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, it was produced by Mallory (as RM400), Eveready (EPX400), and other manufacturers mainly for light meters in smaller SLR cameras. Like all mercury cells, it provided an extremely stable voltage over about 95% of its discharge — the reason it became standard for analog light meters. IEC designation: MR42.

The PX400 was mainly used 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 like the PX400 were effectively banned by the EU amendment directive 98/101/EC from the year 2000 because their mercury content was significantly above the allowed threshold. The battery directive 2006/66/EC later confirmed the ban again. Worldwide production had already ended in the late 1990s — new PX400 cells have not been legally available since then. The reason is not the battery itself, but the environmental impact of mercury in improper disposal.

Important point first: The battery compartment of the Spotmatic family is very flat at 11.2 × 3.6 mm. An LR44 or SR44 (5.4 mm height) physically does not fit — unlike the larger PX625. The sensible options are:

• Zinc-air hearing aid battery type P312 + adapter: The P312 is flat enough at about 3.5 mm height for the PX400 compartment. Under load, it consistently delivers 1.35 V — exactly the original voltage. Available from Ausgeknipst as an adapter set with P312 batteries or as a P312 refill pack. Disadvantage: after removing the protective foil, the cell only lasts about 4-8 weeks, even without use. Recommended for all PX400 cameras, mandatory for the Fujica ST601/ST701.
• Wine cell equivalent button cells: Special zinc-air cells from photo specialty stores, chemically identical to the P312, but more expensive and less available.
• Alkaline/Silver oxide cell in matching height (e.g. 387S, 392): Physically usable, but deliver 1.5-1.55 V instead of 1.35 V. Uncritical for the Pentax/Honeywell Spotmatic family (see next question), but NOT recommended for the Fujica ST601/ST701 — there the light meter is voltage-sensitive and delivers significantly distorted readings at 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, which is insensitive to supply voltage when balanced at zero. A Spotmatic works correctly with 1.35 V (mercury/zinc-air), 1.5 V (alkaline), or 1.55 V (silver oxide) equally. Here, only the mechanics are the bottleneck — to fit 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 a silicon photo cell with FET circuit. The light meter is explicitly voltage-sensitive and delivers significantly incorrect values with modern 1.5 V alkaline or 1.55 V silver oxide cells. For ST601/ST701, 1.35 V is mandatory — either via a zinc-air adapter (P312) or through a Schottky diode modification of the camera.

For the general voltage regulation for other voltage-sensitive camera types: Higher voltage than specified causes a voltage-dependent light meter to accept more light than is actually present — the camera closes the aperture or shortens the shutter speed and the result is underexposed. Lower voltage leads accordingly to overexposure. Negative film forgives overexposure much better than underexposure — a slight undervoltage is therefore generally the lesser evil. Even more important than the absolute value is the consistency over the discharge: Alkaline and standard silver oxide cells continuously lose their voltage and distort the measurement over their lifetime; mercury, zinc-air, and NiMH cells, on the other hand, maintain the voltage largely constant.