The photo above is a good example. The dragonfly is pretty bright, but the background is much darker. This strong contrast of brightness helps emphasize the edges, making the outline of the dragonfly appear much sharper.
This is actually how most sharpening algorithms work in post-processing software: by increasing the contrast of brightness between edges it makes those edges stand out more and appear sharper. That’s why oversharpened images have those ugly halos. [Read more…] about Quick Tip for Getting Sharper Closeup Photos