The Weston Master V, Mercury PX625 and the alternatives
The Weston Master V, Mercury PX625 and the alternatives

Accurate Exposures in old cameras

I’m going to explore a few topics here that relate to ‘exposure setting’ before electronic exposure became available. Generally 1950-1975.

  1. The Weston Master v
  2. Mercury batteries in exposure meters
  3. The alternative Zinc Air battery

But firstly let’s step back in time, to the days of manual exposures available for film cameras.

In the early days of photography, exposure was calculated from a chart. These charts gave average daytime exposure indications in a variety of different lighting conditions. Most photography was B&W and gave a fair guide.


By Gisling – Own work, CC BY-SA 4.0,

A typical guide would be (from a Rolleiflex camera)

It was both general and some interpretation was needed by the photographer. The use of flash allowed a much better control under difficult lighting. In addition film speeds were low, typically 50 ASA /18 DIN.

I could go into how film ‘speed’ is calculated. (d-min above fog level) but that is for another article.

Lest to say not all film manufacturers used the same measurements. So from the outset there were variations in exposure based on a given film type.

I’m saying before you even started, everything was a variable. Even batch to batch of the same film type the effective sensitivity of the film changed to some degree.

The next step forward was to have some way to measure the light, scientifically.


One of the methods employed was the ability of Cadmium Sulphate (CdS) to react to light (variable resistance) when light fell on it. This  is dependent on the intensity  of the light. So now we had a means of calculating how bright a light source, or ambient light was…well sort of. It depends what you call light. We are only interested in the ‘visible’ light we use to see here. Fortunately the visible spectrum of daylight is an even mix of red/green/blue light that we call ‘white’. Other light sources are not. However even under this assumption  we have to qualify it with ‘colour temperature’ measured in Kelvin. I’m not going to get bogged down here as this is a simple summary of  assessing exposure in photography.

Everything is an average in assessing correct exposure for film. There is no absolute way to determine a correct expose, even if such a thing exists. To add further complication to assessment our eyes are not fixed light receptors. The eye regulates several things that we look at. The three main ones being the iris, local adaptation and lateral adaptation. In a nutshell your eyes are changing constantly to different lighting conditions, colour and contrast.

Also it is with ‘exposure’ in film, is not a linear exposing material. It has a characteristic isn’t reciprocal, known as reciprocity characteristics. Film utilises an area of sensitivity for the recording of light in what is terms the ‘straight line region’ of its characteristic curve. This region is what we use in photography, a region for a correct exposure because largely speaking in that region  its sensitivity to light is ‘reciprocal’ The region is quite narrow and as a ballpark figure is around 2.5 ‘stops’ of exposure ie. from the brightest to the darkest elements in your photo. Our scene needs to fall in that region. Highlights and shadows tail of disproportionately outside of that ‘straight line’ exposure range.  The old advice, expose for the shadows and let the highlights take care of themselves.

I’m hoping you can see that already what tonal range, contrast range, we hope to record in a photo is our selection, we cannot record every tonal range that we see as our eyes are far better at adapting (also we scan a scene and adjust accordingly) So how can we do that? How can we measure the light to fall into that narrow straight line region of the film?

The lightmeter

1) Weston Master V

I’m going to use this benchmark meter in this article. If you aren’t familiar with it…I’m very surprised. I could have picked many other meters but this one…is mine. In its evolution from the l to V it spans the area of expose measurement of decades concerned before digital and is still used by many today.

This meter, takes an ‘average’ light measurement. It assumes (used in a point it at the scene way) that when all the light it receives is jumbled together it equals grey. An exposure based on that will be indicated by the meter. But of course that is not the case. If the sun or sky is in shot…the average is going to be well out, unless the sky is your chosen subject. It is accommodating a contrast range in your scene of perhaps 10,000:1 or more. Film (taking B&W here) can handle a fraction of that. Given that you are printing onto photo-paper 200:1 is about the limit of contrast you can achieve in a print. Within that the eye can detect approximately 80 grey steps from black to white so that is where we need to have reciprocity in the films exposure.

