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How Do I Set The Maximum Lens Aperture On My RB67 PD Metered Chimney Finder?

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I've recently obtained a PD metered chimney finder for my RB67 Pro SD, but can't figure out how to set the maximum aperture   for the lens in use. Oddly enough, I can't find instructions for this anywhere on the net. If anyone has a copy of these instructions I would love to have a look. .

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The PD chimney finder appears to have the same multi-function knob as the PD prism finder, so its likely they work exactly the same.  If thats the case, you should be able to see two small press latches on the periphery of the knob, close to the housing surface.

The smaller latch on the bottom (6 o'clock position) releases the ISO dial so you can adjust film speed: letting go of the catch will lock in the ISO.  The slightly larger catch a bit further up (8 o'clock position) allows you to set the maximum lens aperture on the secondary (smaller) aperture scale (at roughly 10 o'clock position). Once you set the lens max aperture on that scale and release the catch, the meter is ready to take readings.

A meter reading is obtained by pressing the white button to activate power, then turning the main knob section (with shutter speed scale) until you get the center green LED to light in the finder display. The correct available aperture and shutter speed combinations can then be read from the adjoining aperture scale and shutter speed scale visible at the top of the knob as you look down at the chimney. Transfer the matched aperture/shutter combo you prefer to the lens barrel control rings.

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The Mamiya CdS and PD metered chimney and/or prism finders work similarly in concept, the manipulation of nested dial scales is just slightly different.

I think the older CdS operation is a bit simpler, because the knob sets all the scales directly (pull out entire knob to set lens max aperture, pull out just the outer ring to set ISO film speed, knob in normal position turns in toto for readings).

The newer PD prisms have much improved silicon blue meter cells but a more fiddly control setup requiring the two miniscule unmarked scale release catches described earlier. The arrangement of ISO vs Max Lens Aperture vs Metered Lens Aperture/Shutter scales is similar to the CdS layout, the added latches increase settings security but also complicate operation. Mamiya during the PD era was fond of cutesy hide-in-plain-sight control "upgrades": try figuring out how to open the film door of the (wonderful) C220F TLR without reference to the instruction manual.

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