You will find many, many, many people and books and websites that tell you with great certainty that you should use sans serif typefaces for posters. For example, Mary Briscoe claims in Preparing Scientific Illustrations:
A serif type can be distracting, especially in a large title.
Why this should be so is, sadly, never explored or explained.
Michael Alley writes in The Craft of Scientific Presentations:
(A) uniform font such as Arial is appropriate. The presenter should avoid a font such as Garamond that has thin strokes.
Nancy Duarte in Slide:ology may have been talking about slides, but the reasons she advocates sans serif would also apply to posters:
The debate still rages about which type is most suited for legibility. ... I conducted my own research from the 28th floor of a Las Vegas hotel. ... Out of the 40 or so billboards visible from the hotel room, the only ones I could read were set in sans serif type.
These all claim that sans serif is more legible for tasks where the words are few, the print is large, and distance to the reader is great. And that pretty much describes posters to a ‘T’.
Even those that advocate serif typefaces for the main text will suggest using serif type for big text. Colin Purrington's Advice on designing scientific posters argues:
Use a non-serif font (e.g., Helvetica) for title and headings and a serif font (e.g., Palatino) for body text (serif-style fonts are much easier to read at smaller font sizes).
Similarly, Creating effective poster presentations advises, “Use a serif font (e.g., Times) for most text – easier to read,”, but in the very next bullet says, “Sans-serif (sic) font (e.g., Helvetica) OK for titles and headings.”
Surely all those smart people must be on to something. Indeed, when I look back at my own posters, most of them have been set in a sans serif typeface, and it’s partly why this blog features a lot of sans serif at the moment.
Should posters always use a sans serif typeface? Not necessarily. There are good reasons to use a serif typeface for a poster, the most important being:
Because you like it.
I appreciate this post from the I Love Typography blog, which says in part:
In my opinion, a lot of time is wasted attempting to prove that one is better than the other for setting extended text. I suggest that you ignore the vague and inconclusive findings of such ramblings and decide for yourself.
For posters in particular, the legibility of text is not determined so much by the little extensions from the letters, but by the size of the print. If you make it big enough, people will be able to read it.
Design is all about making decisions. It’s all too easy to fall into a “default” mode, and use the same things over and over without really making any explicit decisions of your own.
Pictured: Will Eisner’s femme fatale Sand Serif, from The Spirit, whose name is a typographical pun. Serif ‘A’ from Eva the Weaver on Flickr, used under a Creative Commons license.