Long time exposures begin when your film exposure times have a duration longer than one second. Most film manufacturer’s speeds printed on the box are only good for exposures in a normal range. Within the normal exposure range, the total film exposure is a product of light intensity and duration of exposure.
When film exposure times have a duration greater than one second, you begin to enter the world of long exposures. Most film speeds printed on the box are only good for exposures up to 1 second long. Any longer and the effective film speed may start to deviate markedly from the manufacturer’s speeds. That’s because film exposure, which is a product of light intensity and time, starts to behave differently with But, exposure times shorter than one ten thousandth of a second, or longer than one second, fall outside the normal range. When you step outside normal exposure times, film behaves differently. The total film exposure is no longer the simple product of light intensity and time. You have to give even more exposure time than indicated by a light meter.
This change in film exposure behaviour is called reciprocity failure. You need to adjust your exposure times by adding in reciprocity failure when you are making long exposures.
Lefroy Brook Series Long Time Exposures
I used long time exposures on film to make these three images of Lefroy Brook, near Pemberton. It was well after dusk. Below the karri forest canopy and the brook was deep in shadow. Shallow pools of water on jet black rock was lit by smooth, overhead light.
The water level was flowing at a pre-summer low. River bedrock was exposed in parts, but where the rocks remained wet they were lightly coated with algae. The algae’s stickiness trapped the fallen leaves from the forest canopy far above. Across the rock’s dark surfaces random patterns of leaves had formed, shaped by the gentle flow of water.
Choosing 6×12 Roll Film Back over 4×5
The main point of interest in my composition would be the leaves’ colourful patterns against the dark rock tones. I chose the best position to set up my tripod and 4×5 wooden field camera. The leaves are comparatively small and I need to get in tight with my composition. However, I can’t get too close. If I place my camera right over the subject, then it affects the camera to subject angle and changes the light’s reflective quality. Another problem with being too close to my subject is that there is no safe tripod position. I had to choose a longer focal length lens than normal.
The best composition and most comfortable subject working distance was achieved with my Nikkor 210mm lens. But I did not want a full 4×5 frame for the image. The long 6x12cm format best suited the composition I had in mind. I was using a Horseman 6×12cm roll film back loaded with Velvia transparency film, rated at 50 ISO.
Problems with Long Lens Compositions
The 210mm lens is slightly longer than the normal focal length for 4×5, and certainly for 6×12cm. Its depth of field is shallow and the closer the subject, the more shallow it becomes. 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.
Likewise, 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.
Large Format Camera Movements
One of the major advantages of the 4×5 is that I am able to alter the plane of focus. I could do this by utilising my large format camera movements on my Wista 4×5 camera. Doing so achieves a greater perception of sharpness in both the foreground and background. Coupled with a moderate lens aperture of about f22 this would afford me just sufficient depth of field. I now had the composition I wanted on my ground glass. But what about exposure?
Calculating Time Exposures
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 EV 4, representing a 2 stop difference. The white water was slightly higher by one stop, at around 7 EV. The base rock was mostly very dark and was 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.
Time Exposures and Energy Threshold of Film
Photographic film must receive a base threshold amount of light energy before it can begin to record any tones or detail 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. This is compared to a greater portion of image-making light energy received from the brighter areas within the image, and therefore recorded rapidly by the film.
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. The highlights activate disproportionately more image forming silver halide crystals than the darker, shadow regions. This characteristic forms part of a film’s sensitivity curve. It can be described as a curved “toe” at the base, a relatively straight line midsection and a curved “shoulder”.
Every film has its own exposure index, film curve and developer combination. In this low light situation photographing Lefroy Brook, I knew Velvia’s contrast response to long exposures matched my vision to bring attention to the leaves whilst maintaining a dark background. My experience with Velvia extended over several decades as the only colour film I used. Sometimes it pays to know a film well, understand all its quirks. When you regularly swap from one film to another in an attempt the find the best one, you run the risk of knowing very few films well.
Long Exposures on Film is Different to Digital
Are long time exposures on film different to digital imaging? In my experience, yes.
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. If you related that to a film curve you would see more straight-line than toe or shoulder.
An advantage of digital is that sensor ISO sensitivity can be adjusted many times higher than the fastest film. I am guessing this has more to do with the computer algorithms within the camera than the actual sensor. However, as ISO increases so does Noise, a digital artifact that affects image quality. Advanced digital cameras do have a noise reduction program.
One curious observation I have made with the few digital cameras I have worked with is that the in-camera digital image processing time can be as long as the original exposure. So, on a long time exposure of several minutes the camera remains unusable until it has process that exposure – several minutes later.
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.
Print Only – Posted Worldwide
Lefroy Brook SF21, Pemberton unmounted photographic print on photographic paper. Carefully packaged in a postal tube, insured and posted worldwide for $450.
Lefroy Brook SF23, Pemberton unmounted photographic print on photographic paper. Carefully packaged in a postal tube, insured and posted worldwide for $450.
Lefroy Brook SF31, Pemberton unmounted photographic print on photographic paper. Carefully packaged in a postal tube, insured and posted worldwide for $450.
Lefroy Brook Series Set of 3 Prints Only – Posted Worldwide
Lefroy Brook Series SF21, SF23, SF31, Pemberton unmounted photographic prints on photographic paper. Carefully packaged in a postal tube, insured and posted worldwide for $1250.