Discontinued alternatives for photographic mercury batteries (PX625 / MR-9): zinc-air batteries and step-down diode adapter rings

1. History and environmental ban of mercury batteries (PX625/PX13)

From the 1960s to the 1980s, almost all top-end film SLRs and rangefinder cameras (such as the Leica M5/CL, Olympus OM-1, Canon F-1, Ricoh GR1 predecessor, Rollei 35, Nikkormat FTn) were all designed to use the PX625 model Mercury Cell:

1. Holy 1.35V constant voltage discharge curve:

• The mercury cell has unparalleled physical and electrical properties: its output voltage remains locked to an absolutely stable 1.350 volts throughout its entire service life of 2-3 years! It wasn't until one minute before the battery was exhausted that the voltage dropped off a cliff.
• The cameras back then used an extremely simple Wheatstone Bridge or cadmium sulfide (CdS) voltage-dividing photometry circuit without any modern voltage-stabilizing integrated circuits. They relied entirely on the 1.35V constant voltage of the battery itself as the photometry reference level.

2. Global Environmental Protection Ban:

• Mercury is an extremely toxic heavy metal, and discarded batteries can seriously pollute groundwater sources and cause Minamata disease. Since the 1990s, countries around the world have signed the Minamata Convention on Mercury, and the production and sale of PX625 mercury oxide batteries has been completely banned worldwide.

2. The fatal disaster of directly shoehorning modern 1.5V alkaline/silver oxide batteries

Many unscrupulous merchants on the market sell LR625/A625G alkaline batteries with the same appearance and size, deceiving users and saying "they can be used directly", which will lead to extremely tragic photometry collapse:

1. Serious systematic deviation of voltage 0.2V higher:

• The nominal voltage of modern alkaline batteries is 1.50V, and the silver oxide battery (SR44) is 1.55V, which is much higher than the original design of 1.35V.

2. The light meter seriously overshoots $\to$, causing the film to be severely underexposed and dead:

• Higher voltage drives more current to flow through the moving coil of the light meter pointer inside the camera;
• The light meter will "mistakenly believe that the scene is extremely bright" and the pointer will drift upward significantly;
• The photographer followed the pointer prompts to narrow the aperture or speed up the shutter: The developed film produced a serious systematic underexposure of a full 1.5 to 2.5 stops! All the details in the dark areas are completely black and collapsed!

3. Second torture of the inclined discharge characteristics of alkaline batteries:

• The alkaline battery voltage will slowly drop from 1.5V to 1.1V with use, causing the camera to be underexposed by 2 stops on the 1st day, underexposed by 1 stop on the 30th day, and overexposed on the 60th day. The metering error cannot be saved by fixed exposure compensation.

3. Three scientific and perfect 1.35V alternative solutions in practice

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