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I like the Lezyne Femto Duo: it's a simple design with a range of blinky and constant settings that you fit to your helmet to use as a secondary light for both front and rear. It's mostly good news, but the Femto has historically had issues with waterproofing due to its combined lens/switch design and it continues to be an issue here. Unless you're planning to avoid wet weather riding you'll need to be vigilant to dry them out if they get wet.
Effectively this is just two single Femto lights with the backs removed, screwed onto a single helmet mount. Simple enough, and it works. You stick it on your lid by feeding a Velcro strap through the vents, so if you're using an aero/pisspot helmet you might be out of luck unless you stick the mount on with VHB tape or something similar. Anyway, if you have holes, it's easy. The width of the mount keeps it pretty stable but it does mean you can't fit it to the underside of your drop bar and use it as a running light there, which would be a useful second string to its bow.
Using the lights is simplicity itself: hold down the lens to turn them on, click the lens to change modes, hold down to turn off. The two lights work independently so you can have any combination of the four flash modes and static beam. Lezyne claims a 60-hour runtime on two 2032 button cells, so if you're commuting half an hour a day you'll probably make it through the winter without having to replace them, and they're cheap as chips when you do.
You can't legally use this as your only light but it's more of an extra bit of visibility at a different height, to complement your main light. The fact that it's higher up means it's visible over stuff, and through stuff, that your main light might not be. So it's a helpful extra visibility aid after dark. And it's so light that you'll not even notice it on your helmet.
There's a thorn in the Femto Duo's side, though, and that's waterproofing. The lens has to move against the body to work as a button and water, it seems, can creep in there. After five minutes with me in the shower (and it's not even a very good shower) there was water in both front and rear compartments. Not much, but it will corrode the electrics and short the batteries if you leave it in there.
I dried my Femto Duo out on the radiator and it suffered no ill effects at all. If you're fastidious about doing that each time it gets wet then you'll probably have no issues, but really you want the Femto to be a fit-and-forget light, not one you have to remember and care for.
Good extra visibility aid and easy to use, but waterproofing issues mark it down
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road.cc test report
Make and model: Lezyne Femto Duo
Size tested: n/a
Tell us what the light set is for, and who it's aimed at. What do the manufacturers say about it? How does that compare to your own feelings about it?
Lezyne says: The Femto Duo is a two-in-one safety light integrated into a versatile helmet mount. Evolved from our popular Femto series, the unit features a front and rear light coupled by a composite matrix junction that's compatible with nearly all vented-style helmets. The LEDs are housed in a super compact, lightweight and durable machined aluminum body, and the high-grade polycarbonate lenses provide side visibility and double as the power button. The front emits 15 lumens, while the rear puts out 7 lumens; both feature three different output modes. With easy to replace CR2032 batteries, the Femto Duo is a simple front and rear lighting system for increased nighttime visibility.
Tell us some more about the technical aspects of the light set?
MAX LUMENS: 15
BATTERY: CR2032 (4 included)
Nicely made, the Femto feels more solid than in previous incarnations.
Super simple to use, the only issue being it's easy to accidentally turn them on in a bag. Most of the time they're on your hat though.
Simple Velcro strap, easy to fit to most helmets. A pity the clamp design doesn't also allow you to fit to the bottom of your drop bars.
Five minutes in the shower and there was water inside both the front and rear parts of the light. It creeps in around the edge of the lens. Not much but enough to be an issue.
Put them on flashing and they'll last most of the winter.
They're not going to be any good for off-roading in the dark but that's not the point.
Water ingress issues will mean a fairly short life if you're not careful.
Very light, you wouldn't know it was there.
Not expensive for what you get.
Tell us how the lights performed overall when used for their designed purpose
Pretty well, some extra visibility at a useful height.
Tell us what you particularly liked about the lights
Simple, light, good battery life.
Tell us what you particularly disliked about the lights
Waterproofing needs improvement.
Did you enjoy using the lights? Yes
Would you consider buying the lights? I think the waterproofing issues would put me off.
Would you recommend the lights to a friend? Probably not unless they're a fair-weather cyclist.
Use this box to explain your score
Performance is good and they're not expensive, but the issues with water ingress mean they're not ideal for UK winter conditions.
About the tester
I usually ride: whatever I'm testing... My best bike is: Kinesis Tripster ATR, Kinesis Aithein
I've been riding for: Over 20 years I ride: Every day I would class myself as: Experienced
I regularly do the following types of riding: road racing, time trialling, cyclo-cross, commuting, club rides, sportives, general fitness riding, fixed/singlespeed, mountain biking, Mountain Bike Bog Snorkelling, track
Dave is a founding father of road.cc, having previously worked on Cycling Plus and What Mountain Bike magazines back in the day. He also writes about e-bikes for our sister publication ebiketips. He's won three mountain bike bog snorkelling World Championships, and races at the back of the third cats.