Reading Usable Help
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Gordon R. Meyer
See Figure Boring and Obvious, Above
Most technical documentation is predominately delivered using the written word. That is, on the whole, there's really very little use of photography or illustrations. Screenshots are a possible exception, but when used well they're also relatively few-and-far between. (The overuse of screenshots is the most obvious sign of an amateur's attempt at technical writing.)
But even in professionally-produced instructions, when a graphic is used to support the written description is it truly "worth a thousand words?" Often not, particularly if it just shows a menu command being selected, which can easily be described in a brief sentence. That's a shame because well-chosen graphics can allow users to get the gist of a procedure without having the read the detailed written instructions. One way to facilitate this is by proving meaningful and useful captions for all illustrations, photos, and screenshots.
An unfortunate convention of technical documentation is to caption supporting graphics solely with a reference, such as "Figure 3.1". This convention divulges the orientation of the author -- that the written instructions are more important -- because such a caption is only useful to someone is learning by reading. Instead, try an approach that speaks to the person who is skimming your documentation by providing captions that set context and provide additional information. For a useful discussion about caption writing, which is truly an art unto itself, start with Writing Photo Captions For The Web.
Thanks for the link, InfoDesign.