I purchased an older 50mm prime lens to use on my Nikon D3100. I know it will not meter or focus (and I can handle manual focus and exposure etc.) but I cannot get it to fire the on-camera flash. Any ideas?
I use a Nikon D3000 which should be similar.
In the camera menu (shooting menu, I think) you can control the power of the flash manually. Change the flash to manual rather than TTL, then set the power you would like (e.g. 1/2, 1/4, etc.).
It's slow, and I can't think of a quicker way for the on-camera flash, but it will at least allow you to use it.
Please let me know if that works. Good luck.