From reading more about the OTR i3 at BLF, it sounds like the light isn't even rainproof.
The B158 claims IPX8, which is a little remarkable for a zoom. It is probably not at all waterproof if the focus mechanism is used under water. But Brinyte largely makes diving lamps, so I'll take their IPX8 claim as entirely possible.
It's not hard to make a zoomie waterproof. Most cheap zoomies even have the o-ring slots to do so already built-in. Lube up the stock o-rings and add an o-ring or two into the missing slots and even a cheap zoomie becomes airtight.
So why don't manufacturers include the missing o-rings to make their lights waterproof? Why does the i3 use split gaskets instead of sealed gaskets or o-rings? These lights are actually intentionally not airtight. There is a simple reason for this.
How zoomies operate
The typical zoomie operates by having a lens mounted on a movable bezel. When the bezel is retracted, the lens is close to the LED giving flood mode. When the lens is extended, the LED sits on the focal point of the lens giving spot mode. Extending or retracting the bezel changes the internal volume of the light.
Unfortunately, changing the internal volume of an airtight light causes the air inside the light to change pressure. As air pressure inside and outside the light tries to equalize, air pressure will cause the bezel to automatically return to whatever position it was in when the light was sealed (when the battery was inserted).
If you put the battery in when the bezel was in flood mode, and then try to slide the bezel to spot mode, the bezel will move on its own back to flood mode.
The fix for this problem is simple:
The light must have a vent so air pressure inside and outside can equalize. This is necessary to allow the zoom mechanism to operate smoothly in all modes. This vent need not be large. An absolutely tiny opening will work.... but it does need to be there. Most cheap zoomies accomplish this by omitting one of the o-rings. The On The Road i3 does this by having 2 split gaskets instead of fully circular o-rings around the lens.
The most convenient and most common style of zoomie is simple "push-pull" operation, where the bezel extends and retracts by pushing or pulling. Easy to use one-handed and you can go from flood to spot in less than half a second. I have never seen a push-pull zoomie that was watertight. The zoom mechanism in such lights simply isn't stiff enough to resist air pressure moving the bezel.
Another type is the "twist-zoom". These are built just like push-pull zooms, but have threads between the bezel and body. Extending the zoom requires turning the head. This type of zoom mechanism can be made waterproof as the threads may provide enough resistance to prevent air pressure from moving the bezel. The downside is this mechanism is far less convenient than a push-pull. To be stiff enough to resist air pressure, these bezels often require two hands and multiple turns to operate.
Personally, I much prefer the push-pull type zoom mechanism for its ease of use, convenience, and effectiveness. Even with a vent I consider them generally good enough for rain or light splashing. If I need anything more waterproof I'll reach for a non-zoom light.
