Long exposures film or digital? When film exposure becomes greater than one second, you begin to enter the world of long exposures. Apart from the need to steady your camera, usually with a tripod, images created from long film exposures start to behave differently to shorter “regular” exposures and this requires a modified approach to successfully anticipate their outcome. Take the above example of three images which I made late one evening along the Lefroy Brook near Pemberton. The sun had set and the brook was deep in shadow with the light coming from directly above in a break between the karri forest canopy. I was exploring along the shallow pools of the brook which was now flowing at a pre summer low. The river’s bedrock was exposed in parts, but where the rocks remained wet they were slippery and they trapped the fallen leaves from the forest in a scattered pattern across their dark surfaces.
I set up my tripod and 4×5 wooden field camera. The best composition and most comfortable subject working distance was achieved with my Nikkor 210mm lens. I was using a Horseman 6×12 roll film back loaded with Velvia transparency film, rated at 50 ISO. The 210mm lens is slightly longer than normal focal length for 4×5 (and certainly for 6×12). It’s depth of field is shallow and with a close subject this characteristic becomes magnified. If I focused on the nearby leaves the remaining image was so blurry you lost the pattern of the leaves and their relationship with the moving water. If I focused on the far leaves, the foreground image suffered the same blurring, and again I lost the visual relationship I was after. Stopping the lens aperture all the way down to f64 is one way to increase this depth of field, but at the cost of introducing unwanted softness through diffraction. Even at f64 I could not achieve enough depth of field for the image I had in mind. One of the major advantages with the 4×5 is that I am able to alter the plane of focus so that a greater perception of sharpness is achieved in both the foreground and background. Coupled with a moderate aperture of about f22 this would afford me just sufficient depth of field.
But the rapidly fading light was adding to my problems. Not only was it becoming so dark that seeing any image on the ground glass was becoming seriously difficult (it’s already upside down and back to front), but the exposure was going to be long. According to my spot meter the brightest leaves had an EV of about 6, the darkest about 4, representing a 2 stop difference. The white water was slightly higher at around 7 EV. The base rock was mostly very dark and barely registering on my meter with EVs in the 2 to 3 range. My meter indicated an exposure of about 1 minute would be required at f22 with ISO 50 film. I added another 20 seconds to account for lens bellows extension and then I immediately doubled that to about 2 minutes 40 seconds, to take into account my experience with the reciprocity effect in long exposures with Velvia film. In addition to reciprocity, I knew that there would be an increase in contrast in the final transparency compared to what I was seeing on the ground glass which I wanted to use to my advantage. By the time I had completed all three images in the fading light my exposures were getting up into the 5 minute range.
Film has to receive a base threshold amount of light energy before it can begin to record an image from very dark regions within an image. The contrast of the film increases in long exposures because, proportionately, less of the light energy received by the film in the shadow or dark regions of the image is available for image making, compared to a greater portion of image making light energy received from the brighter areas within the image. That is to say that even though the film receives reflected light from both light and dark objects for the same amount of time, the resulting film density is not proportional in both light and dark regions, as the image highlights activate disproportionately more image forming silver halide crystals than the darker, shadow regions. This characteristic forms part of a film’s sensitivity curve: a curved “toe” at the base, a relatively straight line mid section and a curved “shoulder”. Each film has its own exposure index /curve/ developer combination. In this low light situation photographing Lefroy Brook, Velvia’s contrast response to long exposures matched my vision to bring attention to the leaves whilst maintaining a dark background.
Had I used a digital camera to attempt these images the resulting raw files would have been quite different to film, there would have been an almost normal appearance of the brook and leaves as if it was photographed in full daylight. Low light exposures with digital cameras sensors coupled with their particular algorithms can give a greater perception of overall scene brightness, indicating a much more proportional exposure response than film (ie more straight line than toe or shoulder). From my experience, this is one instance where film, by virtue of its inherent characteristics, behaves quite differently to digital. In this series of prints of Lefroy Brook I was able to achieve, in camera, with transparency film, the desired contrast of a long exposure and the necessary adjustment of the plane of focus afforded by a 4×5 field camera.