To expand on what Lowell said...
There is a limit to meter sensitivity
The meter in your camera has a lower limit of EV 1 (presumably @ ISO 200). That means the amount of light striking the meter cell for 1 second at f/1.0. This is different than the ISO sensitivity of the sensor. Consider the following case:
- Available light to the lens EV (200) = 1
- Selected aperture = f/5.6
While this combination works fine for open aperture metering with an "A" series or higher lens, there are problems using green button/stop-down metering. When stopped down to the selected aperture for reading, the amount of light
at the meter is only EV(200)
-4 (1/10,000th the light it requires)! The meter is not linear at this light level, if indeed, it responds at all.
For shooting in dim light using non-"A" contact lenses, using M mode with a sensitive hand-held meter might be the best solution.
Steve