ALT This is a chart of the human contrast sensitivity curve. A blue line curves down to the right where the X axis is contrast sensitivity, and the Y access is spatial frequency, increasing toward the right. An increase in spatial frequency means elements are smaller and thinner. On the right of this info graphic are samples of text from very large and bold to very thin and small, with red lead lines indicating approximately where those samples fall on the contrast sensitivity curve. All of the text samples are at the exact same CSS color of #c7c7c7, the top very large and bold headline is legible but as the fonts become thinner and smaller they literally fade out as if becoming lighter gray, even though they are all at the exact same color. This happens because human contrast sensitivity is substantially driven by spatial characteristics. The amount of luminance contrast required is directly related to the font weight or line thickness.