Long Exposures: Film Reciprocity Failure
One of the things that I love about photography is the ability to capture images you cannot see with the naked eye. I particularly love long exposures of seascapes, with silky smooth water and streaky clouds.
Cronulla rock pool
Reflecting on my journey, I recall capturing this image in Cronulla over a year ago with my trusty digital SLR. However, the transition to film photography brought forth a new challenge. If I were to recreate this shot on film today, I'd still rely on a tripod and a cable release. Yet, the crucial difference lies in the need to adjust the exposure. Film and digital, with their distinct qualities, demand a different approach, adding a layer of intrigue and engagement to the process.
Let’s delve into the key concepts of exposure, a fundamental aspect of photography. Understanding this concept empowers you to control the amount of light that enters your camera, thereby influencing the final image. The basic principle is simple: halving the area of your aperture opening (increasing your f/stop by one stop) necessitates doubling your exposure time (increasing it by one stop) to maintain the same amount of light, resulting in a consistent photograph.
For example, we have just photographed a rock on the stream's edge, and the correct exposure for the photograph was f5.6 at 1/8sec. Suppose we want to show more of the landscape in the shot. To do this, we would wish for a greater depth of field, say f16. To take a shot at f16 (which is three stops away from f5.6 (f5.6, f8, f11, f16), we would then want to let in three more stops of light which would give us a shutter time of 1sec (1/8, 1/4, 1/2, 1).
Relationships like this are said to be reciprocal. If we close the aperture down to a stop, we halve the amount of light, which we compensate for by doubling the stutter time. So, we have done 2*1/2=1, so there is no overall change in the image's exposure.
Reciprocity failure, a term often encountered in film photography, refers to a deviation from the expected relationship between aperture and shutter speed. It's a byproduct of how film records light. At its core, film photography is a chemical reaction, with light providing the energy for the reaction to occur. This failure, inherent in all films to varying degrees, typically manifests around the one-second mark. However, it's worth noting that Fuji Across 100 stands as a notable exception, not succumbing to any reciprocity failure for exposures under 2:00, enlightening us about the intricacies of film photography.
I liken it to shaking up a soft drink bottle and then opening it, and the reaction starts explosively and slowly drops away. So here you have given the bottle energy to perform the reaction, like having a lot of light hitting the film. Now, if you were to open the bottle without shaking it, you would get a little bit of fizz to start with, then a slow release of gas that will take a long time to release the same amount of gas as when you shake up the bottle, which is similar to how reciprocity failure works.
We can adjust the exposure time for long exposures (longer than a second) to compensate for the reciprocity failure; if we do not, we will end up with an underexposed photograph.
Below is a table with adjusted exposure times compensating for the Reciprocity Failure. These times are given in the data sheets for each film. The Ilford sheets have a graph showing the adjusted times, while Kodak and Fuji have a table showing how many stops to adjust your exposure. So, all these times are for a meter reading with ISO 100.
| Meter | Across | Tmax Delta |
Tri-X HP5 |
T-Max Delta 400 |
|
| 1 | 1 | 1 | 1/4 | 1/4 | |
| 2 | 2 | 2 | 1/2 | 1/2 | |
| 4 | 4 | 6 | 1.5 | 1 | |
| 8 | 8 | 12 | 4 | 2 | |
| 15 | 15 | 30 | 12 | 6 | |
| 30 | 30 | 1:00 | 36 | 12 | |
| 1:00 | 1:00 | 2:00 | 1:30 | 30 | |
| 2:00 | 2:00 | 5:00 | 3:30 | 1:00 | |
| 4:00 | 6:00 | 10:00 | 8:00 | 2:00 | |
| 8:00 | 12:00 | 20:00 | 18:00 | 5:00 | |
| 15:00 | 25:00:00 | 50:00:00 | 40:00:00 | 10:00 | |
| 30:00:00 | 45:00:00 | - | 1:30:00 | 20:00 | |
| 1:00:00 | 1:30:00 | - | - | 3:30:00 | |
From Night Photography by Lance Keimig
Now, just looking at this table, one might be led to believe that the best films to use for long exposures would be Kodak T-Max or Ilford Delta 400, as they have the shortest exposure times of any of the films given a meter reading at ISO 100. However, there is another side effect that occurs with Reciprocity Failure, and that is increased dynamic range.
It's important to note that there's a creative aspect to consider when aiming for a broader tonal range in your photograph. Long exposures can automatically provide this, opening up new possibilities for artistic expression.
Let's see where this increased dynamic range comes from. To address these cons, consider the following example. We have judged that the correct exposure for our shot is 4 seconds at ISO 100; the shadows are sitting at 16 seconds (+2 stops), and the highlights are sitting at 1 second (-2 stops). Here, we would have the dynamic range of 4 f stops of light. The table below shows the time for the exposures of the highlights, mid-tones, and shadows, as well as the dynamic range in f stops of light (rounded).
| Film | -2 stops | On Meter | + 2 stops | Range in F stops |
| Fuji Across 100 | 1 | 4 | 15 | (2+2)=4 |
| T-Max/Delta | 1 | 6 | 30 | (6+2)=8 |
| Tri-X/HP5 40 | 1/4 | 1.5 | 12 | (2+4)=6 |
| T-Max /Delta | 1/4 | 1 | 6 | (2+6)=8 |
The original meter reading would have had a total range of 4 stops. Here, we can see that only Fuji Across 100 keeps its tonal range while the others expand. The tonal range will only increase from here, so losing details in the shadows or blowing the highlights is pretty easy if you are not careful. Now, not all is lost, as there are still two ways you can compensate for this: use Across 100 or adjust your development time or concentration.
My experience so far has been limited to using Fuji Across 100 for long exposures, and as long as they are under 2 minutes, there is no need for compensation. However, I am eager to delve deeper into the topic of controlling the tonal range on the negative through compensating development. Stay tuned for a future blog post on this.
Note: In this post, I have only considered B&W film photography. When you start to use colour film for long exposures, you may experience extra difficulty when you experience colour shifts. This can happen because beach colour film contains three light-sensitive emulsion layers, which sometimes have different reciprocity properties. This colour shift can be corrected via colour correction filters. Each film's data sheet should contain the specific filter that should be used.
