Final ISO when using Flash

Teak wrote:

Hello;

I hope this is the right forum for my question.

Is there any way to determine the final ISO the camera will use when using onboard or hot shoe flash?

Last night while taking some pictures with D600 / 50 1.8g and SB400 indoor under flourescent lights (CFL) using manual exposure, the viewfinder showed ISO1400. The picture as taken (with bounced flash off ceiling) had an ISO of 400.

Would be nice if one knows of the ISO "headroom" so can change the shutter speed or aperture for better exposure.

Thanks.