If you're scanning images, whether film, slides, or prints, you're almost certainly going to run into the problem of dust and scratches at some point. It's one of those unfortunate facts of life of analogue photography. No matter how carefully you wipe film or slides before scanning them, it doesn't always work.
If you're using Lightroom, I have another guide for removing dust and scratches in Lightroom. Lightroom does quite a capable job with its healing brush. But Photoshop does an even better job of it and has much more flexibility.
One option is to use Photoshop's content aware Spot Healing Brush. But the downside of that is that it can affect the area around the dust spot and create its own eye catching artifacts. That can be especially noticeable on scans that have visible film grain or shots of fabrics like clothes or furniture.
Photoshop also has a built-in Dust & Scratches filter. You could be forgiven that that's all there to it. But as you'll see, the Dust & Scratches filter is a pretty heavy-handed tool and doesn't do as good a job as it sounds. Yes, it can get rid of the dust spots and scratch marks, but it also makes the rest of the image horribly soft.
So here's a technique that takes the good from that (removing the dust and scratches) but avoids the bad (softening the image). It's most effective on black and white photos--precisely the kind of images that the dust removal features of scanning software usually have trouble with. It's a technique I picked up years ago from an early version of Martin Evening's incredibly useful guide to Photoshop. If you're a Photographer who wants to make use of the power of Photoshop, I highly recommend picking up a copy.
The History Panel
You'll want the History panel open. If it's not currently showing, go to Window > History. The history panel logs everything you do the image. We're going to put that to use.
For this example, I'm going to use this old photo I took of Belém Tower in Lisbon, Portugal. The original was taken with black and white film, and over the years it has managed to get some dust on the negative as well as some scratches.
Apply the Dust & Scratches Filter
The first step is to apply Photoshop's built-in Dust & Scratches filter. Go to Filter > Noise > Dust & Scratches.
There are two sliders in the Dust & Scratches panel. Radius applies to adjacent pixels. Threshold refers to the local contrast. If you set it to Radius 1 and Threshold 255, you'll basically have the original image. If y
You don't have to go far to see what a mess the Dust & Scratches filter can do if used alone. This is with a radius of 46. As you can see, the image has become very, very blurry and is totally unuseable.
What we want to do is find the lowest setting where all of the offending dust spots and scratches disappear. That's going to vary depending on the image content, the resolution of the image, and how bad the markings are.
For black and white images, I tend to put the threshold at about 10 and then adjust the radius. In doing this, you can use both the 100% zoom loop (and select which area it applies to by clicking anywhere in the image) as well as the original image. It's often easier to zoom in on the original image before doing this.
In this example, I'm going to settle for Radius 14 and Threshold 10. But the image is still far too blurry. Yes, the dust and scratches have mostly disappeared, but the image isn't useable anymore. But hit OK to have the filter applied. Yes, your image is going to look terrible, but we'll fix that.
I'm going to zoom in here to make the remaining steps easier. As you can see, the image is pretty blurry.
Put the History Brush to Work
This is where the clever trick is. What we're going to do is use the History Brush. Basically, that uses a special brush that applies history states. Instead of painting with a color, you're painting with past states. Whoever dreamt up this idea way back when Photoshop was being developed deserves a beer.
In your History panel, you'll see that the last line is for the Dust & Scratches filter you just applied.
The first thing you want to do is go back to the previous step, the one right before you applied the Dust & Scratches filter. You do this by clicking on that line of the log. In this example, it's the Open line.
Your image will revert to the original version, revealing all the dist and scratches again.
Next, we want to tell the History Brush which state to use. We want to selectively apply the changes made by the Dust & Scratches filter. So we check the box next to Dust & Scratches. You'll get a small History Brush icon showing in the box to the left of the Dust & Scratches line.
Next, select the History Brush tool from the tool panel at left.
Change the brush size to something a bit bigger than your dust spots. Then change the mode. If you're trying to get ride of white dust spots and scratches, as in this example of a scan of negative film, set the mode to Darken. If you're trying to get rid of dark spots and scratches, as in a scan from a slide or print, set this to Lighten. What this does is limits the corrections only to those pixels that need fixing, and it leaves everything else around it mostly untouched.
Next you essentially want to dab the History Brush on the dust spots and scratches. Despite the precaution we've taken in setting the mode, it's often a good idea only to paint the brush as selectively as possible. If you just paint over everything--especially in tight textures like film grain or fabric--you can still end up with noticeably smoother patches that catch the eye. The image I'm using here was taken using pretty fast film, so it has a fair bit of film grain. Just painting indiscriminately over it doesn't work well.
Continue dabbing the History Brush on as many of the dust and scratch marks as you want on the image. You can then move on to whatever other edits you want to do or simply save it.