1 What is it
It is applied to the raw file immediately after demosaicing and modifies the data in linear gamma to limit halo generation. This means that it will only work on raw files .
2 Is it the definitive sharpening tool?
Even though you can get excellent results with the default settings, it is not meant to be the only tool used for sharpening. Rather, it should be considered as a necessary first step to improve the distinction between noise and details prior to the image being processed by other tools further down the line. It can therefore be used in conjunction with other sharpening methods such as the Unsharp Mask or RL Deconvolution. The combination of sharpening tools will depend on what you are trying to achieve and what is acceptable by way of artifacts.
It will also depend on the final size of the image and whether it is going to be printed or not. This is because in both situations you may be able to apply stronger sharpening knowing that in the final image it will not be so apparent.
Here are two typical sharpening tool combinations:
- with the Unsharp Mask: use a small radius and watch out for artifacts as you increase the amount of sharpening.
- with RL Deconvolution: find an appropriate radius value (through trial and error to avoid halos) and choose a high contrast threshold so that you only enhance the largest details and edges. This will allow you to keep artifacts (common with this tool) to a minimum. You will probably need to apply a higher amount of sharpening as well.
3 How it works (settings)
Any changes in the settings are applied to the whole image, no matter what the zoom level, so depending on your system, it may take some time to process the changes. However, once any changes have been made, you can zoom and pan without any further processing delay.
You can also use the Sharpening Contrast Mask to see which details will be sharpened. Sharpening will occur in the white areas but not in the black areas. The mask is visible both in the Preview area and in the Navigator panel.
3.1 Contrast Threshold
By default the tool analyzes the image and calculates a threshold to prevent sharpening noise.
You can leave the automatic Contrast Threshold calculation on, or you can turn it off and set it as required: moving the slider to the right means that details will have to have higher contrast before they are sharpened. Higher Contrast Threshold values also reduce the amount of sharpening applied to noise, which tends to have lower detail contrast.
When in-camera blurring occurs, the effective resolution is no longer dependent on pixel size and instead, has a radius that increases in proportion to the amount of blurring.
The purpose of this tool is to reduce this blurring by trying to automatically guess the radius needed to counteract the effect.
You can also choose to adjust the radius yourself bearing in mind that if the value is too low, there will not be enough sharpening and if it is too high, it will lead to strong haloing on the edges. Don’t forget that you are only trying to undo the in-camera blurring.
Even though the default radius is spot on most of the time, there may be scenarios where you need to be extra careful to avoid noise and over sharpening. For example when an image is going to be processed in programs designed for focus stacking, astronomical images, super-resolution images etc. In these cases, it may be better to turn off the auto radius calculation and manually set the radius to a lower value to avoid artifacts (check the result by zooming in on the image). There will not be as much sharpening but there will be fewer or no artifacts that could be enhanced by subsequent processing in other software.
Note also that if you are editing a long-exposure raw file (with hot pixels), the auto radius calculation works much better with the hot pixel filter turned on (located in the Raw tab):
3.3 Corner Radius Boost
Often images are softer or more blurred in the corners than in the center. The Corner radius boost slider allows you to compensate for this by increasing or decreasing the radius in these areas.
Moving it to the right increases the sharpening in the outer areas and sliding it to the left will reduce the sharpening.
Once the radii have either been automatically calculated or set manually, the tool carries out a series of calculations to try and compensate for any blurring. It is an iterative process, which will produce artifacts if carried too far. Checking the ‘Auto limit iterations’ checkbox will limit the number of iterations or you can do it manually using the Iterations slider.
To get the best out of this tool, the camera white level needs to be correct, especially if the image has sharp transitions between clipped and non-clipped highlights. If you experience problems, please refer to the section on White Levels in Adding Support for New Raw Formats.