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The battery is at the front too.
The right (seen from the front) silver button gives access to the battery compartment. Slide this button to the left (again seen from the front) and the battery will be revealed.
I bought one of those of this list and here is the words from the seller - it works just like he describes it:
Set the max aperture of the lens on the dial on the prism's right side. Set the film speed on the dial on the left. On the right dial you also have the ability to compensate the exposure -/+. This is done by setting the dials outer disc to prefered value.
Set the prism to "On" by moving the button on to so that it indicates "On". Point to the subject and read the EV value wich is then manually transfered to the lens.
There is also a button to check the battery on top of the prism. Briefly press the button. If the EV value does not change more that 1/3 the battery is ok.
I believe you are describing the old CdS/Zeiss Ikon prism.
It is similar in some respects, but not the same as the VFC6/PME which, i believe, is the one Marc has.
I don't know where to get a manual, sorry! But you may be able to locate an edition of Ernst Wildi's "Hasselblad Manual". He describes how to operate these viewfinders, plus many more.
Anyway, just like Ruben said, you do indeed have to set both film speed and the maximum aperture of the lens that is on the camera.
You do this using the two levers inside the circular recess on the meter's left side (seen from the rear). The values set are visible in the display on the meter's left rear. The aperture value must appear between the two lines in the "MAX" window.
You meter by pressing the button on the meter's right front (still seen from the rear) for one second. The metered value (in EV) will be displayed by a LED in the viewfinder. If two adjacent LEDs light up, the metered value lies between the two.
The LEDs will extinghuis after 15-20 seconds; there is no "off" switch.
You test the battery by pressing the same "metering button" for one second too. A green light will show in a "BAT" display on the prism's right rear (still seen from behind).
No green light: time to replace the battery.
It may be of interest,Hassi do a acumat bright screen viewing prism
if you have this on your camera you have to modify the reading on your finder other wise you will under expose,if you have not got this type of screen its a great improvement on the standard item.
Makes looking thru the lens a real joy.
Hope this is of interest Dave