Speedlight Question

[ianbarber]

Although I am not a big user of flash, I have a situation that always puzzles me.

When I put my Flash (Nikon SB-900) onto my camera (Nikon D700), the exposure meter in the camera always shows an under exposed setting.

I was told this is normal and not to worry about it but i was wondering if anyone could explain why it does this.

 

[HoppyUK]

The meter is probably reading the ambient light.

Flash works differently. Instead of the camera matching its settings to the light, you make the settings and the flash light output is moderated to match those.

It does this by means of a pre-flash that goes out immediately after you press the release. This is measured and used to set the power for the main flash for when the shutter opens. The whole operation happens so fast you cannot distinguish the two flashes by eye.