I believe many ardent typographers would answer, "not much".
Readability is a semantic and syntac attribute of text while legibility is a visual attribute of type. The smaller the type size the heavier the stroke and the wider the set. The larger the type size the lighter the stroke and the narrower the set.
The introduction of photo-mechanical and photo-electronic composition made photographic scaling possible, and the introduction of splines perpetuated the possibility (URW Nimbus, Linotype's early no name splines, Adobe Type 1 and Type 3, Bitstream Speedo, Apple TrueType, Monotype Qubic, Compugraphic Intellifont).
The problem with splines is that the single master is designed for some size and that single master is then scaled to sizes it was not designed for. If the design is for 12 point and a 6 point small cap is faked from the 12 point capital, the 6 point will be illegibly light.
There are two technologies that deal with this, one is Adobe's Multiple Master technology and the other is Apple's Variations technology for up to 64 design dimensions simultaneously. Adobe's technology was made for simulating fonts Adobe did not have permission to embed in PDF.
Adobe's technology was sold in Adobe Type 1 Multiple Master type products, supported in QuarkXPress 3. Adobe's technology is used as internal format in Fontlab whereas the external format is saved out as single master. OpenType 1.4 does not support Multiple Masters.
Apple's Variations technology was developed for complex multilingual composition, for instance, in shaping of kashidas in Arabic. It is simplicity itself to use, simply use the sliders in the Typography Palette for SFNT Spline Font files that have Variations.
Note that the QuickDraw Graphics Extension and ATSUI Apple Type Services for Unicode Imaging support as well Apple Variations as Adobe Multiple Masters. To test this, download Intaglio from Purgatory Design and enable the Typography Palette for Apple Skia and any Adobe Type 1 Multiple Master of your choice.
Henrik
Reference:
http://www.freepatentsonline.com/5185818.html