Have you ever turned up to present some work on a projector, only to discover that some oddity of the computer and the projector means that you can only present in (say) 16 by 9 aspect ratio, and your slides are in 4 by 3? In the worst case scenario, which has happened to me, your entire presentation is stretched to fill the space, meaning circles become ellipses and fonts look like garbage.

I now recommend that if presenting in an uncontrolled environment that you take along two copies of your slides: 4x3 and 16x9 aspect ratios. This poses a little bit of a problem, though — which do you optimise your presentation for?

There are two broad approaches you could take. The more difficult approach is to re-typeset your presentation with the two different frame sizes. This is quite difficult to do correctly, since any relative or absolute sizing or placement you might do needs to work in both situations. E.g., [width=0.5\textwidth] might work in the 4x3 aspect ratio, but for the 16x9 version you’ll end up with a probably-too-tall image.

The easier approach is to design to 4x3 and then just expand the background to the left and right, and live with the additional empty space. This is the approach I take, especially when short of time. In beamer I’d do this as follows:

  \setbeamercolor{background canvas}{bg=}

Hopefully that’s a clear enough example without further explanation. (I do wish that the empty background canvas were default in beamer; I haven’t looked into whether there’s a good reason why not.)