Asked on a forum today: "Can someone explain what determines a photo being too sharp, or not sharp enough?"
This is a great question!
Firstly, it's necessary to distinguish between "focus" and "sharpening". "Focus" is what happens in camera - eg when your lens really nails every single eyelash in a portrait. It's really not possible for an image to be "too well-focused". The better the focus, the better the result.
"Sharpening" is the artificial stuff we add after the event. If you shoot Jpeg, your camera might add some sharpening immediately after it captures the image. And you certainly get to add sharpening in post-processing when you get your images back to your computer. There are sharpening tools available in every imaging software, and the idea is to enhance the distinction of edges in the image, to make the focus seem even better than it was.
You might think I'm trying to bore you with semantics, but this is very relevant to the question.
If somebody says "that photo doesn't seem sharp enough", they're almost certainly talking about focus. Maybe it's a focal plane issue, or shutter speed causing motion blur; either way, it's a camera issue. Artificial sharpening can disguise (not fix) the problem to a degree, but if your camera didn't capture eyelashes, there ain't no software in the world that can automatically create them.
But if somebody says "that looks too sharp", they are definitely talking about sharpening. It means that sharpening in post-processing has been applied too aggressively, or too broadly, or too minutely, depending on whether the problem is Amount, Radius or Threshold respectively. You can learn more about sharpening in my article here: Understanding Unsharp Mask.
As with most things in post-processing, sharpening is largely a matter of personal taste. You don't need to look far on the web to find clashes of opinion about the sharpness of images. Ultimately, you need to find your own preferred level of sharpening, and incorporate it as part of your "style".
The difference between web sharpening and print sharpening is confusing too. For reasons that I don't truly understand, we seem to be able to sharpen our web images much more strongly than our print ones (visually, I mean, not the actual settings). I think web sharpening might have evolved stronger and stronger over the years - I wonder if a time-traveling photographer from ten years ago would think that we are sharpening our web images too much in 2010!
Web sharpening, as I've said before, is easy, because WYSIWYG - if it looks good, it is good. If you're sharpening a web image, and it looks too strong, just ease it back a bit ... no worries! But print sharpening takes a bit of practice, because you can't tell from the screen exactly how it will look on paper. After a while you develop an eye for it, though.