The Chemistry of Film Photography

By Struan Caughey

Introduction

In December 2020 Nikon, over 100 years after its founding, discontinued its final film camera, the D6. This was due to the market dominance of digital cameras and the company shifting full focus to DSLRs and mirrorless cameras. While the technology behind film photography is dated, the art form has been making a resurgence [1]. This has led to companies reemerging in this space, such as Kodak re-releasing Ektachrome film back in 2018 after having discontinued it not six years earlier [2] and potentially having another two releases this year [3]. Polaroid is another company that in 2008, announced that they were ceasing sales of all their instant film products, citing financial issues [4]. This led to three polaroid enthusiasts purchasing the film production equipment from the business to set up their own company, naming it the ‘Impossible Project’ [5]. They went on to buy Polaroid’s trademark, and after several name changes, they rebranded to just ‘Polaroid’ in March 2020, producing new cameras once again [6].

Having been given my grandmother’s film camera in 2019, a 1970s Pentax ME, my own interest was piqued, for much the same reasons as other people: the simplified process of shooting, the nostalgic look of the photos, but especially to me, for the scientific magic going on within this small unit. Despite digital cameras being far more complex, I am studying a BSc in Computer Science, so the analogue magic behind these old cameras is much more of an enigma for me, resulting in the rabbit hole which culminated in this piece.

There are three main chemical stages of producing film: manufacturing, shooting, and development — all three of which are very light sensitive and precise. They also vary for the three main types of film: colour negative, slide, and black and white. We will primarily focus on colour negative film and the C-41 developing process for this article.

Film photograph of Te Tirohanga o te Tōangaroa Hall. Photo by Struan Caughey

Film Construction

A basic colour negative film has seven layers to it, however, with many film stocks you can often find additional layers or the combination of multiple layers into one. The standard stack is as follows [7]:

1. Gelatin protective layer

2. Blue-sensitive silver halide with a yellow forming dye coupler

3. Yellow filter

4. Blue and green-sensitive silver halide with a magenta forming dye coupler

5. Blue and red-sensitive silver halide with a cyan forming dye coupler

6. Antihalation layer

7. Base You do not have all the separate colour layers for black and white film, so the assembly is much simpler.

Each Layer’s Purpose

The gelatine is there to protect the film from scratches and damage.

Layers 2, 4 and 5 contain silver halide crystals that are photosensitive. The photosensitivity is caused by an electron being excited by a photon, moving it into a conduction band. From here the electron can be attracted by a ‘sensitivity speck’ on the surface of the crystal to form metallic silver, which is the latent image. The sensitivity speck will often be a defect, other material, or an electron trap within the crystal which makes this part more sensitive. A similar but reversible technique is used for glasses that shade in bright light. The speck on these crystals can then be utilised in the development stage along with a dye coupler that reacts, making the layer a given colour. This forms a negative of the image taken (light areas will appear darker and vice versa). This will be expanded on in the development section [8].

When a photon hits the film, it passes to a specific layer depending on its frequency. For photons in the blue spectrum, they will interact with Layer 2, which is the blue-sensitive silver halide layer. The blue photons which are not absorbed will get absorbed by the yellow (complementary colour to blue) filter, layer 3. Red and yellow photons will proceed to the lower blue/green and blue/red colour sensitive silver halide layers respectively, with the blue light having been filtered out.

Layer 6 is an antihalation layer. This absorbs all the light which has been transferred through the previous layers to prevent reflection of the camera body, which could lead to image artefacts.

The final layer is the film base, which gives structure and rigidity to the film as well as acting as a protective layer from the back.

Taking the Photo

The size of the silver halide crystals has an effect on two things when taking a photo, the first of which is the film’s sensitivity. The larger the crystals, the less exposure to light that the film needs to render an image. This can be referred to as the ‘speed’ of the film, and can be quantified through measures such as ISO or the older ASA. The higher the ISO, the larger the crystals, the faster the camera can shoot, and the darker the conditions can be while forming a usable picture. The second effect and the downside to this is the graininess of the image. When each crystal of a high ISO film is activated, a larger area will become coloured. This is where the term ‘grainy’ comes from in photography, literally from the size of the silver halide crystals or ‘grains’. This can be seen as artistic by some photographers; however, most would prefer to avoid this look [9].

To prevent accidental exposure of light to the film, it is kept in a light-proof cartridge. When loaded into the camera the first two photos are often intentionally exposed while loading, after which the housing, which is lightproof, is closed to protect the rest of the roll. To remove the film, we reverse the film back into the cartridge before opening the housing, ensuring our photos are not spoiled.

Developing the Photos

Once we have our film with the latent images imprinted on them, we need to make the film stable in daylight. This requires several chemical reactions to complete, which we will break down. Several processes can achieve this, each of which behave differently. Two of the main ones are C-41 and E-6. E-6 is designed for colour positive slides, whereas C-41 is for colour negative film. Each film is designed for a specific process; however, you can sometimes use the non-stated process in a technique called cross-processing. For this, we will just look at C-41.

The first thing required when developing film commercially is to have a darkroom. This will either be wholly black or contain a safety light, which emits light that the film is not sensitive to. The film will proceed to go through six steps [10]:

1. Presoak

2. Developer

3. Bleach

4. Fix

5. Wash

6. Stabilizer

7. Dry

During this time, the process will have to be at 39°C ± 1°C as there are multiple reactions going on at differing depths within the film. If the film is too hot or cold then you may find certain colours develop more or less than others, yielding undesirable results. Because of this, the first process is the presoak. This is in 39°C water and will act to both clean the film and bring the film up to temperature.

