I have a Nikon D780 and a Speedlight SB-400 which I use only very occasionally. It used to work great, but today it essentially stopped working. What's strange is that everything appears normal: the "Ready" light is on, the camera recognises the flash, etc. When I take a picture, the flash goes off, but at such low power that the effect is pretty much like no flash at all. (The subject was at a distance of only about 2m at night with the ISO setting set to 1000 and a 4.5 aperture, so very well within what the flash can handle.)
I'm not sure how to even troubleshoot this since that flash is supposed to pretty much "just work". It has almost no physical UI controls and most setting adjustments are done from the camera menu.
I was seeing a "Zoom head position warning" icon, but when I played with the head, the flash would work occasionally, but only maybe 10% of the time. For example these two pictures:
were taken with the exact same settings and the flash went off both times. Shutter speed was 1/50s.
Any ideas of what I could try?