Guide number
The guide number is the main characteristic that describes the power of the light pulse of the flash. It is described as the maximum distance (in metres) at which, at ISO 100 and f/1 lens speed (aperture 1), a flash is able to illuminate an "average" subject sufficiently for a normal exposure; in other words, at what distance from the flash it will be possible to normally shoot the scene at the specified ISO and aperture.
There are formulas by which, knowing the guide number, you can derive the practical shooting distance for each specific value of sensitivity and aperture. The simplest formula used to calculate the distance at ISO 100 is: S=N/f, where S is the distance, N is the guide number, f is the aperture value. For example, for a guide number of 56 and an f/2.8 lens, this distance would be 56/2.8 = 20 m. Increasing or decreasing sensitivity by a factor of 2 would increase or decrease the specified distance by approximately 1.4 times, respectively. If you need to calculate the distance as accurately as possible, you should refer to more detailed formulas that can be found in specialized sources.
Separately, it is worth noting that the leading numbers of flashes, usually, are indicated by manufacturers for specific focal lengths of lenses. This is due to the fact that the shorter the focal length and, accordingly, the wider the viewing angle, the more light is needed to illuminate the scene being shot and the more powerful the flash pulse should b...e (at the same distance). Therefore, when choosing by the guide number, it makes sense to pay attention to the focal length indicated by the manufacturer and select a model with a power reserve — especially since guide numbers are often prescribed for rather “long-range” lenses (with a focal length of about 80-100 mm in equiv. 35 mm).
Power source
The type of element used to power the flash.
— AA. Powered by replaceable cells of standard AA size, known in everyday life as "finger-type batteries". The main advantage of such a power supply is the ability to quickly replace “dead” batteries with fresh ones: the replacement process takes one or two minutes on the strength (whereas for the built-in battery, charging is often the only option, which takes a long time). AA elements are sold almost everywhere. On the other hand, with such a power supply, you will either have to regularly spend money on disposable batteries, or separately purchase AA batteries and a charger for them; anyway, additional costs cannot be avoided. Another drawback is the dependence of the battery life of the flash on the quality of the batteries: when using cheap cells that are not designed for serious “loads”, the number of pulses per charge can be significantly (several times) lower than stated in the specifications. However, these shortcomings are generally not critical, and this type of food has become quite widespread. Most modern flashes use
2xAA,
4xAA,
6xAA, depending on power.
— AAA. Powered by replaceable cells (batteries or accumulators) of standard AAA size, known under the informal name "little finger" or "mini finger". Such elements are completely similar to the above-described AA and differ only in smaller sizes, wh
...ich makes it possible to reduce the dimensions of the flashes themselves accordingly. However, for a number of reasons, they are used less frequently. Mostly 2xAAA are used.
— Battery. Powered by its own original battery, which is not related to standard sizes, and sometimes also non-removable.
On the one hand, in some aspects, such a power source is much more convenient than replaceable batteries. First, it initially comes in a kit, and the device is ready to work out of the box. Secondly, in the same kit there is usually a charger (or the flash itself connected to the network plays its role). Thus, when using it, you do not need to spend money on buying batteries — everything you need is already included in the kit. In addition, we note that special batteries are often more capacious, more powerful and at the same time more compact than replaceable cells; besides, they are easier to "fit" into the overall design of the flash and reduce the size (although the built-in batteries are different). On the other hand, such power has one key drawback: when the charge is exhausted, the battery will most likely have to be recharged, and this takes time and the availability of an outlet (or other external power source). In the best case, if the battery is removable, you can purchase a spare for it and keep it ready — but this feature is not available in all models.
In light of all this, the main field of application for batteries is "flashlights" for video — sources of on-camera light (see "Type"). It is in such devices that high capacity is of key importance: you have to shine "constantly and a lot", and with a high power of the light source, batteries cannot effectively cope with this task.
— CR123A. Replaceable cylindrical batteries, slightly thicker and noticeably shorter than AA batteries, have a diameter of 17 mm and a length of 34.5 mm. They also differ in operating voltage — 3 V. Some time ago they were quite popular in photographic equipment, but today they are extremely rare.
