They are so convenient and easy to use that you are usually unaware when they fire. Let's take a look at these flash units and some alternatives that give even better results. A flash's power determines what its useful range is. Subjects within that range can be correctly exposed. Those at greater distances will be too dark.
Manufacturers sometimes specify a flash's power as a guide number.
The higher the guide number, the greater the flash's useful range. Because guide numbers change when the sensor speed changes or lens focal lengths change, most manufacturers will instead tell you their flash's maximum range. When they state this range, they assume the flash is fully charged and the lens aperture is wide open. On fully automatic cameras, the flash fires automatically whenever the light is too dim to take a photo by natural light. On more sophisticated cameras, there are various flash modes you can choose from for more creative effects.