Adrian Bacon
Subscriber
I agree with certain reservations. Viewing distance is one...
Viewing distance does play a role.
...and I also think there's a maximum size per format before sharpening detracts from the balance of a photograph. For example if you enlarge an image in editing software and concentrate on a fine detail like grass, it will sharpen up nicely but the image as a whole can appear over-sharpened in smoother areas, with what people have described as a visual mosquito whine.
This is why you selectively sharpen, meaning, only sharpen areas of interest. Just blindly sharpening the whole image leads to things like visual artifacts in areas that should be smooth (like sky). At the risk of sounding like a jerk, anybody who just throws a sharpen filter over the whole image before printing a reasonably large print does not know what they are doing. Just like with shooting with a small DOF field to draw the viewers attention, or using leading lines to draw the viewers eye, intelligently sharpening only the areas of interest has a dramatic effect on the image.
When I’m preparing an image for print and have it resized to the native print resolution for the size print it’s going to be, I routinely have multiple sharpening layers with layer masks masking off just the areas I want sharpened. Each sharpening layer is a different amount of sharpening, so not only do I selectively sharpen parts of the image, I also control how much sharpening is to be applied to each area. This way you can sharpen up the grass (using your example), but sharpen it less than the subject of the picture, and not sharpen the sky and areas that are not important or out of focus at all. I generally make the subject of the photo the sharpest, then progressively dial back the sharpening on other elements in the image the further away from the subject they are.
Image sharpening is a tool in the visual image toolbox just like any other photographic tool that you would use to manipulate the viewer viewing the image.