So the meter has to be used more specifically. Its average to grey has to be selected by you. Usually by using something in the scene, or surroundings that is in the range you want. That is the only way an ‘average’ meter can be used. From this you can see, some selection by the photographer is necessary.

On a note CdS/selenium cells degrade over time. If you have an old meter, 30yrs plus as a guide it won’t be as accurate as it was. I replaced the selenium cell in mine from Sangamo Weston about 4 yrs ago. Many people learn to adjust the reading based on experience but the bottom line is it isn’t to be taken literally. All exposure readings are a guide.

Early on-camera meters worked this way. Later ‘through the lens metering’ removed the general light and used the viewfinder screen (and other ways like the Canon Pellix to name just one.) But it was still an average. ‘Centre Loaded’ metering became the next thing, assuming what you had in the centre of your composition was what you wanted to be ‘correct’…a reasonable assumption. With a single CdS (or other) cell that was what you had to have. This is not the case now we have exposure mapping and all sorts of ‘smart’ metering.

So there it is, no ambient light meter is ‘accurate’ in the sense it will give you the photograph you hoped for on its own. Camera meters worked on percentages of correctly exposed photos. Few managed more than 80% in general outdoor photography. Good enough we thought.

2) Mercury batteries in exposure meters

I’m constantly asked, since the banning of mercury batteries, if the replacement 1.4V Zinc Air (though they are 1.38v under load) are accurate enough as a replacement to the 1.35v mercury batteries. Specifically the PX625. The voltage difference is 0.03v which equates to about 1/3 of a Stop on an average meter. Was an exposure reading of a 1950-60’s camera/meter ever that ‘accurate’ because it depended how you used it also. And that same camera today almost certainly won’t be as accurate as it was for other reasons, it’s old, degraded and not inherently accurate. 1/3 stop however is easily accommodated as to only require a small adjustment say 125ASA setting as against a 100ASA on the film speed; a few test shots will guide you. It should be pointed out that film is calibrated to and average speed in manufacture, not a fixed one. Also, as I stated earlier how that is decided was not an absolute between manufacturers.

3) The alternative Zinc Air battery

Zinc Air batteries were created when mercury batteries were banned. Hearing aids used mercury batteries due to their excellent characteristics of constant voltage and long life. However, zinc air has good constant voltage characteristics but not such a good life. It is, so far, the only battery that can deliver close to the 1.35v mercury. Silver Oxide is great but 1.55v. In many meters and cameras, that use a comparator circuit, it is fine to use a 1.55v battery. In others (and there are 100’s) it is not. There are plenty of resources on the net that can help. To add to the problem the battery makers decided not to make a Px625 style zinc air battery…so a few of us took up the task. We currently have 3 main options. The Wein Cell, which is a single use PX625 case zinc oxide, the MR9 which is a voltage correcting PX625 case (re-useable) that takes silver oxide or the PX625 adapter which gives the outer shape (in brass) of a PX625 into which a zinc air battery fits (re-usable). Much research has been done on these over the years. see my page .


If you have read this article you will see, no meter is ‘accurate’ all require some effort by the user. These cameras/meters are from a pre digital time when the photographer used experience, not just a device. In addition there are so many factors at work nothing was or is fixed in wet photography. It requires skill and knowledge, experience and dare I say luck?

After writing this I realise it needs expanding, and many more factors are at work regarding ‘exposure’ I wanted to keep it simple but not sure it can be. More to come.

Paul Birkeland Green may 2020

Trained by: Kodak, Ilford and Agfa. Photofinishing Densitometry, chemical analysis, Q.C. senior technician, Nashua Photo Products (USA). LBIPP British Institute of Professional Photography 1977-1995. Lecturer in Photography. 1995-2001. Colour processing designer: Jessops Photographic.