Boost (or Zoom) is found on just a few models. This feature permits an extra-intense beam to be projected for a brief period, maybe 10-20 seconds—nice to have when you're really curious about what's causing that rustling sound in those nearby bushes. Just realize this mode exerts a high drain on batteries.
Additional Headlamp Features
Rather than gradually dimming as batteries drain, regulated headlamps offer a steady brightness level throughout the life of the batteries. This is a positive—and deservedly popular—feature.
The downside: When batteries are exhausted, the light of a regulated headlamp can go dark abruptly. This may leave you scrambling to replace batteries in the dark. A dimming light on an unregulated headlamp gives you early warning that batteries are nearing the end of their usefulness.
Red light mode
Many headlamps offer a red-light mode. Red light does not cause our pupils to shrink the way white light can, so it's good for nighttime use.
All headlamps sold at REI are able to withstand some degree of exposure to rain and snow. (They can handle modest drops and jolts, too.) A few can tolerate shallow, short-term immersion.
The ability to adjust the headlamp unit up and down is a nice option. It lets you position the beam exactly where you want it. This is especially handy when reading by headlamp.
On / off switches
If you're examining headlamps at a store, try out the buttons to see if you like how the headlamp cycles through its modes (high to low, or vice-versa). Also, some switches lock to prevent the headlamp from being inadvertently switched on inside a pack.
Headlamps designed to work with lithium batteries are a good choice for cold-weather usage, since lithium batteries outperform alkaline batteries in cold conditions.
Rechargeable nickel metal hydride (NiMH) batteries also work well with headlamps and perform well in cold conditions.
Note: Rechargeable batteries tend to lose power when sitting idle, so it's smart to carry alkalines (excellent at holding their charge) as backups.
External battery packs, top straps
Some high-power headlamps that use 4 batteries position the battery pack on the rear of the headband and run a small cable from the pack to the headlamp. It lightens the load on your forehead but can feel clunky. Top straps (sometimes removable) are offered on some models to add stability.