bouncing the flash

[panocho]

Is there any way to -approximately!- calculate how to adjust a correct aperture when you bounce the flash in a fully manual camera?

With the newer models you don't need so, since the camera reads the amount of light that reaches the subject and stops the flash when that's enough. But with older, fully manual cameras, you only have what the guide number of the flash tells you, but that will only be accurate when you shot the flash directly to the subject. What happens when you bounce it? Any "rule" to more or less compensate the loss of light?

 

[Hertz van Rental]

Calculating the correct exposure is tricky. It's best if you have a flash meter.
Failing that, the following calculation will give you the approximate exposure - but it is only approximate, and is for white surfaces.

f-number = 0.7 x (flash guide no. / d1 + d2)

that's 0.7 times the flash guide number divided by d1 plus d2.

Where d1 is the distance from the flash to the surface it is being bounced off. And d2 is the distance from the surface that the flash is being bounced off to the subject.

Calculating the aperture under normal circumstances, you divide the guide number by the flash to subject distance. For bouncing the flash you are doing the same (d1+d2 is just the total distance the light is travelling, that is the flash to subject distance allowing for the bounce). 0.7 is adjusting this to allow for light lost in bouncing.

For off-white or light coloured surfaces increase the exposure by 1/2 or 1 stop.
That's the nearest you will get I'm afraid, so it will be a bit of trial and error from here on in.

 

[panocho]

Thanks, Hertz!
That's exactly what I needed!