You finally found it. You were digging through a flea market, or maybe browsing online, and you scored a beautiful, heavy, mechanical SLR from the 1970s. The viewfinder is bright, the mechanical shutter sounds like a dream, and you cannot wait to load your first roll of film into it. But then you unscrew the little coin-slotted cap on the bottom plate, peer inside the empty chamber, and read the sticker: Requires one PX625 1.35V Mercury Battery.

Wait, mercury? Yes, actual mercury. If you are new to shooting vintage film cameras, this is a rite of passage. Many of the most legendary analog cameras ever made, like the Olympus OM-1, the Canon F-1, the Pentax Spotmatic, and the Minolta SR-T series, were designed around mercury button-cell batteries to power their internal light meters.

As you probably guessed, you cannot buy these anymore. They were legally banned around the world in the 1990s because throwing little pucks of toxic heavy metals into landfills was an environmental nightmare. It was a huge win for the planet, but it left vintage camera nerds with a really annoying problem.

Why You Cannot Just Use an Alkaline Battery

If you run down to the local pharmacy, you will easily find alkaline batteries that look identical in size and shape to the old PX625 mercury cells. They are usually labeled as LR44, PX625A, or EPX625G. It is incredibly tempting to just pop one in and call it a day. The needle moves in the viewfinder, so it must be working, right?

Not quite. The problem here is voltage, and how the camera's internal electronics understand it. The original mercury batteries output exactly 1.35 volts. Modern alkaline batteries output 1.5 volts. That difference might sound microscopic, but a vintage camera light meter is essentially just a crude calculator. It takes the voltage from the battery, runs it through a photosensitive cell, and tells you what shutter speed and aperture to use. If you feed the calculator 1.5 volts instead of 1.35 volts, it gets confused. It will think the scene is much brighter than it actually is, causing you to underexpose your film by about two to three full stops.

To make matters worse, alkaline batteries do not have a stable discharge rate. A mercury battery would output a perfect 1.35V for years, and then drop dead instantly. An alkaline battery starts at 1.5V, and as it drains over a few months, it slowly drops to 1.4V, then 1.3V, then 1.2V. This means your camera's light meter will be wrong on day one, and the amount by which it is wrong will change every single week. It is a recipe for ruined photos.

So, what can we do to safely power these gorgeous old cameras? Luckily, the community has figured out a few fantastic workarounds.

The Easiest Fix: WeinCell Zinc-Air Batteries

If you want a drop-in replacement that requires zero effort, thinking, or DIY skills, you want a WeinCell MRB625. These are custom-made zinc-air batteries specifically designed for vintage cameras. They are exactly the right physical shape, and most importantly, they output exactly 1.35 volts. If you put a WeinCell into a Canon FTb or a Rollei 35, the light meter will behave identically to how it did in 1975.

There is just one catch. Zinc-air batteries need literal air to generate power. They come with a little sticky tab over the air holes. Once you peel that tab off, a chemical reaction starts and the battery comes to life. But once it starts, it cannot be stopped. Even if you turn the camera off and put it in a drawer, the battery will run completely dead in about three to four months. They are great if you are going on a trip and plan to shoot a lot over a few weeks, but they are expensive to keep replacing if you only use the camera occasionally.

The DIY Fix: Hearing Aid Batteries and O-Rings

If a WeinCell sounds great but you hate the price tag, you can build your own. WeinCells are essentially just repackaged hearing aid batteries. If you go to the store and look for size 675 hearing aid batteries (also called PR44), you will find that they are also zinc-air batteries, and they output between 1.4V and 1.35V, which is close enough that your film will look fantastic.

The only issue is physical size. A 675 battery is much narrower than a PX625, so it will rattle around in your camera's battery chamber and lose its connection. To fix this, head to a hardware store and buy a #9 rubber plumbing O-ring. Slip the O-ring around the edge of the hearing aid battery, and boom suddenly it is the exact same size as the old mercury cell. Pack of hearing aid batteries cost almost nothing, so throwing them away every three months hurts a lot less.

The Permanent Fix: The MR-9 Adapter

This is honestly my personal favorite method, and the one I use in my own vintage SLR bodies. You can buy a tiny, precision-machined brass adapter called an MR-9 adapter. It is perfectly shaped like a PX625 battery, and you insert a much smaller, modern silver-oxide battery inside it.

The magic is inside the brass casing. A true MR-9 adapter has a microscopic voltage-reducing diode built right into the metal. It takes a modern 1.55V silver-oxide battery (specifically a size 386 or SR43W) and instantly steps the power down to a perfect, flat 1.35V. Silver-oxide batteries are amazing because, just like the banned mercury cells, they hold their voltage perfectly flat for years and then die suddenly. You buy the adapter once, and then you just throw cheap silver-oxide batteries in it once every year or two. It is absolutely brilliant.

When To Just Ignore the Battery Entirely

I feel like I have to mention the elephant in the room: sometimes the battery isn't the problem at all. The light meters in these old cameras rely on CdS (Cadmium Sulfide) photoresistor cells. Over forty or fifty years, these cells naturally degrade. You might spend time perfectly adapting a battery to 1.35 volts, only to realize the needle in your viewfinder is dead anyway, or wildly inaccurate just due to old age.

If your internal meter is dead, do not panic. These cameras are fully mechanical. The shutter and the aperture do not need battery power, they run on springs and gears. You can totally shoot without a battery by using the Sunny 16 rule, trying a light meter app on your phone, or grabbing a dedicated external light meter. Actually, hot-shoe mounted light meters have become incredibly popular lately. They slide right onto the flash mount of your camera, weigh almost nothing, and are often more accurate than the 1970s technology that was built into the camera anyway.

Let's be real, you shouldn't let a banned battery stand between you and enjoying a beautiful mechanical camera. Whether you go with the hearing aid hack, invest in a voltage-dropping adapter, or just bypass the internal meter entirely, bringing these heavy metal legends back to life is worth the minor hassle.

If you've given up on a dead internal meter and want to go the handheld route, or you are just looking for your next fully mechanical vintage beauty to try these tricks on, check out our current inventory. You can browse through our great selection of vintage light meters, or find a fantastic fully mechanical SLR camera that's ready to be loaded up and shot today. Keep shooting, and don't let 1.5 volts ruin your day!