Today's title asked the question: "can Adobe Photoshop CS5's Smudge Tool be used as a sharpening device. Well, looking at the two insets, flanked by the original image and the repaired image shows the answer is yes. The (left) original image comes right out of Adobe Lightroom 3. LR3 was used on most of the image to adjust color, tone, lighting and other things. The Adjustment Brush was used to lighten the shadow area under the cap. That resulted in a distinct light/dark light. In the second inset the line is gone. How was it done? To find out how, hit the "Read More".
The Smudge Tool (there is no keyboard shortcut - it's nested with the Blur Tool [looks like a droplet] just under the Gradient Tool (G) in the Tool Bar) is one of CS5's least used tools. If you're the type of person who's gone through every tool and every sub-tool you've probably played with the Smudge Tool and dismissed it. I'll kind of put it right next to the Liquify Filter (Filter/Liquify) as a misunderstood piece of the last several versions of Photoshop.
The way to use the Smudge Tool (or the Liquify for that matter) is to think small. Rather than making major movements, the Smudge Tool is best used at tiny details. The insets of today's image are blown up more than 300%. The line being fixed is clearly visible in an 11" x 14" enlargement. Your eye is drawn to it because "something" is wrong. There's a (comparably) bright line on the face of the conductor. Once the line is "smudged" there is no bright area to draw the eye. Is it perfect? Not quite, but remember, the insets are much larger than you'd ever see them in the image.
Using the Smudge Tool it increase apparent sharpness is best done using a graphics tablet. It can be done with the mouse, but the movements, in this case, are unnatural. If you've never used a tablet, it takes a little getting used to, but learning it is a worthwhile experience. I would never try to "fix" the line in today's image in one stroke. Using many small strokes is a much better way to go. Using small strokes also gives the benefit of being able to Go Back (CTRL Z) just a little rather than losing a major stroke.
Not all strokes have to be in a straight line. Tiny imperfections can be taken out using a circular motion. I wouldn't use it for blemish removal, but if you're using the Smudge Tool anyway... A couple places this technique was used in today's image are: The blue of the Conductor's coat. The stripes of the engineer's coat. The pillar between the two men. The wording on the engineer's cap (you still can't really read it, but it does look a little clearer.
So, there you have it. Another infrequently used tool in CS5 that, when used properly, can make life easier.