Flash shooting distance range
From The Glossary of Digital Photography
The distance from the flash that a correctly exposed image can be created when the flash is firing at its minimum and maximum flash power and duration. The range will vary based on lens aperture and ISO setting on the camera. If a subject is farther from the flash than the maximum range, the image will be underexposed. The aperture can be opened up, the subject can be brought closer, or the ISO setting can be increased to fix this problem. If the subject is closer than the minimum flash shooting distance, then the aperture can be closed down, the subject can be moved away, the flash can be bounced off of a white card, the ISO setting can be reduced, or the flash output can be reduced with a filter over the flash or even by placing a white handkerchief or white cloth napkin over it.
