Ask a new designer to kern a word and they will reach for a number. Minus ten here, plus five there, applied evenly, as if the alphabet were made of identical rectangles. Then they will look at the result and quietly feel that something is wrong without being able to say what.
The eye counts area, not distance
What we perceive as even spacing is not equal distance between letterforms. It is equal area of white between them. That is why a capital A next to a capital V can almost overlap and still look correct, while two capital H letters need real daylight between them to look like they are merely adjacent.
Type is set for the eye, and the eye has never once measured anything.
Where it actually matters
At body size, leave it alone. The type designer has already solved it, and your adjustments will make it worse across ten thousand words. Kerning is a display problem: logotypes, headlines, anything set above about thirty pixels where a single awkward pair becomes the only thing anyone can see.
- Squint at the word. Bad pairs show up as dark clumps or bright gaps the moment detail drops away.
- Read it upside down. It stops your brain from reading and forces it to look at shapes.
- Fix the worst pair, then reassess. Kerning is relative, so one change moves everything.
There is no correct value. There is only the point at which you stop noticing, and that point is the answer.
