nce in a while one recalls an invention or product that was so esteemed in its day as to make its disappearance seem inexplicable. A classic example is the property of back-leak compensation (BLC), as employed on the Weston Master V exposure meter with the Invercone attachment. The design of the Invercone resulted from a rare combination of one man's experimental ingenuity and the collaboration of a responsive manufacturer. The venture was to give photographers an exposure-estimating tool which, in the writer's opinion, has not been equalled. But before launching into a description of BLC it is worthwhile to understand something of the photographic environment in the Fifties and Sixties; the period which gave birth to the Invercone. In colour photography, reversal film reigned supreme, principally because the alternative colour print process was attended by high cost and poor quality. This popularity was not suppressed by the difficulty of accurate exposure estimation demanded by the former. The motion film industry had already been using colour reversal film for some years, and faced the dual challenge of 'correct exposure' and the need for a consistent screen brightness from scene to scene. The solution to this problem was to come by courtesy of incident light measurement. ncident light It is claimed that the first commercial incident light meter was the Avo-Smethurst incident meter: Smethurst, by the way, was once acting editor of BJP. The meter used a flat diffuser over the normal light-sensitive cell Ð known today as the 'cosine type receptor'. Given the universal limitations that attended all meters in the day, this was fairly successful in front-lit situations (but poor at other lighting angles). Later it was found that a better performance in side lighting could be obtained with the flat cell meter by use of the 'duplex method', where the exposure given is the average (geometric mean) of two separate readings covering the main and fill-in light sources. In America, the Norwood Director exposure meter (later known as the Brockway and eventually as the Sekonic) was an early and successful attempt to solve both motion picture needs and side-lighting problems. The Norwood used incident light measurement employing a translucent hemisphere over the meter receptor cell. The hemisphere is actually a three-dimensional model of 'real world' objects, light reaching the inside of which is used to estimate exposure. The performance of the hemisphere is said to be 'compensating' because it integrates light received from a multitude of directions associated with both the main light and fill-in sources. Returning to stills photography 40 years ago, exposure estimation was particularly difficult in back-lit situations even with an incident meter such as the Brockway. Such scenes have an inherently high contrast range, which is often greater than that which the film can properly handle. Nonetheless, shots of this type were considered stunning Ð provided that the appropriate exposure could be psycho-photographically balanced. That great innovator Jack F Dunn experimented with a system that enabled a standard meter to achieve such a balance in back-lit conditions. Early in the period, workers aimed at obtaining the 'correct exposure' in the highlights, as is evidenced by the Avo-Smethurst meter being known as the 'High-Light'. Later it was realised that a better balance is achieved using a mid-tone bias, where the highlight and the shadow portions of the scene are equally sacrificed. Dunn presented a paper at the Nineteenth Hurter and Driffield Memorial Lecture in 1957 at the Royal Photographic Society: that work, Developments and Trends in Practical Exposure Determination, outlined how a mid-tone bias could be obtained using back-leak compensation (BLC), and led to a collaboration with D Connelly, of Sangamo Weston, in the commercial realisation of the Invercone (V). This was the most innovative period in exposure meter development, which has sadly lain dormant ever since. Instrument technology has improved vastly due to electronic advances, but makers have not developed their sensing techniques. That the Weston Invercone is still held in such high regard is significant, and the writer knows of no meter of its type available today that is its equal. reliminary check It was decided to unfold the mystery behind the Invercone by an experimental method that would compare its performance with a modern meter, in this case the excellent Sekonic 308b. A test-rig was made so that the response of a meter could be accurately measured against three light condition variables: those of sun elevation, meter orientation and lighting angle. It is not fair to compare readings from the Sekonic meter directly with those from the Weston meter as the latter uses an out-dated galvanometer movement (which is not linear). It was therefore decided to adopt the Sekonic to accept the Invercone attachment. Importantly, a graph of the indicated exposure versus the sun position relative to the meter produces essentially the same shape curve for both the Weston and the Sekonic, so validating the test plan to use the Invercone on the Sekonic meter. nderstanding BLC Because the Invercone is larger than the meter body (see product illustrations), the main source light (sun) can reach the Invercone receptor even when it illuminates from behind the meter. The light so collected scatters within the cone, eventually reaching the cell and reducing the meter's tendency to call for excessive exposure in back-lit situations. It is sometimes said that the Invercone behaves in the same way as a hemispherical receptor, but this underestimates Weston's design. Quite apart from the BLC effect, less receptor area is presented to the sun in side lighting than with a hemisphere, so calling for slightly greater exposure when the main light is at an angle of 40-80¡. The same pair of curves that reveal this result also exhibit very different behaviours for the 90-180¡ region, where the Invercone (blue curve) exhibits increasingly less droop than the Lumisphere (red curve). The extent of the droop difference is approximately 1EV. eter angle If any specific advice is given at all, most manufacturers appear to suggest that an incident meter should be held