It used to be if you were shopping for a flashlight, you could buy something cheap and disposable, or a Maglite in any of several sizes. If your needs were especially demanding, emergency service work, for example, you might have even considered something that ran on expensive camera batteries and burned out its halogen bulbs every few hours.
Today's flashlights are almost as different from any of those options as modern smartphones are from early car phones. Some can provide usable illumination on objects a kilometer away. Many fit comfortably in a pocket and have the output of a car headlight - 1000 lumens. Some can run on low for weeks on a single battery. There are even keychain lights with the output, though not the focus of the old 3D krypton Maglite. The options are overwhelming.
The first step to narrowing those options is to clearly define the intended use case for this particular flashlight, then decide what features that use case does and does not require. An everyday carry light must be small and comfortable enough to carry every day. A police duty light must be bright, reliable and have a predictable user interface. A professional's work light must have adequate performance, durability and reasonable operating costs. In all cases, the user interface should be to the user's liking, which is guaranteed to vary from user to user.
Battery type will be the next major consideration for most people. A common expectation is that flashlights will run on disposable alkaline batteries like they have for decades. This is a slightly odd historical artifact; few other electronic devices with high power requirements and expected runtimes of several hours are expected to run on low-powered disposable batteries. Alkaline batteries, even large ones don't cope well with high loads; three alkaline D cells can power a 1000 lumen flashlight, but actually have worse runtime than three NiMH AAs under the same load. Many support alkalines for convenience, but their performance is usually much improved when using quality NiMH rechargeables such as Eneloops, or for long-term storage, disposable lithium batteries such as the Energizer L91 lithium AA.
Smartphones, laptops, power tools and electric cars run on rechargeable lithium-ion batteries, and so can flashlights. The sizes for these batteries are standardized, and the most common is the 18650 (18x65mm). The 18650 powers Tesla cars, USB power banks, laptops and many flashlights. Its use for highly competitive applications has led to manufacturers applying their very best technology, and its energy storage for a given weight and volume significantly exceeds all other battery types commonly used in flashlights. A single 18650, with about twice the volume of an AA significantly exceeds the performance of the three D cells in the previous paragraph and has one-ninth the volume. Most 18650-powered flashlights can also run on disposable CR123A batteries. There are also lithium-ion equivalents to AAA, AA and CR123A batteries: the 10440, 14500 and 16340. Lights that support these usually have higher output and shorter runtime with the lithium-ion battery. Lights that do not support them may be damaged by their higher voltage. As buying rechargeable batteries and chargers represents an increased barrier to entry, lights with an included battery and charging via Micro-USB are increasingly common. Most phone and tablet chargers can be used to charge these.
A feature commonly found on older flashlights that's rarely seen on modern ones is adjustable focus. It comes in two flavors: a traditional reflector that moves relative to the light source, and a light-bending aspheric lens similar to that of a theatre spotlight. Movable reflectors don't work well with LEDs. Maglite still uses them, but they're rare elsewhere. Aspheric lenses usually produce a perfectly even spot, which most people find less useful than the bright hotspot and less intense spill of a traditional reflector. They're also not very efficient, absorbing up to 40% of the light. Both types reduce durability and seriously hamper waterproofing. Most people won't miss this with a light that puts out 1000 lumens and throws 200 meters, though it's commonly found on cheap lights sold on eBay and Amazon that lie about putting out 1000 lumens.
Zoomie Full Flood
Zoomie Full Throw
Olight M20SX Javelot beamshot
There's still a tradeoff to be made between wide flood and long-distance throw. Both flood and throw depend on the total light output in lumens, but the real measure of throw is beam intensity in candela. This intensity, not the total output is also what's blinding up close. Most smaller lights tend toward flood, but the more powerful of those achieve plenty of throw for most situations. For long-distance outdoor use, a larger reflector producing a smaller, more intense beam will produce the best results, though it will be of less use up close.
There are a huge variety of options on the market for different use cases, but the default flashlight for the average person in 2015 should run on 18650s, charge via USB and put out around 1000 lumens on the highest mode. Good options include the Olight S30R, the Nitecore MH20 and the Fenix UC35. Don't mind using an external charger? There are a huge variety of options. Want something small instead? Perhaps you'd like to light up your entire camp site or you really need to see something half a mile away. There's a flashlight for virtually any illumination task.
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