Some years ago before Adobe refined their sharpening tools like to current Smart Sharpen, an often used technique was to convert Mode to LAB color and then select just the Luminence channel and apply sharpening to it. This produced an effective sharpening on a principle somewhat similar to high pass and without making graininess more apparent.

I've been playing around with this for a couple of days and my (completely unscientific) reaction is that it does work well. My best results so far have been from doing several passes with the high pass filter set to a radius of 10 pixels or less until I get sharpness to a point just short of where I want it, then finish with one or two passes of the smart sharpen filter set at 50%/2 pixel radius. I like the results better than smart sharpen alone, but I still need to tweak the technique some more to get it just right.

Also, it seems to me that digital images respond better to this procedure than scanned negatives (both 35mm and 6X6), but there are so many variables involved that it's hard to say where the difference lies......

I sharpen the luminance channel on a sub-pixel level generally using a 0.3 pixel radius and whatever amount is required by the individual image which can be as much as 500 though generally less.

On a noisy or grainy image, this does sharpen the edges of the grains in gradients too, so in the past I did small blurs on the luminance channel - just to the point that the grain or noise blends, but not so much that detail is impacted. usually a radius of 0.4 to 0.7. Yes, sub=pixel again. The a and b colour channels can take considerably more - 1 to 3 full pixels.

In the past few days, I have been investigating identifying edges, and only sharpening there. Smart Blur has an edge mode that works very well. Duplicate the background layer, run Smart Blur and use Colour range to select the edges. Delete the layer and the selection drops to the edges on the background layer. Use the luminance sharpening as above. The effect is very much like accutance in film - edge contrast. The images look sharp - not sharpened.

I recorded this as an Action, so it is quick and easy. I am trying a variation using Select-Modify-Expand to widen the selection by a pixel or two, but have not decided. It clearly improves the sharp look, but can introduce artifacts. The Action pauses when the USM dialogue is open so I can vary the amount as needed.