Will 1.4 volts be close enough?

[leicarfcam]

All meters read a little differently. There are differences in how they average a scene, how much weight is given to the centre, etc. And that's without taking battery voltage into consideration.

It's called angle of view. When you change it, you get a slightly different reading. If you have a meter that has an accy attachment you can see it..

If you find your meter underexposes a bit for a certain film at box speed, compensate on the ISO dial when you load the film. Easy. Fast. No big deal, it's been done since meters first appeared. A strict time schedule has nothing to do with compensating for your meter, surely? I think we may be talking at cross purposes! ;-)

If I find my meter is off over or under I calibrate it. I've never had a meter that reads fine at say 100asa but is off at say 400asa.

Have you ever been on an assignment with a very strict time schedule? If so you don't have time to stop and make adjustments because your meter is off. You check everything before the shoot..

 

[Xmas]

It's called angle of view. When you change it, you get a slightly different reading. If you have a meter that has an accy attachment you can see it..



If I find my meter is off over or under I calibrate it. I've never had a meter that reads fine at say 100asa but is off at say 400asa.

Have you ever been on an assignment with a very strict time schedule? If so you don't have time to stop and make adjustments because your meter is off. You check everything before the shoot..

If the voltage of the cell is not close to Mercury then the meter will only be accurate at one light level an off in one direction bright and the other direction dull.

You need to test at calibration extremes.

 

[leicarfcam]

If the voltage of the cell is not close to Mercury then the meter will only be accurate at one light level an off in one direction bright and the other direction dull.

You need to test at calibration extremes.

All batteries have a nominal voltage slightly higher than the voltage required of the equipment but drops to the specified voltage once a load is placed on it. The mercury battery had a nominal voltage of 1.4v just like the zinc-air battery. As has already been stated the only difference is in the lifespan of the zinc-air as opposed to the mercury. That is the reason I prefer using a diode in line and using silver oxide batteries. I get the same voltage curve (as the mercury cell) and the long life.. This also translates to a more accurate reading when the battery is near it final stage. The zinc-air tends to be a bit unstable at that point whereas the silver oxide is rock solid stable right up to the point of battery death.

Something to remember is that battery death does not mean the battery is depleted of any voltage. Battery death is when the needed voltage drops below what is needed to drive the meter...

I do agree tests are in fact done at calibration extremes.. Old batteries will show their weakness at the extreme load end.
Most camera meters have a current load of approx. 50µA-200µA so there isn't a lot of difference. Hand held meters usually higher.. In case you are wondering µA is micro-amps..

 

[Xmas]

All batteries have a nominal voltage slightly higher than the voltage required of the equipment but drops to the specified voltage once a load is placed on it. The mercury battery had a nominal voltage of 1.4v just like the zinc-air battery. As has already been stated the only difference is in the lifespan of the zinc-air as opposed to the mercury. That is the reason I prefer using a diode in line and using silver oxide batteries. I get the same voltage curve (as the mercury cell) and the long life.. This also translates to a more accurate reading when the battery is near it final stage. The zinc-air tends to be a bit unstable at that point whereas the silver oxide is rock solid stable right up to the point of battery death.

Something to remember is that battery death does not mean the battery is depleted of any voltage. Battery death is when the needed voltage drops below what is needed to drive the meter...

I do agree tests are in fact done at calibration extremes.. Old batteries will show their weakness at the extreme load end.
Most camera meters have a current load of approx. 50µA-200µA so there isn't a lot of difference. Hand held meters usually higher.. In case you are wondering µA is micro-amps..

Measured my kit to hand

OM1n dull light /1.8 lens 15.0 micro amps
V625PX (Hg off load) 1.5v - it is an old cell

The meter current would be below nudging my meter with a lens cap on but a lot higher in bright sun the CdS cells vary their resistance radically note I did the current test with a fresh alkaline to avoid any internal resistance problems.

I don't actually use camera meters so don't have zinc batteries to hand.

I did do a calibration test for the meter with zinc cells and from dull to bright the OM1n was better than 1/2 a stop with an OM4, the CdS resistors are old and do drift.

Old batteries can develop internal resistance which alters the meters effective calibration especially in bright light.

Selecting a diode is dependent on current taken and you need to solder it in or make an adapter.