It is normal. It is actually a problem with stopped down metering. When the user stops down the aperture of the lens, the TTL metering light sensor receives less light than when it is wide opened.
In your case, you shot in an already low light situation. After stopping down further, at some point, the light metering cell simply cannot "see" anymore which is called to be "out of the metering range" (for this case, it is out of the lower limit). In case of wide opened metering mode, there is a warning to tell the user that the lowest metering range has been reached and the sensor is unable to measure further, but in stopped down metering, there is no aids and indication to tell the user that, for the current Pentax's design.
Do note the primitive problem of stopped down metering is still the light loss caused by teh aperture cut which cause even more troubles to take the metering readings in low light conditions.
Originally posted by stanjo K100D with SMC-M 50mm/1.7.
I know how to work the manual lens, but noticed a weird problem with stop-down metering in low-light conditions.
in "M" mode, I first tried the DA 18-55 lens metering in a room lit by a 100W bulb. I tested f5.6-22 aperture range and the exposure metering came up with speeds from 0.3sec-6.0secs (f22/6.0 secs)
Now, with the M 50/1.7 lens with stop-down metering, I got the following results:
f5.6 - 0.3 secs
f8.0 - 0.7 secs.
f11 to f22 - all get 0.7 secs when stop-down metered.
So, above f8.0, the metering is wrong.
Have you experienced the same problem?
Regards,
Stan