In the studio (or anywhere, for that matter), flash-to-subject distance can be altered to affect exposure. The series of images below illustrates how this works. The picture in the center was properly exposed; the others were not.
Six feet away - The middle picture was taken with the flash head six feet away from the model. A flash meter was used to obtain proper exposure. Four feet away - Without changing the camera's settings, the flash was moved two feet closer to the model (i.e. four feet away), resulting in the over-exposed image immediately on the right of the middle image. Three feet away - The grossly over-exposed picture on the far right was taken with the flash three feet away from the model. Eight feet away - The picture on the immediate left of the middle image was shot with the flash eight feet from the model, and exhibits underexposure. Ten feet away - The far left picture was taken with the flash ten feet away, and is the darkest of all.
The picture in the middle is properly exposed. The other images are over-exposed (right) and under-exposed (left) due to changing the flash-to-subject distance.
Studio photographers commonly move their lights closer to or further from the subject to control lighting ratio. When they do so, they check the amount of light striking their subject with a light meter (in the case of flash, with a flash meter), so they won't end up with the kind of over- or under-exposure that is shown above.
THE INVERSE SQUARE LAW
Photographers who are new to studio lighting are sometimes astonished at how a relatively-small change in flash-to-subject distance can result in such a large variation in light intensity.
Light diminishes over distance in accordance with the inverse square law, which states that doubling the flash-to-subject distance reduces the light falling on the subject to one-quarter. There was therefore four times the amount of light striking the model above when the flash was three-feet away than there was when the flash was six feet away. This explains why the far-right image is so over-exposed.
CHANGING FLASH-TO-SUBJECT DISTANCE IS NOT THE ONLY MEANS OF CONTROLLING LIGHT INTENSITY
The operating controls of studio flash units usually permit the photographer to select different power output settings for the flash head, controlling light intensity without having to move the flash unit closer to or further from the subject to increase or decrease its illumination level.
Flash diffusion and bouncing the flash off a reflective material can also reduce intensity without changing the flash-to-subject distance, although technically you are changing flash-to-subject distance when you use bounce flash. You are just not physically moving the flash unit itself. The use of neutral-density gels over the flash head can also result in light reduction.
Everyone is happy when the flash-to-subject distance is correct for proper exposure.
DOES THIS WORK FOR MY CAMERA'S BUILT-IN FLASH?
A camera's built-in flash is automatically controlled to avoid problems of exposure due to distance. A sensor in the camera measures the light bounced back from the subject and instantly turns off the flash when it determines that the right amount of light has reached the subject for proper exposure. So, when you move your camera closer to or further from your subject, the camera compensates for the change in distance and controls the flash accordingly.
This control, however, has a limited range for most cameras, and will over-expose the subject if the flash is too close - generally closer than two or three feet. A typical built-in flash is not powerful enough to illuminate an object that is more than eight to fifteen feet away, depending on the model. The size of a particular camera's flash capacitor, battery and flash tube determine the flash's range. Your camera's manual will provide you with the flash's effective range.
Built-in flash automatically compensates for distance to achieve proper exposure.
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