Designing for the Eye
Optical Corrections in Architecture and Typography
By Niko Kitsakis, June 2025
This article highlights a special aspect of both visual design and architecture: Optical corrections (or optical adjustments, if you prefer). If you found my piece about legible typefaces interesting, you’ll likely enjoy this one as well. Since I included many visual examples that are size-sensitive, make sure you read this on a big screen and not your mobile device.
Optical Illusions
The Müller-Lyer illusion, pictured below, makes you think that, of two lines, one is longer than the other when in fact they are of the same length. It appears in virtually every introductory book on graphic design and, of course, in books on perception and psychology. You might not have known it by name, but you must have seen it before:
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Is the blue line on top longer than the one at the bottom? Click the arrows to find out.
It used to be said that, in order to see the illusion, you must have grown up in an environment where lines very similar to these would explicitly appear in your everyday surroundings – in architecture and furniture, for example, or when you had played with building blocks as a kid and so on. This is, incidentally, what I too was taught at my school of visual design some decades ago. However, I always had a hard time believing it, and according to some new research, this is indeed not the case: Not only can the illusion be perceived by anyone, no matter their cultural background, it even extends (to some degree) to the animal kingdom and can be experienced in a haptic version as well.
The reason why I never really believed that the Müller-Lyer illusion (and others like it) would work only on people who grew up in our Western culture is that, in the first place, it seems visually far too universal to begin with. After all, they’re just lines. You can draw them in the sand, and they can appear naturally in many circumstances. When I look at another person, my brain will assign certain values to the length of that person’s arms, legs, and torso, and memorise them as some variety of line as well. If the brain didn’t do that, we would not be able to tell if a body or facial features were out of proportion. So in my view, something discernible only by people from a specific culture would have to be far more elaborate than an optical illusion of just a few lines.
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