if it's at all possible.
And this is where the dehaze filter comes in. One of the characteristics of the dehaze filter is that it can exaggerate optical flaws in the image. Most of the time that's a negative, but it can also be put to good use.
Just like Lightroom's dedicated dust and scratches view helps the offending areas to leap off the screen, the dehaze filter can be used to similar effect for dust spots. And once you've found them, you can use Lightroom's healing brush to eliminate them. It's a very quick and easy process.
So long as do this entire process in Lightroom, the dehaze filter is completely reversible thanks to Lightroom's non-destructive workflow. What you don't want to do is rasterize the file to send it to Photoshop or another external editor, because that will hard code the results of the dehaze filter and you won't be able to undo it.
Here are a couple of examples where the dust spots were there but were too subtle to be easily seen in the master file. Ramping up the dehaze filter to +100 made them pop. I could then clone the spots away and then reset the dehaze slider back to 0.
In this first shot, the overall image is naturally very high-key. It was shot through heavy rain, looking into the distance across the Golden Horn towards Galata Tower in Istanbul.
In the original, the dust spots are basically invisible thanks to the overall highlights. But by turning the dehaze slider up to +100, a number of dust spots all over the top half of the frame become clearly visible and therefore easily squashable with the healing brush tool.
Here's another example where the dust spots are hiding in the thick haze of the sky in the late afternoon in Hue, Vietnam. But turning the dehaze filter up to +100 shows up several others, particularly over in the far left of frame.
In neither of these examples would I leave the dehaze filter turned on to the degree I'm using it here. But it's very easy to reverse, simply by resetting it back to 0.
Why Do It This Way?
You could use other methods to create the same effect, but they involve much more work.
With the first example, even cranking up the clarity and contrast sliders to the maximum and bringing the blacks slider as far left as it'll go only ended up with one dust spot--the one in top right--being revealed. Only by also messing with the exposure and highlights as well is it possible to find the others. And that's a lot of sliders to remember to get back to their original settings. The visual spots tool, used for dust and scratches, similarly didn't show up the spots.
The dehaze filter, on the other hand, provided a one-stop, easily reversible tool to find them the spots quickly. It's not the reason the dehaze filter exists, but it's a handy use for it.