Nikon D700 viewfinder dark

It sounds as if there is a problem with aperture blades not fully opening.

A quick test...

With a known good lens on the camera, set it to shutter priority mode. Point the camera at a plain wall or ceiling, half press the shutter button and note the aperture reading suggested by the camera. (Play around with the shutter speed settings until you get something around about f11.)

If possible, switch to another good lens and do the same. (You will need to point the camera at the same wall or ceiling!) You should find that without altering any settings, you will get the same, or very similar, aperture reading as before.

Now try the suspect lens. If there is a a fault with the aperture, the camera will almost certainly suggest that a significantly different aperture setting is required.

Hope this helps!