Replacing a Mercury Battery

Many of the cameras made by Canon (i.e. Pellix, FT, EX/EE, FTb, et al), Nikon, Pentax and other manufacturers during the 1960s and early 1970s were designed to use a 1.35 volt mercury battery which is no longer available in the U.S.A., due to concerns about the impact of heavy metals on the environment. Examples of this battery include the Mallory PX-13, Eveready EPX-13, and Burgess HPX-13.

How to substitute the 1.5 volt 625A

One modern substitute for this battery is Duracell's 1.5 volt PX 625A. The higher voltage of the substitute battery causes the camera to meter more light than is actually available, resulting in under-exposure of the image. This under-exposure is approximately equal to one and a half stops of the apperture ring. In order to compensate for this error in the meter, the photographer should first set the camera to the aperture as indicated by the meter, and then widen the aperture by one and one half stops. If the meter says, for example, that the film should be exposed at f/4 for 1/60 of a second, proper exposure would result after setting the camera for f/2.5 at the same shutter speed.

An alternative method of obtaining correct readings with this meter would be to adjust the speed of the film (as recorded on the film speed selector). To utilize this method, first take the ISO film speed and divide by 5. Then multiply the result by 2. Finally, set the film speed selector for the result of this calculation. As an example, suppose that you are using a film that is rated at ISO 400. You would divide the actual film speed (400) by 5 (yielding a quotient of 80) and then, after multiplying the quotient by 2, obtain an adjusted film speed of 160.

(400 / 5) X 2 = 160

If the film speed selector is then set for ISO (ASA) 160 the meter will yield an accurate f-stop reading.

A direct substitute: the 1.35 volt Wein Cell

The Wein Cell (Wein MRB625) is a mercury-free, direct replacement for the Mallory PX-13 and PX-625 mercury batteries. This battery is sold by many camera stores and is usually priced only slightly higher than the more commonly available PX-625A.