vertical (with the cone axis parallel to the ground). Yet, many photographers have found that in certain situations better results can be obtained by holding the meter with its hemisphere pointing towards the sky at an angle of about 30-45¡. To an extent, there is evidence that this gives some correction for the absence of BLC but, as so often in photography, it is not as simple as that. The accompanying three-curve graph shows the reduced droop that is obtained when the meter is held at an angle of 45¡ (as well as an interim angle of 22¡ for the Lumisphere). The reader is urged to compare the starting points for both graphs, and to observe that the basic reading for frontal lighting is not affected by tilting the meter, but the 180¡ back-lit condition now shows that the red curve for the Lumisphere has changed from a droop of 2.9EV to a mere 0.9EV. By contrast, the Invercone was less deceived by holding the meter at 45¡, reducing in droop from 1.9EV to 1EV. Even tilting the Lumisphere just 22¡ produces a greater shift than this (1.6EV overall). nvercone advantage It is significant too that the red and blue curves now cross over, because the Invercone has done a better job of preserving appropriate back-light exposure while the Lumisphere has received a general over-stimulation. This demonstrates that the properties of BLC and cardioid compensation are not the same, but their effect on exposure estimation can be similar in the appropriate lighting conditions. The reader may wonder what is going on here and ask whether this difference is rather academic. Before coming to an early conclusion, study should be made of the third graph here, which is for a low sun elevation of 30¡ with, by definition, an increased the lighting ratio. The red curve (Lumisphere) now shows a massive droop of 3.2EV while the blue curve with the Invercone exhibits a smaller droop of 2.1EV. It was previously shown that holding the conventional cardioid meter at 45¡ gives an excessive boost to the back-lit reading, but holding the meter at half that angle, approximately 22¡, has yielded a general droop of 1.5-2.1 EV, which is more realistic. est summary A number of summary results can be drawn from these tests. Firstly, BLC reduces the tendency for meters to cause over-exposure in back-lit situations. Secondly, as the sun's elevation reduces and contrast increases, the self-compensating effect of BLC is increased Ð so avoiding over-exposure in this critical phase. Thirdly, the need for and effect of BLC both increase with the lighting contrast. Fourthly, by holding a standard meter with a hemispherical (Lumisphere) receptor at an angle of about 22¡, a similar (but not the same) effect to that of BLC can be obtained. Finally, due to the cardioid behaviour of a hemispherical receptor, no significant change in the basic meter exposure indication is found for frontal lighting when a meter is used at 22¡. hotographic Results To illustrate these observations, a high contrast back-lit scene was used. In the trio of pictures, the sun is the main source while fill-in skylight is hindered by tree and building cover so contriving to increase the contrast. The exposures were estimated by use of the standard Sekonic 308b (non-BLC) meter with a Lumisphere receptor held at the angles indicated in the captions. The pictures demonstrate that a meter attitude of approximately 22¡ in low sun back-lit scenes gives the best correction for this meter. This supports the predictions drawn from the graphs. In a more typical back-lit condition (the pair of pictures), the reading from a vertically held meter once again shows over-exposure, whereas when the meter is held at 45¡ there is a lesser amount of under-exposure. Although the meter angle of 22¡ (not shown) gives a slightly better result, there is less difference between the 45¡ and 22¡ orientations due to the reduced lighting contrast ratio prevailing in this back-lit scene. ractical Implications Except for perhaps the most skeptical reader, it is clear that BLC is very desirable in strongly side-lit and back-lit conditions as it has the action of promoting mid-tone bias to the exposure. This means that the meter places the exposure close to the transparency film speed-point density, and that deep shadows and white-on-white highlights are equally sacrificed if the range is long. This is a case where technicality and aesthetics must be in harmony, and if that balance is achieved the results will look convincing. This approach is also valid for colour negative film (despite its difference in speed point determination), both from exposure estimation and capture quality considerations, as it assists good shadow gradation and colour balance in this region. It is now possible to see a basket of reasons for users' claims that exposure estimation is better when tilting the meter receptor upwards. In the light of these observations the writer is of the opinion that most workers will see an improvement in exposure estimation if the following techniques are used... 1) In cloudy, flat or non-directional light, hold the meter vertically. 2) When working wholly within the shadow area of a building, use the meter vertically. 3) In sunny, white sky, conditions with light coming from side-to-back (the 90-180¡ range), use the meter at 22¡. 4) In all high contrast, sunny blue sky, situations, hold the meter at 22¡. 5) In low sun elevations (below 45¡), hold the meter at 22¡. he Logical Question Readers may wonder why, if BLC is so beneficial, do makers not exploit it? The answer is not the same for all makers of course, but in the writer's view there are three factors for this omission; a regrettable lack of new development in meter sensing techniques, over-indulgence of instrument technology and slavish compliance with national and international standards. On the last point, it is all too easy for makers to hide behind standards, and for standards committees to perpetuate the status quo Ð as Connelly discovered. It was only after great effort that he eventually succeeded in getting the cardioid receptor introduced in the American Standard, but he did not succeed in getting the property of BLC so embodied. Without doubt, this would have been seen as too brand-specific. It was a loss to us all. Source:BJP