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OK, so you got your DSLR camera, you make wonderful landscapes or city shots... but you still miss something. Your portraits are not "sound". You need an external flash unit. But, which one?
First thing you have to decide is what kind of pictures you want to do. If you go for a typical case, you can consider indoor portrait photography, which typically has controlled conditions:
- A single subject or small group
- Lowest possible ISO, typically 100, to avoid noise generation
- A mid-range lens (between 80 and 150mm) to reduce distortion; take into account the multiplication factor of your camera body. This focal distance may be limited to a few meters by the available space.
- background (walls?), relatively close to the subject(s)
The guide number
Great, now you know which kind of photos you want. Now, you have to calculate the guide number. This number is quite standard among all flash manufacturers, and it refers to the light output power that the flash can produce. Calculation is (initially) quite simple: you just have to multiply distance to subject by aperture setting.
In our case, we could have around 3 meters to the subject, then f5.6 to blur the background slightly. Result would be a guide number of 16.8. Easy, ain't it? In comparison, my camera (a Canon 500D) has a guide number of 13, so it would not provide enough light power for this photo.
If you take a worse case, a small group, you have to close the aperture (to f8, for example) to get everyone in focus, and to step a bit behind, maybe to 4 meters. Then, needed guide number is 32.
... and the small print
But clearly, this is not all. Captured image will also depend on ISO setting (or synchronization speed, but this is another story) and the viewing angle used (related to focal distance). This is why I sand that the guide number was only "quite" standard...
Even when most flash manufacturers use ISO 100 as a reference for the calculations, this is not the case with focal distance. Flashes with zoom usually present the data at 105mm (so that the light is concentrated), but smaller devices will refer to their maximum focal length - say, 50mm. Illuminated area may be too wide for you, and some light will be lost - surrounding your real capture.
Of course, you may also work with higher ISO setting, at ISO 200, for example. Noise may still be acceptable, and you will get double the light... just as if you would open one aperture step more for your calculation of the guide number.
Any conclusion...?
As a first approach, a flash with a guide number around 30 should be enough for indoor portrait (or a similar power need). If you want more power, you may find units with guide numbers of 43 and 58 (in case of Canon brand...). Price may be another important decision factor, and you may find third party units (Sigma, Metz, Nissin, Yongnuo...) that are also good to begin with.
Later on, you may want to check further topics, like reload times, ETTL adjustment, remote (radio) triggering...
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