What is sharp?
To understand sharpness you must first understand the definition of an edge. An edge is the line of intersection of two surfaces. A highly sharpened edge is one where the two surfaces are highly polished to form a very fine edge. Sharpening is the means to that very fine edge.
There are countless ways of testing knives and tools for sharpness. We believe the easiest way to test sharpness is to use the tool or knife. If it does not cut fast and cleanly, it needs sharpening. On a kitchen knife, the knife should be able to cut vegetables with almost no downward pressure. On a woodworking tool, the tool should cut wood fibers cleanly without leaving marks or crushing the wood fibers. On a fillet or skinning knife, it should be able to cut very quickly without having to saw through the meat.
If you really want to get down to fine-tuned levels of sharpness there are a few more tests you can use. My personal favorite is to take a piece of paper and hold it vertically. If you try to cut it with a dull knife, the paper will crumple beneath the knife. A sharp knife will cut it cleanly when use a slicing motion to cut through the paper. A razor sharp knife can cut the paper cleanly by just pressing down on the edge of paper without any slicing at all.
Another test is to shave the hair on your arms. While we recommend caution using this method, it can be very useful. A dull or even moderately sharp knife will just fold over your arm hairs without cutting. A well sharpened knife will cut almost all of the hairs in one pass. A very sharp knife will cut all the hairs in its path. This level of sharpness can only be attained using the finest abrasive materials.
Sharp Edges Shave Hair
Another factor effecting edge sharpness is the angle it is sharpened. The lower the angle, the sharper the blade becomes. However, the lower the angle, the weaker the edge becomes. Very low angle blades like a razor blade or a fillet knife will ultimately have a sharper edge than high angle tools such as an axe, but that edge will not last as long and will require more frequent resharpening.
A More Technical Approach to Testing Knife Sharpness
If you want an objective way to measure sharpness, the Edge-On-Up Sharpness Tester will give you accurate and repeatable results. Rather than test by cutting hair or paper, the Edge On Up uses a certified testing media and gives an actual number that can be used to compare edges.