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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: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Fri, 04 Apr 2008 20:40:19 -0400
I'd forgotten about the light limiting doors.  Somewhere I have a 
largish GE meter handed down from my father-in-law (probably 50's 
vintage) that (I think) has a little metal flap with a window that folds 
over the sensor for the high range.

Chuck Norcutt

Moose wrote:
> 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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