A brief note on underexposure

So you’ve just gotten back a roll of color negative film that is muddy, grainy, and dark. You did everything right, or you think you did.

Although camera shutters never speed up with age, the photocells in older cameras age badly. Old cameras and handheld meters, pre-1980, often have meter cell decay. Cadmium-sulphide (CdS) photocells pass more current when more light is hitting them. And when there is no light hitting them, they are essentially “off” switches. Unfortunately, over time, the material breaks down and loses its resistance to electricity. This may manifest first as battery drains – and eventually will manifest itself as underexposed pictures. This really hit Polaroid auto pack cameras hardest, but you will see it in anything with a CdS cell, from old Luna-Pro meters to 35mm SLRs. Some lower-end point-and-shoots have CdS cells as well (by the 1980s, most good meters had gone to silicon cells).

An oft-overlooked second cause of underexposure is the wrong batteries. Millennials and GenZers who grew up in an era where there were only alkaline and silver batteries might not know that most SLRs using button cells were designed to run off 1.35v mercury cells (say a PX625). Mercury cells have extremely flat voltage until they die, which means that a meter need not have further voltage regulation (or much of it). They also have an almost infinite shelf life. Unfortunately, producing and disposing of them were not good for human health or the environment, and they went away.

Putting a 1.5v alkaline of identical size, like an A76 (or 1.55v S76 silver) cell in one of these mercury-cell cameras – even if it is the same physical size – will overwhelm the meter and lead to a stop or more of underexposure until the battery drops to 1.35v. But over time, the alkaline battery will drop below 1.35v and start to overexpose. So unless you catch the battery at just the right moment in its life, it won’t work too well.

Ironically, this same thing would not happen when you stick an old mercury cell in a modern camera – because there is more resiliency in the circuits of a modern camera to account for battery voltage fluctuation (alkalines start high, stabilize at a lower voltage, and eventually die).

I blame this battery-voltage problem a bit on battery manufacturers, who glibly published guides showing that their alkaline button cells fit into all manner of older cameras. They do fit, but they don’t work well. Of course, it’s a moot issue because you can’t buy 1.35v batteries except as highly corrosive zinc-air hearing aid batteries. You don’t want those in your camera. The second they run out of power, they begin to ooze nasty goo.

In terms of countermeasures, you can counter the CdS cell aging by recalibrating the meter. This will give you some more years, but it won’t last forever. If you don’t want to recalibrate, just try cutting the ASA in half on the meter.

Recalibration can also work for incorrect battery voltage, provided that the meter has enough adjustability, but the easier solution is to have a zener diode installed in line with the battery. This drops 1.5v to 1.35v. There are some adapters that incorporate these – you would use a slightly smaller silver cell in the adapter.

Anyway, I hope this helps you understand those muddy negatives were not (entirely) your fault.

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2 responses to “A brief note on underexposure”

  1. gregory ugloff says :

    Hi we have in Russia one plant who still produce that mercury px625s for the military purposes. They sell from 100 pcs. Pm me I can point you to them if you interested.

  2. Jim Grey says :

    I’ve decided to just use the silver-oxide batteries and lean on the exposure latitude of the films I choose (consumer C41 and b/w films like HP5) to make up the difference. Some of my cameras have bridge circuits that step the voltage down as needed, some don’t. I always get good enough exposures. I’d prefer to use a battery with the right voltage but I hate spending so much money on those infernal zinc-oxide cells.

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