About Adobe’s Optical Kerning - Extensis.com


About Adobe’s Optical Kerning

Monday, der 8. November 2010 by Thomas Phinney

Typographer and type designer Steve Mehallo (hi Steve!) asked me the other day whether I had anything to do with Adobe’s optical kerning. First, I should explain what this unusual technology for font spacing actually is….

Kerning is of course adjustments in spacing between particular pairs of letters (well, glyphs really). Adobe refers to the kerning information built into the font as “metrics kerning,” and it’s the default in their applications. These days font designers set up that metric kerning with “classes,” so despite the old definition of kerning, the way it’s stored in fonts today often isn’t really “pairs” but more along the lines of “kern all the letters with this right hand diagonal shape (V, W) this much against all the letters with that left hand round shape (O, C, G, Q, and any accented variants).”

Different kerning effects on Mac system Palatino
Different kerning effects on the Mac system version of Palatino, a passable quality font that gets some benefits from optical kerning.

Adobe’s “optical kerning,” seen as an option in InDesign, Illustrator and Photoshop, completely ignores the “metrics kerning” built into the font. It instead looks at just a couple of key instances of the base spacing in the font, and then mathematically calculates spacing for every glyph pairing in the current text.

This optical kerning uses technology patents originally from Peter Karow and URW. Said tech in turn was based on technology originally developed for TeX by Donald Knuth. I did not have much to do with the Adobe implementation, but I did occasionally work reasonably closely with Eric Menninga, the fellow who did Adobe’s last version of it for InDesign (though it had previously been seen in part in PageMaker).

My take is that optical kerning is a pretty cool thing, but one has to be careful with it and use it when appropriate. Originally when InDesign came out a decade ago, document performance would have been an issue, but that problem has largely gone away with increased processor speed. But there are some other issues.Optical kerning is most useful when dealing with:

  • An average or worse font, of all fonts in the world. Even the Mac system version of Palatino (above graphic) is missing some useful kerning adjustments. This category likely includes >90% of “free fonts,” though a few have spacing which is beyond saving by optical kerning.
  • Kerning between glyphs of different point sizes or different fonts; kerning in a font doesn’t deal with this.
  • Specific glyph combinations which were not addressed in otherwise well-kerned fonts.


  • It wreaks havoc on the spacing of script fonts, particularly connecting script fonts.
  • Fonts which are well-spaced and well-kerned typically receive minimal benefits from optical kerning. (See Parisine in the graphic below.)
  • At text sizes, in a really well-spaced and well-kerned font, optical kerning can actually destroy the “rhythm” of vertical strokes. In such cases it is more useful at “display” sizes rather than “text” sizes.

Different kerning effects on Parisine
Different kerning effects on Parisine, a well-crafted font which does not much benefit from optical kerning.

Different kerning effects on Hill House
Different kerning effects on Hill House, a free font with little or no built-in kerning. Optical kerning is a significant improvement.

So overall, there are several good reasons Adobe made “metrics” kerning the default rather than “optical” kerning. Most importantly, most of the typefaces bundled with Creative Suite are in that last category, and they just look better with metrics kerning. But you still might need to slip in occasionally to hand-kern some particular comnbination, or turn on optical kerning between two glyphs that weren’t kerned at all in the first place, but should have been.