Nikon D90 light meter broken or not being exposed?

Question

I have a Nikon D90 that had an unfortunate fall. Since then, if I have it in any mode except manual it throws up an Err.

If I use it in manual and follow its exposure guide in the view finder it vastly over exposes (white). However, if I ignore the exposure (let it say the exposure is way too dark), best guess then adjust based on the preview for that pic, it works.

So it seems the either: a) the exposure sensor is broken; b) that the sensor isn't actually being exposed to light through the lens and is only, eventually, being exposed to some incidental light through the view finder - hence the white out; or c) something in the electronics that interprets the sensor.

Given the autofocus still works (in manual) and I can see through the viewfinder just fine I assume the mirror, focusing screen, and autofocus sensor are all functional. But the fact that it does eventually read some light suggests it is still 'on'/responding/plugged in, perhaps?

Can anyone think of anything else I could try before I try before attempting to get into the prism? :s

Answer 1

It could be just about anything.

Your Nikon D90 has a 420 pixel RGB light meter that is essentially a second, very low resolution image sensor. The chances are pretty high that the connection between the light meter and the camera's main processing unit has been damaged. It could be something as simple as a ribbon cable coming unplugged, or as devastating as a crack in your camera's main PCB.

When you are in Manual Exposure Mode and the camera seems to be getting a very weak light meter reading, it could be noise generated by the electronics at some point in the connection path between the light meter and the camera's CPU. Just as a high resolution CMOS sensor and its associated electronics can generate 'dark current' that manifests itself as read noise, the much lower resolution CMOS metering sensor and its associated electronics can as well. If the break in communication is between the light meter and the electrical components that amplify the signals from it, then the noise generated by the amplifier might be what your camera's CPU is interpreting as a very weak signal from the light meter.

As this answer suggests, it could also be a communication error between the lens and the camera. Such an error may or may not be associated with physical damage at the interface between the camera and lens. It could also be occurring anywhere in the electronic path between the lens contacts and the camera's CPU.

Answer 2

Sounds to me like you may have a problem with the electronic aperture linkage between the lens and the mount - maybe one of the pins isn't making proper contact.

If you're getting an error in every meter mode except manual, that suggests that the camera doesn't know what the aperture's supposed to be, which suggests the lens isn't managing to tell the camera what the maximum aperture is. That may also explain why your manual metering is way off - if it's assuming that it's wide open setting is at f32 - or whatever its smallest possible number is as a default, - then that would explain why it overexposes.

Since the camera doesn't particularly care what the brightness currently is for getting a metering mode to work (as opposed to giving an error) that suggests its probably not either of your ABC options - the camera has no way of knowing whether the light value it's reading is correct, so there's no reason why it should give an error. On the other hand, if it's not getting a maximum aperture value back - and it knows that it isn't - then it should give an error for any of the metering modes where it needs to know what that is (basically, most of them except manual)

Might be worth checking that the lens mount contacts are working OK. (and may be worth trying a different lens to see if it's a mount/internal or lens problem).