Adobe Photoshop CS5 for Photographers - Martin Evening [191]
Alternative history brush spotting technique
This spotting method has evolved from a technique that was first described by Russell Brown, Senior Creative Director of the Adobe Photoshop team. It revolves around using the Remove Dust & Scratches filter, which can be found in the Filter Noise submenu. If this filter is applied globally to the entire image, you end up with a very soft-looking result. So ideally, this filter should be applied selectively to the damaged portions of the image only. The technique shown here has the advantage of applying the filtered information via the history brush such that only the pixels which are considered too dark or too light are painted out. This modified approach to working with the Dust & Scratches filter avoids doing any more harm to the image than is absolutely necessary. As you can see, the technique works well when you have a picture that is very badly damaged and where using the clone stamp would be a very tedious process. What is really clever is the way that the Lighten (and Darken) blend modes can be used to precisely target which pixels are repaired from the stored Dust & Scratches history state.
Replacing film grain
If the photographic original contains noticeable film grain, you may encounter a problem here since even with a selective application of the Dust & Scratches filter, you may end up softening the image as you retouch it. To counteract this it may help to apply a small amount of noise after you have applied the Dust & Scratches filter. Add just enough noise to match the grain of the original (usually around 2–3%). This will enable you to better disguise the history brush retouching.
Lighten/Darken blend mode
The Lighten blend mode was used to remove the dark marks in the image. Similarly, one can use the Darken blend mode to remove any light blemish marks. For example, if you were to retouch a scanned color negative, the dust spots would all show up as white marks.
1.
This scanned photograph serves as a good example with which to demonstrate the history brush spotting technique. A lot of dust marks are clearly visible in the enlarged detail views shown here.
Threshold adjustment
I mentioned that the Dust & Scratches filter can be rather destructive. One way to mitigate the destructiveness of this filter is to raise the Threshold value in the filter dialog.
2.
I went to the Filter menu and chose Noise Dust & Scratches. I checked the filter dialog preview and adjusted the Radius and Threshold settings until I could verify that most of the dust marks would be removed. I clicked OK to apply this filter to the image.
3.
I then went to the History panel and clicked on the previous unfiltered image history state, but set the Dust & Scratches filtered version as the history source to paint from. I then selected the history brush and in the tool Options bar changed the history brush blend mode to Lighten. As I painted over the dark spots, the history brush lightened only those pixels that were darker than the sampled history state. All the other pixels remained unchanged. I continued using the history brush in this way until I had removed all the dust spots in the photograph.
Flat field calibration
One way to remove image sensor pixel defects and artifacts such as dust spots and lens vignettes is to use the flat field calibration technique shown here. Some specialist digital capture systems, such as those used in astronomy and microscopy, already offer their own software solutions to achieve this. However, Photoshop CS5 now includes a new Divide blend mode that allows you to carry out flat field calibrations for any type of digital camera system. Be