Next, we have the most important step. This is the developer one variant of which is comprised of a paraphenylene diamine-based chemical known as CD-4 [11]. A reaction occurs between this chemical and the silver halide crystals, turning them into silver metal. Those crystals that have already got silver in them (due to an incident photon having impacted it) will be catalysed, so they should develop faster and darker. This oxidises the developer, and the oxidised developer reacts with the dye coupler, resulting in the colour forming dye coupler turning from clear to the desired colour.

The film is then removed from the developer and it is placed in a bath of bleach. This reacts with the silver, reforming it into silver halide, which can be dissolved by the fixer. Some people will skip this step, leaving the silver crystals in the film. These will not be dissolved by the fixer, resulting in a black and white image on top of the regular colour image [12].

The fixer is composed of several chemicals that strip the silver halide from the film, leaving just the coloured dyes and silver metal behind. Sometimes the bleach and fixer are combined into one bath, known as Blix [13]. This is more common for at-home kits than in commercial operations.

This is followed by a wash to remove the existing chemicals from the film before a stabiliser is used. The stabiliser used is often formaldehyde; however since the late 90s most film includes the stabilisation process within the film itself so is often omitted [14]. The purpose of this was to stabilise the dyes, harden and clean the film, as well as place a hydrophobic coating on it to prevent watermarks. There are still some stabilisers used but this is an optional step and only cleans the film and waterproofs it.

Lastly, the film is dried in a low dust area before being scanned, enlarged or stored.

Overview

To summarise the process, a photon hits the film, exciting an electron within the silver halide and causing it to ionise into silver metal, which makes an invisible latent image. This film is then developed, turning the silver halide crystals into silver, oxidising the developer. The silver halide crystals which have been impacted by a photon already have some silver in them that catalyses this reaction, so these areas will develop first. This oxidised developer will then be able to react with the film’s dye coupler to form a negative image. The film is then put through bleach to convert the silver back into soluble silver halide, before all the silver halide is removed using the fixer. Lastly, the film is washed and stabilised before being dried. Now you have a negative of your image ready to be scanned!

Having now understood the process, the magic of film photography is even greater. The complex set of equations required to turn a latent image in silver crystals into a computer desktop background is truly fascinating. As film gains in popularity, changes in this process may still be on the horizon and I cannot wait to see where this space develops from here.

References

[1] K. Fung. “Exploring exactly why film photography is on the rise” DailyCal.org. https://www.dailycal.org/2020/11/08/exploring-exactly-why-film-photography-is-on-the-rise (accessed July. 21 2021)

[2] “To the Delight of Photographers and Filmmakers Everywhere, New EKTACHROME Films to Begin Shipping” Kodak.com.https://www.kodak.com/en/company/press-release/ektachrome-film-begins-shipping (accessed July. 21 2021)

[3] SilvergrainClassics. “New Kodak Products in 2021? + The Year in Review: SilvergrainClassics Fireside Chat #1 29.12.2020” https://www.youtube.com/watch?v=nwdwdcOG4QU (accessed July. 21 2021)

[4] A. Clark. “Polaroid files for bankruptcy protection” TheGuardian.com. https://www.theguardian.com/business/2008/dec/19/chapter-11-corporate-bankruptcies-corporatefraud (accessed July. 21 2021)

[5] M. Zhang. “Polaroid Acquired by The Impossible Project’s Largest Shareholder” PetaPixel.com. https://petapixel.com/2017/05/12/polaroid-acquired-impossible-projectslargest-shareholder/ (accessed July. 21 2021)

[6] Polaroid [@Polaroid], “This is Polaroid — now...”, Twitter, Mar 17, 2020. https://twitter.com/Polaroid/ status/1243292725065199616 (accessed July. 21 2021)

[7] “Colour-film structure” Britannica.com. https://www.britannica.com/technology/technology-ofphotography/Colour-film-structure (accessed July. 21 2021)

[8] M. Witten. “The Chemistry of Photography” (2016). Senior Theses. 84. https://scholarcommons.sc.edu/senior_theses/84 (accessed July. 21 2021)

[9] N. Snape. “Is grain or noise a bad thing in photography” NeilSnape.com https://neilsnape.com/is-grain-or-noisea-bad-thing-in-photography/

[10] Kodak. “KODAK FLEXICOLOR CHEMICALS” https://imaging.kodakalaris.com/sites/default/files/wysiwyg/ pro/chemistry/z131.pdf (accessed July. 21 2021)

[11] “COMPOUND SUMMARY (4-Ammonio-m-tolyl) ethyl(2-hydroxyethyl)ammonium sulphate” PubChem. ncbi,nlm.nih.gov. https://pubchem.ncbi.nlm.nih.gov/compound/25646-77-9 (accessed July. 21 2021)

[12] A. V. Hurkman, “Color Correction Look Book: Creative Grading Techniques for Film and Video” Chap. 2 ISBN-13: 978-0-321-98818-8. [Online]. Available: http://www.bookref.com. https://www.oreilly.com/library/ view/color-correction-look/9780133818482/ch02.html (accessed July. 21 2021)

[13] “CS41 “COLOR SIMPLIFIED” 2-BATH KIT FOR PROCESSING COLOR NEGATIVE FILM AT HOME (C-41 CHEMISTRY)” CinestillFilm.com. https://cinestillfilm.com/collections/product-catalog/products/cs41- simplified-color-processing-at-home-quart-kit-c-41- chemistry (accessed July. 21 2021)

[14] “Do I need an additional stabilizer bath with the 2-Bath Powder Cs41 Color Kit?” CinestillFilm.com.https://help. cinestillfilm.com/hc/en-us/articles/360025784411-DoI-need-an-additional-stabilizer-bath-with-the-2-BathPowder-Cs41-Color-Kit- (accessed July. 21 2021)