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  1. #21
    Steve Smith's Avatar
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    Quote Originally Posted by Tony-S View Post
    Are you saying a 1.5V battery will result in proper metering in the Spotmatics (all in the series)? In my Canon F-1n a 1.5V battery alters the meter readings, such that if I'm shooting IS0 100 film I have to set my ASA dial to 40 to get proper exposures. I thought all these old 1.35 meters needed the Schottky diode modification to meter properly with 1.5V batteries.
    Yes. Any camera with a bridge circuit (such as all the Spotmatics) will show correct exposure with the meter in the centre which is its resting place with no current flowing through it. With no current, the voltage is irrelevant.

    Basically, any camera which shows correct exposure with the meter needle in the middle is insensitive to cell voltage. These cameras will also have the meter needle rest in the centre position with the power off.
    Cameras which require you to set one indicator relative to the meter's needle position are usually dependent on the cell voltage for accuracy. With these cameras, the meter's needle rests at one end of its travel when switched off.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  2. #22
    Hatchetman's Avatar
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    [QUOTE=Tony-S;1335964]
    Quote Originally Posted by Rol_Lei Nut View Post

    Are you saying a 1.5V battery will result in proper metering in the Spotmatics (all in the series)?
    This is correct.

  3. #23
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    OK, I just came across this at Butkus :

    Depending on the circuits in an exposure meter a constant voltage is actually necessary or not. A Pentax Spotmatic, for instance, has a ‘bridge’ measuring network and does not need an exact 1.35 volts. This camera works perfect on a 1.55 volts alkaline or 1.6 volts silver-oxide cell.

    Is this true for ALL versions of the Spotmatic? If so, then all I need is a PX400 form factor 1.5V silver oxide battery?

  4. #24
    Tony-S's Avatar
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    Alright, thanks!

  5. #25
    Steve Smith's Avatar
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    Quote Originally Posted by Tony-S View Post
    Is this true for ALL versions of the Spotmatic? If so, then all I need is a PX400 form factor 1.5V silver oxide battery?
    Yes, all versions. Some people will tell you that it doesn't apply to the F but it does. Most Spotmatics have a standard Wheatstone bridge circuit. The confusion arises because the F has a different circuit where the meter has dual coils working out of phase. Current through one, controlled by the aperture, shutter speed and ISO setting, pushes the meter in one direction. Current through the second coil, controlled by the metering cell, pushes the needle the other way. Correct exposure is shown when the current through both coils is equal so there is zero net movement of the needle. If the supply voltage changes, the current through both coils changes proportionally which does not cause the needle to move to a different position.

    http://pentax-manuals.com/manuals/se...ot_f_meter.pdf


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  6. #26

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    Pulled the trigger on the Km and a 50/1.7 this morning. Free shipping this weekend,so I'm glad I didn't get to it yesterday. It should be in my hot greedy hands tuesday or wed. at the latest.

  7. #27

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    BTW, the free shipping started yesterday . . .

  8. #28

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    Now find yourself a nice Spotmatic to go w/ the KM!

  9. #29

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    It arrived today just before lunch. Seals and mirror bumper look great , leatherette tight and pretty clean for a 37 y.o. camera , few minor rub marks on the bottom of the bottom plate. Put a battery in and the meter responds and seems about right, guessing by sunny 16. The lens is sweet, clean , smooth and no signs of wear on the rubber. Thinking I'll run my last roll of ektar through it to check it out . hopefully there will be a good sunset today or tomorrow , if not there's a little car show nearby on Saturday. I'm very happy today

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