The cult of exposure
As a photographer it’s important to understand the difference between exposure and lightness
Exposure is a very easy topic to have an argument about in photographic circles. This is a little strange because it is a basic tenet of photography, and has been since Ferdinand Hurter and Vero Charles Driffield undertook their pioneering work in the late 19th century to establish the science of sensitometry, which characterised the response of photographic emulsions to light, producing a characteristic curve, which in film days plotted the density of the processed film against exposure.
Exposure is the density of the light energy at the focal plane which, since the advent of Système Internationale units, is measured in lux seconds. Since then, film characteristic curves have been presented in a somewhat standardised way with exposure plotted on the x-axis (generally on a logarithmic scale) and density on the y-axis. It is the exposure at which the density reaches a specified value (known as the ‘speed point’) that defines the speed of that emulsion/ development combination. Since the advent of digital photography, the notion of density no longer applies, and to provide an analogous system of speed, has been replaced by a specified value in the output file. That value represents the lightness and darkness in the output colour space and in colour science goes under the name of ‘lightness’. The speed setting on a camera represents the relationship between exposure and the lightness of the output image.
So, why do discussions on exposure become so contentious? I think that there are several related reasons. If one looks at popular web-based tutorials it is clear that they fail to distinguish between exposure and lightness, and a generation of photographers has developed believing the two to be synonymous. Once the distinction is lost, it becomes impossible to understand the way in which exposure affects the final nature of the image. Exposure management then becomes more akin to a black art, instead of being a matter of method based on science. People talk about ‘crafting’ a ‘correct’ exposure, which since exposure is a simple measurement of a physical quantity – that is, light energy density – is much akin to talking about crafting a correct temperature. To my mind, the very idea that there is a correct exposure is somewhat misguided. When choosing which exposure to use, one is generally deciding on the best compromise between a number of competing factors.
Choosing an exposure
As an example, I use the photograph shown above. The exposure for this shot was set after consideration of a number of factors, none of which were the required lightness of the output image. My intention was to use panning to provide a linear blur to the background to create an impression of speed. This required an exposure time of 1/100sec in order to achieve the effect I was looking for. The light was bright, which meant that, in turn, the f-number was set by the maximum exposure that my camera will accept, using its lowest ISO setting of 64. In general, if one wants to maximise image quality, the general rule is to use the highest exposure you can, subject to pictorial constraints on f-number and shutter speed. In this case, given that I was limited by the exposure the camera can capture, this was not an issue.
‘ To my mind, the very idea that there is a correct exposure is somewhat misguided’