If you use Adobe In­De­sign—and if you don’t, you can ig­nore this page—you might have no­ticed the popup menu that gives you a choice be­tween “met­rics” and “op­ti­cal” font spacing.

De­spite the be­nign-sound­ing name, op­ti­cal spac­ing will man­gle your font. It is akin to putting your finest cash­mere sweater in the wash­ing ma­chine. So al­ways use the “met­rics” option.

This set­ting con­trols the way In­De­sign lays out your type:

• Met­rics spac­ing re­lies on the char­ac­ter-spac­ing in­for­ma­tion in­side the font—that is, the in­for­ma­tion about char­ac­ter widths and kern­ing pairs that the font de­signer put there. When you use met­rics spac­ing, you’re see­ing the font the way the de­signer intended.

• Op­ti­cal spac­ing com­pletely junks all this char­ac­ter-spac­ing in­for­ma­tion. In­stead, it ap­plies a patented spac­ing al­go­rithm that guesses what the width and kern­ing of every char­ac­ter should be. To no one’s sur­prise, it of­ten guesses wrong.

In this type de­signer’s opin­ion, the spac­ing of a font—that is, the de­sign of the white spaces—is far more con­se­quen­tial to its ap­pear­ance than the de­sign of the black shapes. If you buy a pro­fes­sional font (see font rec­om­men­da­tions), and then run it through the op­ti­cal-spac­ing wringer, you’re throw­ing away most of what you paid for.

Still, don’t take my word for it. Com­pare these sam­ples, show­ing Eq­uity with met­rics spac­ing on top, and op­ti­cal spac­ing on the bottom:

• In met­rics spac­ing, the spaces be­tween nu and un are vi­su­ally bal­anced. But in op­ti­cal spac­ing, the u is pushed to the left, so nu looks tighter than un.

• In met­rics spac­ing, the spaces be­tween un and no are vi­su­ally bal­anced. But in op­ti­cal spac­ing, the o (and all rounded let­ters) are squished closer to flat let­ters like n.

• In met­rics spac­ing, the t has the space it needs on the left, so that a combo like ntit looks even. In op­ti­cal spac­ing, the space within nt and it shrinks, be­com­ing darker than the ti space.

• “The op­ti­cal-spac­ing row on the bot­tom looks OK to me.” I’ve en­larged these 11 let­ters to con­vince you that spac­ing dif­fer­ences ex­ist. But yes, in a head­line, these dif­fer­ences are less both­er­some, be­cause there’s still plenty of space be­tween each pair. The real prob­lem arises when you re­turn to or­di­nary point size. These asym­me­tries ac­cu­mu­late across hun­dreds of let­ter pairs, re­sult­ing in body text that’s jit­tery and uneven.

The goal of spac­ing let­ters in a font is to give type­set text an even color, min­i­miz­ing dark and light spots. Op­ti­cal spac­ing does well enough with let­ters that are sym­met­ric—say, an up­per­case H or low­er­case n.

But those are easy. As we can see above, achiev­ing the same even­ness is harder with asym­met­ric let­ters like low­er­case a or r or t. But it’s also vi­tal, be­cause these let­ters oc­cur so fre­quently in text. This is why hu­mans out­per­form the ma­chine on font spacing.

“So what is op­ti­cal spac­ing good for?” Op­ti­cal is the emer­gency op­tion if you find your­self work­ing with:

1. A font that has bad spac­ing (for in­stance, the client has in­sisted on such-and-such free font) or;

2. A font that’s be­ing pressed into ser­vice be­yond its de­signed ca­pac­ity (for in­stance, a body-text font be­ing used for a headline).

If such an emer­gency arises, break the glass. Try op­ti­cal spac­ing. Judge with your eyes whether it helps.

Oth­er­wise—met­rics.