Sharpness

Anything that lets you get closer, extension tubes excepted, causes a loss of sharpness.  Fortunately, because we underwater photographers usually work at small apertures, this loss can  be held to an acceptable level with the right combination of gear.

The single-element accessory lenses are really pretty bad.  I'd stay away from them, and use the two-element accessory lenses from Nikon and others.  I recommend against trying to use two of these in combination, even though I've included the data in the tables: the results on a table on the boat were so fuzzy in the viewfinder that I've never tried this underwater.

Reversing another lens can yield acceptable sharpness, or not, and it's hard to tell what the results will be without trying.  The situation is made more confusing by lenses like the 105 mm f/2.8 AF Micro-Nikkor, and the 200 mm f/4D AF Micro-Nikkor, where the distance from the front element to the threaded ring on the front of the lens varies depending on where the focus is set, so sharpness will vary with magnification as well.

Even if a tele-converter is perfect, it will magnify defects in the image produced by the prime lens, and this contributes to a loss of sharpness.  In practice, I've found that a quality tele-converter can produce crisp images if the (indicated) f/stop is smaller (numerically greater) than f/8 or f/11. I've had better results with 1.4x tele-converters than with 2x ones.