Robert D Feinman

When optimizing a scanned image for printing, it is important to sharpen  to restore details lost in the scanning process.
This is not "cheating" since proper sharpening can actually restore detail  lost by the mismatch between the scanner's pixel size and the grain of the slide or negative.
Sharpening performs an operation that cannot be easily duplicated photographically. It subtracts the bluriness around the edges.
Here is a raw scan at my scanner's max resolution of 1600dpi, reproduced as actual pixels.
In the final step we will view this at print size, but this enlargement makes the changes from sharpening easier to see.

We used the sharpen filter on the entire image with settings of:
Amount 165%
Radius 2.7 pixels
Threshold 7 levels
Under most circumstances the radius should be set to about 1/100 inch.
So since we are planning to print this as a 300dpi image a radius of 3 pixels  would be 1/100 inch. We used slightly less to minimize undesirable side effects.
By setting the threshold to 7 we avoid  sharpening the film grain, especially in the sky.
The image is much "sharper" than the original, but note the fringe at the top of the hill where it meets the sky.
If this was a scan destined for web display the image would be at 72dpi and a radius of about 0.7 pixels would probably be best.

To explain what has happened we have created a difference image of the previous two and greatly increased the contrast. This shows only the altered pixels in colors. Unchanged pixels are black.

Notice that there is a fringe on both sides of the ridge of the hill and the fence bars, etc. Also small highlights in the foliage have become donuts.

Difference from Sharpening

Here is the image with sharpening applied only to the luminosity (lightness channel). This is achieved by selecting the image in the channels  pallet and control-click on the thumbnail. This creates a selection where the brighter parts are more selected than the dark.
We then apply the sharpen filter to the selection. This tends to sharpen the parts of the image that the eye sees more clearly without overdoing the rest of the image. Notice the effect is less that the overall sharpening even though the same values were used.

Luminosity Sharpen

Once again we show the difference between the original and the previous  sharpened image.
Notice the same general features have been sharpened, but the halos are not as big.

Luminosity Sharpen Difference

One of the worst problems with sharpening occurs at sharp color boudaries such as the horizon.
In order to minimize this we use the color range tool to select the sky and invert the selection. The sky now will not be sharpened and the ridge of the  hill will not bleed into the sky.
We sharpen with the same values as before. Notice the fence looks sharper  but there is not a halo near the sky.

Color Range Sharpen