I upgraded to D90 from D3100, on the D3100, I could switch to manual from TTL when using a Macro Filter to adjust the power of the flash, but there's no such setting on the D90. I don't have a flash gun and can the lowest shutter speed I can hold the camera at is 1/25.
См. статью: Nikon D90 - How do you turn on Manual Flash Settings? (Built In Flash)? - 1