We "see better" than the optical system of our eyes allow. Image processing happens to the signals from our retinas before it arrives at the visual cortex and especially before it arrives at higher levels within the brain. This means that logical letter forms may not be ideal and that actual experiments need to be performed to understand what works well when considering visual perception.

For example, Kuffler units within the retina are triggered by the presence of intensity transitions so we have edge detection and enhancement happening very early in the visual system. We don't actually see edges as clearly as we think we do. A number of optical illusions are due to this fact.