Battery type PX400: Compatible cameras

The PX400 is a 1.35-volt zinc-mercury oxide button cell (Zn-HgO) measuring 11.2 × 3.6 mm. From the 1960s onward, it was produced by Mallory (as RM400), Eveready (EPX400), and other manufacturers, mainly for the light meters of smaller SLR cameras. Like all mercury cells, it delivered an extremely stable voltage over around 95% of its discharge. That is why it became popular for analog light meters. IEC designation: MR42.

The PX400 was used primarily in the Pentax Spotmatic family: Pentax Spotmatic SP, SP-II, SP-IIA, SP500, and SP1000, as well as the sister models sold under the Honeywell Pentax label for the US market. It was also used in the early Fujica SLRs ST601 and ST701. You can find the complete searchable list of all compatible cameras at the top of this page.

Pure mercury-oxide cells such as the PX400 were effectively banned from 2000 onward by EU Amendment Directive 98/101/EC because their mercury content was significantly above the permitted threshold. Battery Directive 2006/66/EC later reaffirmed the ban. Worldwide production had already ended by the late 1990s, and new PX400 cells have not been legally available since then. The reason is not the battery itself, but the environmental impact of mercury when improperly disposed of.

First, an important point: The battery compartment of the Spotmatic family is very shallow at 11.2 × 3.6 mm. An LR44 or SR44 (5.4 mm high) will not physically fit, unlike the larger PX625. The sensible options are:

• P312 zinc-air hearing-aid battery + adapter: At approximately 3.5 mm high, the P312 is flat enough for the PX400 compartment. Under load, it provides a constant 1.35 V—the exact original voltage. Available from Ausgeknipst as an adapter set with P312 batteries or as a P312 refill pack. Disadvantage: after removing the protective film, the cell lasts only about 4–8 weeks, even when unused. Recommended for all PX400 cameras and essential for the Fujica ST601/ST701.
• Wein Cell-equivalent button cells: Special zinc-air cells from specialist photography retailers, chemically identical to the P312, but more expensive and less readily available.
• Alkaline/silver-oxide cell with the appropriate height (e.g. 387S, 392): They fit physically, but deliver 1.5–1.55 V instead of 1.35 V. This is uncritical for the Pentax/Honeywell Spotmatic family (see the next question), but NOT recommended for the Fujica ST601/ST701. Its light meter is voltage-sensitive and produces significantly inaccurate readings with 1.5 V.

Spotmatic and Fujica are two fundamentally different worlds here and must not be lumped together:

• Pentax/Honeywell Spotmatic family (SP, SP-II, SP-IIA, SP500, SP1000): The light meter is designed as a Wheatstone bridge circuit that is insensitive to the supply voltage when zeroed. A Spotmatic works correctly with 1.35 V (mercury/zinc-air), 1.5 V (alkaline), or 1.55 V (silver-oxide) alike. Here, only the mechanics are the bottleneck: matching the 11.2 × 3.6 mm form factor.
• Fujica ST601 and ST701: According to Camera-wiki, these models do NOT use a bridge circuit, but rather a silicon photocell with an FET circuit. The light meter is explicitly voltage-sensitive and gives significantly inaccurate readings with modern 1.5 V alkaline or 1.55 V silver-oxide cells. For the ST601/ST701, 1.35 V is mandatory, either via a zinc-air adapter (P312) or a Schottky-diode modification to the camera.

For the general voltage rule concerning other voltage-sensitive camera types: A higher-than-specified voltage makes a voltage-dependent light meter assume that more light is present than actually is. The camera closes the aperture or shortens the shutter speed, resulting in underexposure. Lower voltage correspondingly leads to overexposure. Negative film tolerates overexposure considerably better than underexposure, so slight undervoltage is generally the lesser evil. Even more important than the absolute value is consistency throughout the discharge: alkaline and standard silver-oxide cells continuously lose voltage, distorting the meter reading over their lifetime; mercury, zinc-air, and NiMH cells, by contrast, maintain a largely constant voltage.