Hi - I am still a novice when it comes to this and like you read Michelle's wonderful tutorial. I found that to check for the halo effect I increased my window with the unsharp mask images to full size. I then zoomed in to a part of the image where there was an edge likely to have a halo eg: the horizon or contrasting coloured edges, I could then clearly see the halo. I am sure with more experience I would spot it easier, but this is the way I found so I am sharing in case it helps you or others. With regards to the difference between unsharp mask and sharpen which is what seems also to be the question I can only quote from a tutorial I have. "The sharpen command works by increasing the contrast between pixels and does so in an automatic task applied to all pixels.This does sharpen the picture but causes it to become pixelated very quickly. It is worth applying once only if the picture just needs a very small tweak " The shaprpen command works on the whole image and USM is selective within the image because it only sharpens edges depending on what settings you have chosen. From the same tutorialon USM: Strength 75 is considered to be the medium. 50 to give a subtle effect. Radius 0.5 to 1 is good for web images, 2-3 ok for high resolution prints. Clipping: 4 for general use but it depends very much on the picture and differences in levels. Richard.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html