I was not changing the setting on the SB-600, I was changing the exposure setting of the camera and the SB-600 was responding to these changes.

You are right, I should have mentioned that the changes are seen in the DOF between the f-stops and not the exposure.

I think you’re confusing yourself here.

This isn’t really a test of flash settings as much as a test of flash TTL metering.

The flash is adjusting it’s output in TTL (or whatever Nikon’s auto mode is – I’m a Canon shooter!) to expose the scene.

Taking these one by one.

1/60th f2.8 is a little underexposed – this is confusing the issue because it should be the same illumination as f5.6 and f9

When dealing with flash the aperture only controls the DoF – the flash takes care of the exposure by adding or cutting off illumination to match your settings – that’s why f5.6 and f9 look identical *exposure*. You’ve got even more DoF with f13 of course. But an identical exposure.

By f18 the flashgun is running out of power to effectively expose that tiny aperture so it can’t keep up – hence it underexposes. Moreso by f22 where what you are seeing is almost all ambient (at ISO400 1/60th f22 indoors that *will* be dark).

Put the flash on full manual power and redo the experiment. Keep shutter at 1/60th. You’ll see how it overexposes at f2.8 and hits a sweet spot where its correct then underexposes by f22.

PP

Great learning series here.