Optical spacing beats mathematical spacing
Why evenly measured letterspacing still looks wrong — and how to trust the eye over the ruler.
The ruler lies
Letterforms enclose different amounts of air. A capital A beside a capital V overlaps in a way an H beside an I never will. Measure the gaps with a ruler and you will get five identical numbers and five different-looking spaces.
Space is area, not distance
What the eye reads is the area of white trapped between two shapes. Round letters need slightly tighter setting than flat-sided ones, and diagonals need less again. This is why every typeface ships with kerning pairs, and why display sizes always need a second pass by hand.
How to see it
Set the word, blur your eyes, and fix the gaps that flicker. Turn the word upside down if you can — reading gets in the way of looking. The ruler measures distance; the eye measures area, and the eye is the client.
The ruler measures distance. The eye measures area — and the eye is the client.
Set a five-letter word in a serif at 120pt. Space it by eye only, screenshot it, then space a second copy with identical tracking values. Compare them at 30% zoom.