Sharpening is a very delicate subject. It should be used with great care, and mostly to give a little bit of extra pop to already reasonably sharp images. It may be tempting to oversharpen in order to «fix» bad focus or motion blur from camera shake, but as this would be the same as restoring detail that just isn’t there, it should be avoided.
There are three ways to apply sharpening in Darktable: Sharpen, Equalizer and the High Pass filter. They work in different ways but the results are very similar. Therefore I don’t want to say anything about when to use which method. You should learn them all and decide for yourself in each situation.
Also, Sharpening should be done at the very end of your workflow, and ideally you would sharpen for each output resolution. Say you export a small file for Instagram, a medium sized file for a web site and a full size file for printing. If you zoom in to 100% and sharpen the very finest details, this sharpening will never be visible in the Instagram file. Small file sizes require sharpening on a larger scale. And if this sharpening is done in the full size file, it will look more like overdone local contrast than sharpening.
This is the obvious choice. It resembles the good old Unsharp Mask, with the two sliders Radius and Amount. It also has a third slider, Threshold, which sets a threshold for how big the difference between two pixels should be for sharpening to begin. This can help to avoid the sharpening of noise.
Enhances local contrast at freely selectable frequency ranges. For the best sharpening, boost contrast at the highest frequencies visible in your output resolution.
High Pass filter
Boosting edge contrast by adding an edge detect layer in Overlay blend mode. Be careful, as haloing around the edges is easy to introduce.
To learn more about the different sharpening tools, watch the video here: