Originally posted by Anvh ISO is simply the sensativity of the sensor.
that means when you go from ISO100 to 200 that all the light of the image will increase by a factor of 2, that also includes the amount of flash light in the image.
So your flash power will double in effectiveness so to say in terms of light output.
I understand ISO. What I don`t understand is the comment:
``There's a good reason for using a low one because if the flash is not filling enough and you can't or don't want to alter aperture then MANUALY increasing the iso will increase the flash output.``
Yes ISO 200 indicates twice the sensitivity to light of ISO 100 but changing the ISO to 200 does not increase the light output from the flash, in fact it would require half the light output of the flash at 200 ISO that it took at 100 ISO to get the same result. If you want to get more output from the flash for a given scene for fill flash purposes then increase the flash output not the ISO. Remember you don`t want double the exposure for the entire frame just more `fill`.