On metering…

Whilst reading accounts by Ansel Adams of how he took his photographs and how he assessed lighting levels I was struck that he used the term candles/sq ft. I do not think this term describing the amount of light is understood nowadays, and certainly it took a little research and internet browsing before I found the information I needed to understand Adams’ descriptions.

In this developed digital age, we take many camera operations for granted, and that extends to metering the light we endeavour to capture. In some ways, in wasn’t just that exposing/developing/printing was far more expensive frame on frame, but the detail available to us generally in the film/analogue era helped us much more, because it forced us to work slowly and with understanding.

These days, every camera has a light meter built in, that are sometimes extremely complex in how the light in the scenes we capture is assessed and measured before expo

sure. But when in Manual mode, is there enough data for our assessment of exposure shown in the viewfinder, or are we really satisfied whether the image will be correctly exposed albeit with simple – or + indications?

Many cameras have a spot metering facility, at least 10 degrees sometimes but sometimes as narrow as 2 degrees of the angle of view or even less, but a simple +/- indication does not help our overall assessment. It is said that the modern digital meter (eg Polaris) gives us an indication, but the small LCD window doesn’t give us the information that the manual dial of a Weston Master (from the war to the 1950s) or later Euromaster (until the demise of Weston in the 1980s) gives us at a glance.


A Weston meter dial gives us a readout of all shutter/f stop combinations for a given film speed at a glance, that the digital readout can only show by multiple button presses and scrolling through (albeit detailed) data.

Additionally, it can tell us whether the scene falls within the available dynamic range of the black and white or colour film emulsions. With the use of an invercone, we are also not limited to reflected light assessments, but we can also measure the incident light falling on the scene, unaltered from the differing degrees of reflectance of different colours and materials. The internal camera meter is unable to do this.

Interestingly, the meter of earlier versions of the Weston Master measured light from 0.2 to 1600 candles/sq ft, so presumably Adams would have felt at home. The meters in later Westons and the Euromaster series measure light in EV values from 2 to 16


Operation is fairly simple. First of all the inner ring that includes a window should be set to the desired sensor (film) sensitivity. This is normally ISO/ASA and DIN (though early Weston meters are the Weston scale. If you have such a meter I would advise setting it to W 80 instead of 100; the scale seems to be roughly 1/3 stop slower than modern ISO).

Then take a reading by holding up the meter towards the scene, but avoiding the sky area if outdoors. There are usually two scales depending on the light level –  a small cover may have to be adjusted in order to use the correct scale. Then transfer the number (light candles for early meters, EV for later ones) to the outer dial.


The inner dial then shows all the shutter speed/aperture combinations to achieve a good exposure.

Weston considered the useable dynamic range of black and white film to be around 7 stops, and the dial is engraved with ‘U’ and ‘O’ markings to indicate this range, and for those wishing to fully control their camera exposures to control brightness levels. For colour film there are ‘A’ and ‘C’ markings indicating the much more limited range of colour film that was just 2 stops. You can thus use the meter by measuring the brightest/darkest readings of the scene and setting the camera to the median result. If the brightness levels are too wide for the sensor/film, the photographer then has to compromise in order to protect the most valued parts of the scene being photographed.

One can argue that the simplification towards EV nomenclature gives us less detail, but modern meters such as the digital Polaris does give us huge precision.


The readout tells us the required f stop for a given shutter speed, and a graphic indication of whether the correct exposure is 1/3 or 2/3 under or over, that is further divided into 3 divisions. [Now we are getting into the realms of whether the mechanical camera shutter speed can be consistent at these precise measurements.] The digital Polaris does have a built in invercone if required and you can buy a spot meter attachment, though only a 10 degree view.

Ergonomically, the Weston meter is smaller (!) albeit a little heavier, but its readout is all embracing though less precise… and it does give a huge amount of data at a glance.

If you do pick up a Weston meter at a car boot sale there are a few things to watch out for. The ISO/ASA scale of earlier Westons is not ISO but a ‘Weston scale’ that is not quite the same. Generaly you just need to set it one notch lower than your intended ISO ie 80 instead of 100. Be wary of accuracy – the selenium cells are now quite old. Both of my Westons are between ½ and 1 stop slow, but easily compensated by adjusting the ISO/ASA dial.

These are selenium meters. The light itself produces a voltage that varies depending on the amount of light falling on the cell and thus do not need a separate power source. Over time, some camera manufacturers and meter makers switched to cadmium sulphide that was both quicker and could be more sensitive, such as this very cheap Jessops meter.


If you are interested in the Zone system a hand held meter is the way to go so that you fully assess the brightness range of the scene.

Some places can be very challenging to meter especially if you haven’t a hand held meter or cannot conveniently use one. For instance in a church or cathedral as a starting point spot meter for the brightest element (usually the windows) and then open up 2 stops. This usually produces a good result, even though the camera’s overall meter may indicate underexposure of 3 stops or more!

If you’re not that interested in metering as such, Weston meters are beautiful examples of design and engineering – their longevity is testament! If you are interested in metering more accurately, there are of course many more modern and sometimes expensive meters available.