Of all the responses, I think Brendan put his finger on it when he pointed out that with screwmount lenses, the camera switches to center-weighted metering, which is less accurate than, or at least different from, multi-segment metering.
When shooting contrasty scenes, magazine testers (the one in Shutterbug comes to mind) have mentioned that Pentax multi-segment metering tends to overexpose a bit (1/2 stop?), in order to capture shadow detail. Other than that, I found the multi-segment metering in the MZ-5n to be very accurate almost every single time. The center-weighted metering of the MX (my previous camera) could produce good results, but required more practice and interpretation. Pat White
