Originally posted by westmill I have had the exact same problem on the odd occasion.
With me its normaly in the dark interior of a church.
I just put it down to the camera reaching its low light limits. I also, like you, have and use nikon. I use the D300. I also know the Nikon would not have struggled in the same conditions. When faced with this, the only way around it, that Ive found so far is to use flash with the red eye function set. Its a long burn which allows the camera to see and focus clearly. Exposures are also superb. Im reading this thread too, for the same reason as you. Other than maybe a faster lens, which would reasonbly allow more light through for the sensors to work, the preflash option seems to be the only solution so far
I thought that the claim about using red-eye pre-pre-flash to obtain low light focus was interesting enough to test once it got dark enough tonight.
I setup my K5 on a tripod with the AF540 mounted and then taped over all AF assist lights on the flash and body. I measured the ev level with my Sekonic hand-held meter and it was -.6 ev. The lens was the DA 16-50 f2.8, so this light level is well under the low limit for the camera/lens combo and it would not lock on the subject (as expected).
Of course, since it would not lock focus and the menu setting was set to focus priority, you cannot even trip the shutter and therefore the flash will not do anything, much less provide focus assist via the red-eye setting.
I changed the focus priority to release, meaning the camera will fire even if focus lock is not achieved, and then set the flash for pre-pre-flash red-eye and took an image.
Result: subject completely out of focus.
I then changed to AF-C, and set the camera to release priority and took another image.
Result: subject completely out of focus.
Unless I am missing something, the red-eye pre-pre-flash has no effect on achieving proper focus when the light level is below the lower limit specified for the camera/lens combination (+2ev in this case).
Ray