OK it’s time to get my head around sharpening once & for all! It’s something I naturally use at the end of editing every photo but I’ve never really understood what the three elements of “Unsharp Mask” did.
Radius. This controls how many pixels adjacent to the edges are sharpened (sharpening works by the software increasing the contrast along the edges of objects within the image). A small radius enhances smaller scale details eg. stray hairs, eyelashes etc. Usually a setting of 0.6 to 2.5 is used. Setting too high a radius can create unwanted halos around objects.
Factor. This affects the strength of the sharpening (how much darker & how much lighter the edges become).
Threshold. This controls how far apart adjacent tonal values have to be before they are considered an edge ie. before any action is taken. Low values should sharpen more because fewer areas are excluded. Therefore, for human faces for example, higher values should be selected whereas, inanimate objects such as still life images can take a lower value.
When using Affinity Photo always apply sharpening non destructively via the Live Unsharp Mask filter, Layer>New Live Filter Layer>Unsharp Mask Filter. By doing this not only can you switch this layer on & off to see what affect the sharpening has on your image but you can also come back to tweek your settings at a later date.
Using a small radius (eg. 0.5) & a large factor (eg. 3.5) will accentuate more of the finer details eg. the brickwork on a building. Using a large radius (4.0) & a smaller factor (1.0) accentuates the larger details & acts in a similar way to a clarity filter. For portraits or images of low contrast eg. skin, increase the Threshold slider gradually to prevent artefacts from being enhanced in these areas.