<\/div>\n\n\n\n
Part of my fascination with photographic light meters stems from the challenges faced by measuring light levels under natural conditions, and in particular how those measurements can be translated into the information required by the photographer, namely the lens aperture setting and the exposure time. Together, these determine the amount of light falling on the film.<\/p>\n\n\n\n
Design issues<\/strong><\/p>\n\n\n\nThe first photographic light meters were made by instrument manufacturers rather than camera makers, and many of them show that heritage in their design. The first meters were often utilitarian in appearance and not particularly easy to use. Over time, light meter manufacturers paid more attention to physical design and ease of use, and the best light meters became desirable, stylish gadgets for the aspiring photographer. Some manufacturers produced a bewildering array of different models, often with multiple names depending on the country of sale, or for marketing purposes. In contrast, the Weston company and its successors produced the very popular ‘Master’ series of light meters between 1939 and 2010 with only minor modifications, a testament to the quality of the initial design.<\/p>\n\n\n\n
Technical issues<\/strong><\/p>\n\n\n\nLike the human eye, the relative sensitivity of photographic film follows a logarithmic response, so a light meter should respond in the same way. This is most easily acheived by using a detector that itself has a logarithmic response to light, such as a cadmium sulphide (CdS) ‘light dependent resistor’ or a selenium cell. Many light meters of the 1960s and ’70s were based on one or other of these detectors, with selenium cell meters having the advantage that they generated their own power so no battery was needed.<\/p>\n\n\n\n
The second technical challenge concerns the range of wavelengths over which the light meter responds. Photographic light meters were generally designed to have a spectral response similar to the human eye. Once again, this is most easily achieved by choosing a detector that has such a response, such as a selenium cell. The spectral response of a typical CdS detector is not ideal, but is closer to the human eye response that an unfiltered silicon photodiode, for example.<\/p>\n\n\n\nTypical CdS and selenium cell spectral response compared with that of the human eye. Source: Ferranti MS600\/MS601 data sheet, Issue 3, Sept 1974.<\/em><\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\n
Practical issues<\/strong><\/p>\n\n\n\nThere are several practical issues to be addressed when assessing a photographic light meter, particularly one that is many decades old. First, whether the device needs an internal battery or whether, like the selenium cell design, it is self-powered. This is particularly important for a device which is likely to be only used intermittently, and therefore left for long periods of time with batteries installed. Many old light meters are now useless because batteries have been left in until they leaked.<\/p>\n\n\n\n
Mercury cells such as the PX625 were commonly used in light meters, partly for their small size, but also because they delivered an almost constant voltage (1.35 volts) until completely depleted. These are now banned from sale in Europe, so replacing these is problematic. Modern button cells are a different physical size, and their voltage is different and not as stable over the life of the battery. Sadly, many of the high quality and innovative light meters manufactured by the Gossen company are affected by this problem. Work-arounds involving metal shims and Wein Zinc\/Air cells are a possible but expensive solution.<\/p>\n\n\n\n
Second, there should be some means of defining the field-of-view of the light meter so that the user know which parts of the scene are contributing to the measurement. In practice, a sighting device was often only found on top-of-the-range light meters, and most meters were simply pointed in the general direction required.<\/p>\n\n\n\nA sighting attachment on a Gossen Lunasix 3 (left). The Gossen Multibeam (right) had an in-built sighting device.<\/em><\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\n
Although not as common on budget light meters, both these meters had sighting devices.<\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\n
Third, a photographic light meter needs some means of coping with the very large dynamic range, from twilight to full Sun. In some instruments this was achieved by having mutiple scales and switched gain. Perhaps the most effective solution to this problem involved a mechnical means to change the scale at the same time as the sensitivity of the instrument was varied. High-end light meters from manufactures such as Gossen and Weston achieved this by inserting baffles in the light path and using mechanical linkages to change the scale markings. Weston also used large selenium cells so as to give good sensitivity to low light levels.<\/p>\n\n\n\nLight baffle on a Sangamo Weston Master V<\/em><\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\n
Operational issues<\/strong><\/p>\n\n\n\nHaving designed an instrument that has the correct spectral response, is convenient to use in the field and covers the range of illumination required is not the end of the matter. In fact, these are all secondary to the basic question, ‘What are we to measure?’. Simply pointing a light meter at the scene to be measured tells us little about the incident light from the Sun and the sky, as the light measurement obtained depends on the composition of the scene. An alternative approach involves placing the light meter in the scene and pointing it at the camera. This is referred to as ‘incident light measurement’ as opposed to ‘reflected light measurement’.<\/p>\n\n\n\n
Various add-on devices were produced to allow the same instrument to be used for incident light and reflected light, for example removeable diffusers, the Weston ‘Invercone’ and the rolling shutter device used by Gossen in the 1950s.<\/p>\n\n\n\n
<\/div>\n\n\n\n
Sekonic with diffuser<\/figcaption><\/figure><\/li>Weston Invercone<\/figcaption><\/figure><\/li>Fitted to Master V<\/figcaption><\/figure><\/li>With incident light shutter deployed<\/figcaption><\/figure><\/li>Sixtomat set for reflected light<\/figcaption><\/figure><\/li><\/ul>Attachments for incident light measurement<\/figcaption><\/figure>\n\n\n\n<\/div>\n\n\n\n
Whilst this enables accurate measurement of the light falling on the scene it has two problems: first, it is not always practical to locate the meter in the scene being photographed, and second, it still tells us nothing about the amount of light that will be reflected from the scene towards the camera. To do this, we need to make an important assumption about the scene being photographed.<\/p>\n\n\n\n
Conventionally, it is assumed that the typical scene has an overall average reflectance of 18%, so this figure is used in both incident and reflected metering to estimate the amount of light entering the camera. Clearly, this is a gross simplification as anyone who has used a light meter to photograph a snowy scene will be aware. Under such conditions the light meter will assume that a highly reflective snowscape, with an overall reflectance approaching 90%, should be adjusted to correspond to an average scene of reflectance 18%. The result is that the snow scene is badly underexposed.<\/p>\n\n\n\n
Throughout the period when separate light meters were used, the answer to this problem was intelligent use of data from the meter. More recently, advanced matrix metering and in-camera processing has greatly improved the automated determination of exposure, although occasionally even these systems can be fooled by an unexpectedly dark or light scene.<\/p>\n","protected":false},"excerpt":{"rendered":"
This page is a celebration of photographic light meters (aka ‘exposure meters’), the small instruments that aspiring amateur photographers and […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"_links":{"self":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/710"}],"collection":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/comments?post=710"}],"version-history":[{"count":22,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/710\/revisions"}],"predecessor-version":[{"id":836,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/710\/revisions\/836"}],"wp:attachment":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/media?parent=710"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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