The Flash The Guide Number
As a flash is powerful How far can we cover with an external flash without risking under-exposure of a photograph Obviously answer the second question is almost impossible as are many parameters to consider (we are outdoors or indoors Are there elements that can reflect light).
As regards the first application and at a half-response for the second, there are meeting the flash of producers that specify, on all their products, the guide number (GN).
The latter is no more than the ratio of the flash-subject distance and the diaphragm that you are using smilesmultimedia. The calculation is done assuming to use an ISO 100 and a target (on a full frame) 50mm.
If the internal flash of the Nikon D5000 is the guide number of 12 (then with f 5.6 illuminates a subject to 2 meters), while for a Canon 20D get to 13. If we turn to external flashes, the NG grows definitely the Nikon SB-900 AF has an NG equal to 34 and having a NG = 43 Canon 430-EX arrives. Even third-party flash, such as economic but excellent Yognuo, have the NG values ??of respect for example, the YN467-II TT shows a GN of 33 (costanto up to 15 of the price of an equivalent brand).
Increasing the sensitivity of course increases the number Guide if you maintain constant aperture. Conversely, if you want to store the value of NG it will be necessary to reduce the opening.
For example, if you want to keep NG doubling the ISO (and then rising to 200), you will need to bring the aperture from f 5.6 to f 8. At 400 ISO the same number as we get it driving with f 11 and so on.
If, on the contrary, you want to earn a greater NG increasing the ISO, this is the infeed table to be used
So, if you double the ISO and keep the aperture f 5.6, the internal flash of the Nikon D5000 would increase from 12 to 16.8 and then would gain a round meter in the distance to the subject (so we can photograph a subject place instead of 2 to 3 meters).
There is also another category, although particular niche, and that’s what the ring flash, used in macro photography. Their characteristic is to move the light immediately close to the objective lens, or better around. They have the characteristic of reset the shadows and illuminating part to be mounted in front of the lens.