Filters menu in Affinity Photo and then Sharpen menu and "Unsharp Mask filter" and then Set radius and Set factor and Set threshold and then Apply. Great for edges of objects and images, for contrast.
Radius + Factor + Threshold. You can set the radius and factor to max and little will happen unless the threshold is set to a fairly low value though this depends on the image. As you reduce the threshold you will notice more and more of the effect creeps in. You can push the radius setting beyond its maximum slider as well as entering a larger value in the edit field.
Reduce the threshold to 0 and set the others high and all the light areas will be greatly sharpened though this result is not always too pleasing unless that is the result you want. Certainly does not work well with the strands of the hair in the image
You can set it too high and artifacts will appear across the image especially if it is not the best image in the first place such as one of my renderings from a 3D app.
You can apply the first once, you can apply it multiple times with the same settings via the repeat command at the top of the application. You can apply it multiple times but after the third or fourth time, the result will probably not change at all and you will then need to set different values before any repeat. Youtube video on subject
You can fade it and blend it with a different blending mode using the layer menu and fade command. You can create all kinds of amazing designs by using this unassuming command. Set the blend to difference or linear burn or perhaps change the opacity of the effect to 50% etc Youtube video on subject
You can also apply the unsharp mask filter to all the color channels but using the channels panel you can apply it only to the red, so you can create an interesting sharpen effect with the red channel and perhaps a different one for the green and blue. View menu and studio and channels and select the composite red channel and then apply the filter to that. Click the circular arrow button at the top right of the panel to return to RGB. You will now see the sharpen effect only applied to the red Youtube video on subject
OK, so you can tweak the effect in channels etc but what is the use for this powerful too, well you can use it to create an increased image contrast with edges in photos etc. Really brings out parts of the image.
The filter is a destructive effect, you can also use the tool via the layer menu and new live filter layer and sharpen and the panel will then allow for use of blends as well as opacity between the image and the effect and also the settings and you can associate it with one layer. You can use the layers panel to delete the effect as well as double click the entry and change the settings. This is a non destructive effect. You can add multiple entries as well and combine it with other effects.
You can record this to a macro and re-use the settings at a later time. You can also modify the parameters of the effect in the macro during playback. You can use all the parameters such as radius, factor and threshold but you can also set up the parameters to be only radius if that is the most useful setting for you. You can then playback the effect multiple times. Youtube video on subject
The tool works well with flattened images and text.
You can also use the effect with other filters such as Gaussian blur to create some really abstract designs by using a high radius setting and low threshold etc and also perhaps a few repeat applies of the filter Youtube video on subject