The first thing we can observe is what we already saw in the previous illustration:because of the big difference with the black border, we get an accentuated, white edge next to it that smoothes into the image itself. And the higher the Radius setting, the thicker it becomes.
What we can do in this case is make a selection of what we want to sharpen, copy this to a new layer (Ctrl/Cmd+J) and apply sharpening to this layer instead of our basic image.
(Same goes of course for a photograph with a beautiful white border.)
We also see that there is a colour shift, and that this becomes more prominent when the contrast is greater: it is more visible on the left-hand side where the difference is at maximum, and it is non existent where the colours are the same, that is on the right-hand side. It also becomes more prominent when the radius increases and, although the effect diminishes with applying a Threshold setting, it cannot be undone by it.
What is happening is that we seem to apply the Unsharp Mask to the image we see, but in fact, it is applied to each channel of the image separately.
Let's take a look at the channels. I created this image by making a layer of the background, duplicating it twice, and adding a new black bottom layer for visibility. Then, to each duplicate, I added a Channel Mixer adjustment layer, set to monochrome and dragged the sliders to 100%. The top one is the Red channel, the middle one the Green, and the bottom one the Blue channel.
