Nikon SB-400 fires but gives almost no light on a D780 — how can I troubleshoot it?

Question

My Nikon SB-400 used to work normally on my D780, but now it behaves oddly. The flash appears ready, the camera recognizes it, and it does fire when I take a photo, yet the output is so weak that it barely affects the exposure. This happens even in an easy situation for flash use: subject about 2 m away at night, ISO 1000, f/4.5, 1/50 s.

I also noticed a zoom/head-position warning icon. If I move the flash head, it will occasionally work properly, but only intermittently. Since the SB-400 has very few controls, I’m not sure how to isolate whether the problem is the flash, the hot shoe connection, or something else.

What checks should I do to narrow down the cause?

user27440 · Answer

Couple of things you can try:

Try another flash on the hotshoe.

It could be the flash, it could be the camera hotshoe. If you have another speedlight available to test with, putting that on the camera and seeing if it works properly might help you narrow the issue down to the camera or the flash.

Check that the flash is properly seated in the hotshoe.

This actually may not be as easy as it sounds. :) The D750 introduced a weird issue that sometimes a flash can sit too far forward in a Nikon hotshoe for all the pins on the foot of the flash to touch their respective electronic contacts on the hotshoe, so you only get partial communication and some weird behavior. Some folks built spacers for their hotshoes, but a simpler method to try would be to put the flash on the hotshoe, partially tighten the foot, and then slide it back until the locking pin engages (clicks into place).

Check that the flash has recycled.

It takes a while for a flash's capacitor to draw enough charge from the batteries to be ready to fire. Some folks assume they can burst shoot with flash or forget to check the ready light. If it's taking more than four seconds (the SB-400 is speced for a full power recycle at 4s for alkaline batteries, 2.5s for NiMh batteries), it may be the batteries.

Try new/different batteries in the flash.

Batteries deplete and the older rechargeable batteries get, the less charge they may hold. Having a specialized charger that can test capacity on a battery may be worth looking into. But failing that, just getting some brand new alkalines to try in the flash can also help eliminate if it's a battery issue.

If none of this is working, it's possible the flash has some kind of internal damage that may require servicing or replacement.