Mamiya M645 metering prism - auto selection mode - how??

[tkamiya]

On Mamiya M645 (the later ones) metered prisms, there is a switch to go from centered metering, average metering, and auto-selection. In auto-selection mode, depending on scene, the metering switches from center metering to average metering.

Does anyone know what determines one mode to be selected over the other? I'm pretty sure this isn't a sophisticated system like Nikon's matrix metering or anything like it. But I'd like to understand what makes this system 'tick'.

 

[mwdake]

If I remember correctly the meter takes a spot reading then an average reading and if the spot reading is significantly darker then it uses the spot reading; but if only a little darker then it uses a mix of the 2 readings. If the 2 readings are about equal is uses the average reading.

I think Mike Butkus has the manual for the AE type prism on his site, do a google search for the link and please consider his donation request if you use the manual as he does a great service for us all.

 

[tkamiya]

Got it. Thank you.