In the 1960s and 1970s, there were millions of lovely cameras and handheld light meters manufactured—many of which remain perfectly usable today. Except for one little problem. Their light-measuring circuits were designed to be powered using a mercury battery.
What made mercury button cells so appealing was that their voltage stayed absolutely ruler-flat, until the last of the chemicals were depleted. After that, the battery quickly died. Most camera makers omitted any voltage compensation in their meter circuits, and simply used the battery itself as a voltage reference.
Mercury PX-13 battery, curse of vintage camera-dom
By far the most common size used in older cameras was the PX-13 or PX-625 type. Its case had a raised shoulder around its minus end, making it look vaguely muffin-like.
Today we recognize mercury to be a highly toxic metal; and worldwide, mercury battery production has been phased out. Any stocks of mercury batteries now remaining are from old production runs—a safe guess being from sometime in the last millennium.
If you go shopping for a PX625 today, you’ll discover lookalike replacements being sold. But they are alkaline cells, not mercury. And the problem is, a mercury cell is a 1.35 volt battery. An alkaline cell starts out at about 1.55 volts instead.
In a calculator, kitchen timer, etc., this voltage discrepancy is unimportant. But a light meter works by measuring the exact current flowing through a photocell: so the wrong voltage can wreak havoc with accurate readings. A few cameras (notably Pentax) used a meter circuit which was insensitive to voltage variations—but for most meters, wrong voltage means wrong exposure.
Worse, an alkaline battery actually drops off in voltage as it’s used, so the error is not even consistent—really you get the worst of both worlds. (The same drooping-voltage problem applies with 3-volt lithium batteries, in applications where those could be used.)
But silver-oxide batteries are widely available, and maintain a flat voltage (of about 1.58 volts) over their whole lifetime. The long life of silver-oxide cells make them the first choice anywhere it’s possible to use them.
Meter-Battery Voltage: Myths & Reality
Sometimes you read confused internet discussions about whether this o.2-volt error is important. And some rather questionable assertions get repeated. One claim is: “modern film has such wide exposure latitude that it doesn’t matter.” Another is, “you can just change the ASA setting to compensate.”
Fortunately, I am lucky to own one last genuine, mercury PX-13 cell, which still has some juice to it. So I decided to make a definitive test for myself.
I took light meter readings using two classic old-school SLRs (an Olympus OM-1 and a Canon FTb), and compared them to a known-accurate Pentax V spotmeter. Using the intended mercury battery, I got the camera and the spotmeter to agree within about 1/2 stop, over the entire range from full sun to dim indoor light.
But with the higher voltage of a silver-oxide battery, the cameras’ meters gave incorrect readings—and with a strange pattern: In bright sunlight, the indicated readings would yield two and a half stops underexposure! Yet in dim indoor light (at about the limit for handheld shooting) the meter readings were nearly correct. Between those two extremes, there was a variable amount of underexposure.
Well, this demolishes both of the internet myths I mentioned. First, 2-1/2 stops of underexposure is a terrible idea with any negative film I know of. (You’d get ugly grain and totally blank shadows.) Second, there is no simplistic way to adjust the ASA to compensate, because the error is not consistent as you go from bright to dim light.
The errors could certainly be different for other brands of cameras, using different circuit designs. There is no substitute for checking your own equipment against a known-good meter. But obviously the problem is a real one.
Frans De Gruijter has written the definitive article on this problem, along with several solutions, downloadable here (500 kB PDF). This article goes into dense technical detail; but at the very least, look at the graph he provides on page 3, showing the voltage curves for several different battery chemistries.
And there you’ll notice an intriguing possibility: Zinc-air batteries.
Zinc-air is an interesting battery chemistry, giving excellent energy density at low cost—advantages that have made them the preferred power supply for hearing aids. Happily, zinc-air cells have a voltage quite close to that of mercury cells. And this voltage stays consistent over the battery’s lifetime, just as we’d like.
Pull the blue tab to activate the battery
Zinc-air chemistry is also the basis of the “Wein cell,” often sold in camera stores as the correct-voltage replacement for mercury photo batteries. However the cost of vanilla #675 hearing-aid batteries is much lower—about $6 for a pack of 8.
To use either of these types, you must pull off a sticky tab first, which allows air to enter pinholes in the battery case. The battery does not produce any voltage until oxygen reaches the interior. Unopened cells can be stored for many years and remain fresh.
But one downside is that the inside of a zinc cell must remain moist for the chemical reaction to work. In arid environments, the cell can dry out and stop working after just a month or two, before its electrical capacity has been used up.
Putting the sticker back over the air holes will prolong the battery’s life, if you can remember to do it. But with the low cost of hearing-aid cells you might just consider them expendable, replacing them often.
The 675 size hearing-aid battery is a little bit thinner than a PX13 mercury cell; also it lacks the “muffin” shoulder and so is smaller in diameter. Sometimes you will need to add a little spacer ring to keep it centered in the battery compartment.
For this, I just slice rings off the end of a piece of tubing of the proper diameter:
Now, the voltage of the zinc-air battery is not perfect—it can be a shade too high. In fact, both the Wein cell and hearing-aid solutions have some voltage quirks, which I plan to write about in another article. However let’s keep things in perspective:
Over 40 years, any light meter might drift out of calibration—even if supplied with the textbook 1.35 volts. The shutter speeds on a vintage camera could easily be out of adjustment by a half a stop or so. There can be some slop in aperture linkages, so that you aren’t getting precisely the marked f/number. Vintage cameras are not the place to look for 3-digit precision.
But my tests say that a zinc-air hearing aid battery will get you to within half a stop of the exposure reading you’d get using a mercury battery. And any error will be worst in bright sun—the one situation where it’s most reliable to trust those old “Sunny 16″ instincts.
So if all that’s stopping you from taking some nice old camera for a spin is the mercury battery issue, go with the zinc-air cells. It’ll get you out there shooting after one quick, inexpensive trip to the drugstore.
Then you can explore other, techier solutions to the problem later, if you choose to go that route.
Update: More on the quirks of zinc-air battery voltage in this follow-up post.