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[OM] Re: OM 2N vs OM4T(i) - Relative reliability/repairability

Subject: [OM] Re: OM 2N vs OM4T(i) - Relative reliability/repairability
From: Moose <olymoose@xxxxxxxxx>
Date: Fri, 04 Apr 2008 15:26:24 -0700
Leandro DUTRA wrote:
> 2008/4/4, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>:
>   
>> Selenium is photo-voltaic meaning it generates a voltage on its own with 
>> enough power to drive a meter without a battery.  However, the output power 
>> is proportional to the size of the piece of selenium and it requires a 
>> pretty large chunk to move the needle on an exposure meter.
>>     
> Thus the translucent circle around Pen EE lenses?
>   
Yup.

As Chuck says, it takes a fair amount of surface area to get a useful 
voltage out of selenium cells. We tend to forget with modern meters that 
the old ones were not as sensitive as contemporary meters. The Weston 
Masters have about a 4cm diameter cell. My Master V will theoretically 
read down to about EV 3, although the meter movement is so small there 
that I think EV 4 is a more practical limit. Newer meters commonly read 
accurately below EV 0.

My old GE PR1 & 2 meters have rectangular cells that are harder to 
measure, but smaller, I think, in area, so they are a bit less sensitive 
than the Westons. The PR-2, though, had an add-on attachment I've never 
seen that would bring it down to work in very low light.

The other awkward thing about selenium is that the cells have a limited 
dynamic range. So they need lots of area for low light and a mechanical 
way to cut down the light reaching them in bright light. The Westons 
have a little door with holes in it that closes over the cell manually 
for bright light. The GEs have an internal flap that flips over the cell 
internally when the dial passes a certain point.

I have one Weston Master (IV? Euro?) that's given up the ghost, but the 
two GEs and the Master V are still fine.

Moose


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