// Javascript copyright Sean T. McHugh
// Cambridge in Colour Photography
// http://www.cambridgeincolour.com

// x,x0 == new,original light from flash
// y,y0 == new,original light from ambient
// y = y0 * 2^EC , x = x0 * 2^EC * 2^FEC
// FR,FR0 == new,original flash ratio = x/y,x0/y0
// need x+y=x0+y0 to maintain the same exposure
//
// FR/FR0 = 2^FEC --> FEC = LOG2[FR/FR0]
// x0*2^FEC+y0=2^-EC --> EC = -LOG2[y0+x0*(FR/FR0)] --> EC = LOG2[(1+FR0)/(1+FR)]

<!-- Begin Flash Ratio Calculator
function fratio(fratioform) {

fr1 = fratioform.fr1.value*1; // original flash ratio
fr2 = fratioform.fr2.value*1; // new (desired) flash ratio

reqfec = Math.log( fr2/fr1 ) / Math.log(2);
reqec = Math.log( (fr1+1) / (fr2+1) ) / Math.log(2);

fratioform.reqfec.value = Math.round(100 * reqfec) / 100;
fratioform.reqec.value = Math.round(100 * reqec) / 100;

}
//  End Flash Ratio Calculator -->