USM

[BillOz]

Joe,

I did a little test.  By no means exhaustive, but I did it on two photos, one set of which I'll include here for comparison.

First I took a photo, straight out of the camera, no editing.  I saved this as a Photoshop .psd file, to keep as a clean starting point, with no editing.

For 1st version.  File: 350Z-9182-Resize-USM-800.jpg
I first resized to 800 pixels.
I then ran USM on the photo, with the following settings.  75%, 5.0 radius, Threshold 5 levels.
I then saved as a max quality .jpg file.

For 2nd version.  File: 350Z-9182-USM-Resize-800.jpg
I first ran USM on the photo, with the following settings.  75%, 5.0 radius, Threshold 5 levels.
I then resized to 800 pixels.
I then saved as a max quality .jpg file.

The last file is simply a resize of the original, with no editing at all.  Just for comparison

I did no other editing, such as contrast, lighting, etc. on any of the files.

Here they are.  There isn't much difference visible at these sizes, but when you have the files opened side by side, and can zoom in equally on each, it appears as though the version which is sized 1st appears a little more sharp.  Not more resolving/resolution, but appears sharper.  So there is less gradation of lines/edges, and more contrast, therefore appears sharper.

I think the reason for this is that sharpening adds contract to pixels which are next to each other, and so when you sharpen on a higher resolution file, there is less contrast between pixels, because the same line, say a wing edge, is composed of more gradations with a higher resolution file.   The lower resolution file, has already had some of the gradation taken out, and so there is a more distinct line to sharpen.

I originally resized to 1000 pixels, but the file sizes wouldn't meet the limits here, so I completely redid the test with a 800 pixel width resizing.   In viewing the 1000 pixel images, the results are virtually the same.

Basically I guess if you need to resize a photo, to use on the net, or elsewhere, it may be best to resize prior to USM.  How this might be affected by other editing, such as contrast, color, lighting, etc., I'm not sure, because some of these actions also affect apparent sharpness, and apparent sharpness affects apparent contrast, etc.

I hope this makes sense.  If not, do your own test, make your own conclusions, and edit your photos to whatever looks best to you.