By Jim Kittrell

Fit & Comfort
A well-fitting mask forms an airtight (and, subsequently, watertight) seal on your face. To judge the fit, place the mask on your face without using the straps, gently inhale through your nose, and remove your hands. If it's still on your face, it fits. Make sure it doesn't rub against the bridge of your nose--this tends to be among the most common complaints.

Purging
Even a perfectly fitted mask must occasionally be purged (filling the mask with air to force water out). While many divers are content with the old tilt-the-head-back-and-blow-through-the-nose trick, others prefer a mask with an automatic purging device. The upside to the technology is a clear mask with the touch of a button; the downside is added weight and size--you make the call.

Volume
Low-volume masks--those that leave little room between lens and face--generally provide better peripheral vision and are easy to purge. However, they fill up with water more quickly, necessitating more frequent clearing. High-volume masks can tolerate some leakage, but they can be difficult to purge manually.

Lenses and Skirts
Masks have either one or two front lenses: For a broad, uninterrupted view, choose one lens; for low volume or corrective optics, choose two lenses divided by a nosepiece (if you're not bothered by the bisected view). Optional side windows let in additional light and provide peripheral vision. However, some divers, like photographers, spearfishers and underwater surveyors--folks who tend to focus on a single object--find them distracting. Bottom line: Try a few different variations before plunking down the plastic.

Jim Kittrell is the co-editor of Down Time, a collection of essays about the joys of scuba diving.