My understanding (disclaimer - through what I've heard/read and not
from any formal training!) is that serif faces are indeed easier for
persons with vision falling in "normal" ranges.  The serifs form a
horizontal "line" along the top, middle, and bottom of the type line that ties
the letters together and makes it easier for the eye to follow.  However,
for persons with low vision, sans serif faces are easier to read because
there's less "clutter" from the serifs, so the letter shapes are more easily
distinguishable.

The old (pre-ADA) typsetting rule of thumb that I learned is that body text
(dense or voluminous text) should be set in serif face because it is
easier for the eye to follow the lines of type, and that only headlines or
blocked out text should appear in sans serif face.  But that isn't always
the best solution for persons with low vision.

Sans serif faces can actually be relatively easily legible for most people
if enough care is taken to use a large enough type size and adjust
kerning/tracking (spacing between letters - "horizontal" spacing) and
letting (spacing between lines - "vertical" spacing) accordingly.  Larger
letting (more space between lines) is easier to read than dense letting
because the lines stand apart more - but, if you go too far, it's easy to
lose the next line when you get to the end of the first.  Same with
tracking - a little space gives the letters "room to breathe" and can help
persons with low vision distinguish one letter from the next by making
the text less "cluttered."  But again, too much space and it's easy to lose
the flow of the type, making it difficult to read.

From what I've read, there is no silver bullet or easy formula in type
setting for normal vs. low vision - it's all a balancing act.  (Isn't
everything?)

Any accessibility experts out there who can weigh in on this?


___________
Nicole M. Bouvier
Smithsonian Institution Traveling Exhibition Service (SITES)
202.357.3168 x120
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