Optical Adjustment in Typeface Design
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In my Advanced Kinetic Typeface Design class at SUNY FIT, I always start the semester with a circle and a square projected on the screen and ask, "Can you tell me the difference between being correct and being right?" It is a trick question. It would be correct to say the circle and the square are the same height. It would not be right, because what you want is for the two shapes to feel the same size.
Alignment tools measure. They find the bounding box of a shape and measure them with absolute precision, a dependable answer to a question your eye is not asking. Your eye estimates, and what is mathematically correct might not look right.
What is optical adjustment in typography?
Optical adjustment in typography is the practice of drawing letterforms with unequal measurements so they feel “right” to the human eye. Human vision is reliably biased. When the horizontal and vertical strokes of the letter T have the same thickness, our eyes perceive the horizontal stroke as slightly thicker. So, a typographer needs to compensate for this visual bias by optically adjusting the horizontal stroke.
The same thing happens to letters. Set an O and an H to the same cap height and the O reads as smaller, because only a narrow sliver of the curve ever touches the cap line. Draw the O about 3 percent taller at both ends and the two read as equal. A pointed apex, the A or the V, needs at least as much.
Then there is where that crossbar of the H sits. Put it at the exact vertical center, and it looks low. Raise it 2 or 3 percent and it looks centered. That is the align button doing arithmetic correctly and design badly.
Nobody agrees on the number
Optical adjustment is not an exact science. It is closer to an art, and the numbers prove it. Three experienced sources give three different corrections.
| Source | Round letters | Pointed letters |
|---|---|---|
| Tobias Frere-Jones, 2015 | No fixed figure given | More than round, and it changes with weight |
| Communication Arts | 1 to 2 percent | About 3 percent |
| Our own drawings | 3 percent | 3 percent |
Tobias Frere-Jones, who has taught at the Yale School of Art since 1996, declines to publish a number at all. Writing in 2015, he shows overshoot shrinking as a curve flattens, and notes that a V with a blunt apex in a heavy weight needs a different alignment than the same V in the light.
None of these people is wrong. They are reporting where they landed, on their own drawings, at their own sizes.
What the research establishes, and what it does not
Vision research confirms the bias and stops there. Sergio Chieffi and colleagues, publishing in Revue Neurologique in 2019, asked ninety healthy participants to mark the midpoint of a vertical line. On average they marked it 7.49 millimeters above the true center, in line with work going back to Shelton and colleagues in 1990. A crossbar placed at the mathematical center reads as low. That is a property of vision, not a preference.
The heavy horizontal has its own literature. A 2013 paper in the Journal of Vision splits the vertical-horizontal illusion into roughly 6 percent overestimation of a vertical extent against a horizontal one, plus a bisection bias near 16 percent. The effect changes size depending on whether the lines form a T, an L, or a cross.
Here is the limit. The research explains why a correction is necessary. It does not tell you how much. Those are length judgments in a laboratory, not a lookup table for a letterform.
A practical example of optical adjustment
The V in VH New Order, a typeface designed by Cynda Media Lab, tapers on its way into the vertex. The diagonal runs at 100 percent through most of its length, narrows to 90, then to 87 at the join. Without the taper the strokes pile up and the letter grows a dark blot where the eye expects a point. The K and the X get similar treatments.
Those numbers came from more than thirty years of drawing this kind of thing and checking it at final size. They are a starting point. In the next design, they will be different because the weight and apex angle will be different, and the “adjustment” will need to be adjusted accordingly.
Instead of finding these judgment calls intimidating or confusing, you should find them liberating, because they are the decisions that make your design uniquely yours.
FAQ
What is optical adjustment in typography? Optical adjustment is the practice of drawing letterforms at unequal measurements so they appear equal. Round and pointed letters are drawn slightly taller than flat-sided ones, and horizontal strokes are drawn slightly lighter than vertical ones.
How much overshoot should a round letter have? There is no single correct figure. Published guidance runs from about 1 to 3 percent of cap height for round letters and more for pointed ones, and experienced type designers disagree. The amount depends on curvature, weight, and reading size.
C.J. Yeh is founder and ECD at Cynda Media Lab and a co-founder of DSGN BY D. He is also a tenured professor and curriculum chair of Creative Technology and Design at the Fashion Institute of Technology.
Sources
Tobias Frere-Jones, "This Optical Illusion Tricks You Into Thinking That Typeface Letters Are the Same Height," Slate, April 9, 2015. https://slate.com/human-interest/2015/04/tobias-frere-jones-explains-the-optical-illusion-that-tricks-our-brains-into-perceiving-typeface-letters-are-of-equal-height.html
"Know If a Font Sucks," Communication Arts. https://www.commarts.com/columns/know-if-a-font-sucks
Sergio Chieffi, Clara Castaldi, Girolamo Di Maio, Marco La Marra, Antonietta Messina, Vincenzo Monda and Ines Villano, "Attentional bias in the radial and vertical dimensions of space," Revue Neurologique, 2019. https://www.sciencedirect.com/science/article/pii/S1631069119300368
"The vertical-horizontal illusion: Assessing the contributions of anisotropy, abutting, and crossing to the misperception of simple line stimuli," Journal of Vision, 2013. https://jov.arvojournals.org/article.aspx?articleid=2194000