Time is an essential parameter in photography. It can be looked at from a technical or physical, an aesthetic or even a philosophical point of view. Photography (and painting) is also different from other arts in that once it is produced, it doesn’t have an intrinsic duration. In contrast, a movie, a piece of music, a ballet or a play all unfold in time and do have a duration. But in the process of making a photograph, the photographer has to make decisions regarding time and these decisions reflect upon his or her intentions. This series of posts will first look at these different aspects of time in photography. In addition it will visit the topic of time as a subject of photography, which means: “Can we photograph time?”.
Technical Aspects of Time in Photography
Photography is recording of light. As light is either emitted or reflected by objects in form of photons (light-particles/waves), a camera exposes a photosensitive material (either film or a digital sensor) to these photons.
Digital sensors in modern cameras are devices that contain (millions of) sensing pixels. Each pixel is a photon counting device. A common analogy is that of a bucket in the rain, where the rain drops represent the photons. After a pre-selected amount of time the bucket is emptied and the collected amount of water measured. In a camera that pre-elected time is called exposure time and realized by a curtain in front of the sensor (the shutter) that is opened and closed within a selected time interval (the shutter speed).
In order to control how much light is able to reach the sensor, a camera employs a variable aperture in the lens. In terms of our bucket analogy the aperture is like a cover with an opening laid over our bucket . Depending on the diameter of the opening more or less water will enter the bucket. In the recorded photograph, pixel sites that counted few photons will appear dark, those that counted many will appear light.
As we can see, there is an important relationship between the amount of light and the exposure time: 100 photons per one second reaching the sensor amount to the same photon count as 10 photons per second counted for 10 seconds. In both scenarios the pixel will count 100 photons and so both exposures will result in the same brightness value. But the interesting part for the photographer is that both exposures will only result in the same picture if nothing in the scene moves. As soon as elements in the scene move, photons reflected by moving objects will reach different pixels on the sensor during the exposure time and the movement will be visible in the photograph.
In addition, the pixel has no means to determine when or from where a photon arrived during the exposure time. The pixel is thus effectively averaging incoming light over the exposure period. That has the peculiar effect that during very long exposure times objects that enter the scene only for a short time will not be recorded/noticed at all – they “drown” in the average. A famous example for this effect is visible in a photograph from 1838, Daguerre’s “Boulevard du Temple” shown at the top of this post. The photograph depicts a busy street in Paris, but as the exposure time was more than ten minutes all the moving traffic has not been recorded. Only the man having a shoe shine stayed long enough to be visible.
Exposure time is therefore a powerful tool in the hand of the photographer. In the next part of this series I will explain how it can be employed to shape and realize artistic intent. So stay tuned and see you then!