FLASH PHOTOGRAPHY

One of the tools for control of the light on a subject is a flash.

Often a photographer will want to take photographs where there is not enough light to get an exposure.  To record an image under low light conditions several options are available.  One is to go to a film with a  higher ASA such as T-max 3200 or to use a very long shutter speed and a tripod to steady the camera. The other is to bring in additional light either from a flood light or from a portable strobe or flash. 

A flash puts out a burst of light at a very high speed, sometimes as short as 1/5000 of a second.  It is very bright close to the strobe and then fades quickly in the distance.  The light comes so quickly that it creates exposure problems for the photographer.  The main problem with a fast blast of light is found in the shutter of a single lens reflex camera.  The focal plane shutter in a SLR is never completely open.  It is made up of two cloth or metal curtains that open at different times in the exposure sequence.  The first curtain opens and begins to slide across the film from left to right for a fraction of a second before the second curtain begins to follow.  If the source of light for a photo is constant as in sun light this moving slit does not effect the exposure, but with a fast burst of light the curtains will only slide over a small portion of the frame creating a partial exposure and leaving part of the frame blank or under exposed.  When the shutter is set to fast speeds like 1000 or 500 the two curtains follow in quick succession creating a small opening that moves across the film.  When the shutter speed slows down the distance between the two curtains gets larger.  Finally at around 1/60 of a second the delay between the two curtains is long enough to leave the entire frame visible for a short time sequence.  IF the strobe fires while this happens it will expose the entire image area.  Any shutter speed under 60 will have the same effect of having the delay in the two curtains long enough to uncover the entire frame for a length of time.  The slower the shutter speed the longer this gap becomes.  This speed of 1/60 or 60 is called the sync speed.  It is shown on most cameras with an X next to it or in a different color.  The shutter must be set at this speed or lower to avoid cutting off the image. 

EXPOSURE CALCULATION is accomplished by calculation.  The amount of light that comes out of a strobe when it fires would be 100% of intensity at 1 foot from the strobe.  At 2 feet it will be 1/4 intensity.  At 3 feet it will be 1/9th intensity and at 4 feet it will be 1/16th.  At 10 feet it will be 1/100th or only 1% of the full intensity.  Very few strobes are bright enough to cast light more than 60 feet which is 1/3600 th of the full intensity. The brightness is reduced inversely proportional to the square of the distance.  This is called the inverse square law by the scientist.

Exposure is thus determined by the distance the light has to travel.  To make the calculation possible we must begin knowing how bright the strobe is at 1 foot.  This number is calibrated by the manufacturer and given a number called the Guide Number or GN.  To calculate exposure we divide the guide number by the distance the strobe is from the subject....the distance the light must travel.  Formula is f stop = GN / D.  Most strobes have a chart on the back that does this calculation for you while others have a small light meter on them that shuts down the strobe when enough light has bounced back off the subject to make a correct exposure.  Modern system cameras with auto exposure and auto everything often read the amount of light actually hitting the film as it comes  into the lens and shut off the strobe light when enough light has come in.  These strobes can get a very fast speed for the exposure, sometimes fast enough to stop a speeding bullet.  In short it is the INTENSITY of the flash that determines exposure and NOT the shutter speed.