The World of Film Stock: Of Film Speed, Circles of Confusion, and Much More!
After learning about real film cameras like e.g. the Bolex H16 and Arri SR2 and SR3, it is now time to learn about the enticing (and slightly complicated) world of film stock.
As we will be participating in the Nahemi Kodak Awards, we will, as a matter of course, also film with Kodak products and thus start out with their film stock and their specs. We will hence ignore other film stock companies, products, and specs for the time being. If I manage to find time later in the course of this module, I will dedicate another blog entry to the further research of film stock companies.
Filming on actual film stock is considered to be the Holy Grail of filmmaking as it not only stands as a witness to your mastery of the laws of exposure, but also offers a lot of dynamic range, and thus, visual quality in regard to saturation, contrast, and colour range.
The company Kodak offers a wide range of film stock beginning from Super 8, 8mm, 16mm, to 35mm with two and three perforations per single frame length to 35mm and 65mm material. The given measurements in mm thereby represent the width of the film stock. Apart from the black-and-white film stock, all film stocks come as colour-negatives.
In order to discern the various widths of film, Kodak uses four-digit film codes that indicate the type of film. If the number starts with the number five, the film inside the can is 35mm, if it starts with a seven, it marks 16mm, the type of film we will be using for the competition.
When filming with film stock, achieving the correct (or desired) exposure is key. This not only requires you to plan your visual style ahead and in detail, it also requires you to know in what kind of lighting (light temperature) you will be filming and to properly use a light meter in order to achieve the best results.
Depending on whether you film outside (daylight setting) or inside (tungsten lighting), you’ll need to choose a different type of film, based on the film stock’s ISO or ‘film speed’. This film speed describes the film stock’s light sensitivity and thus the speed at which a film will be exposed. It consists of a number and the letters T or D, for either tungsten or daylight setting.
This film speed, however, does not refer to shutter speed or ‘shutter angle’ and how many frames per seconds you choose to film in, but rather the actual reaction time of the emulsion on the film stock when being exposed. The higher the number in front of the letter, the quicker the emulsion reacts to incoming light. The lower the number, the slower the emulsion’s reaction, and the more light the exposed film gets. This is especially important if you want to capture daylight scenes, whereas you would want a faster development (and thus low exposure) to capture night-time or low light settings.
As for Kodak’s film stock, the following options are currently available:
50D – for outdoor scenes and pure daylight settings
500T – for indoor scenes and pure tungsten settings
250D – for fairly low light settings mixed with daylight
200T – for fairly bright settings mixed with tungsten
negative film – for filming in black and white
Regarding the 250D and 200T film speeds, these are covering the middle ground in cases where you have mixed lighting with different colour temperatures. These film speeds are rather forgiving and a good place to start with if you first start out shooting on real film.
But how do you determine the light temperature in every given setting? While you might recognise a tungsten or daylight setting, you would still not be able to determine the exact lighting situation of every given shot. This is where light meters celebrate their grand entrance.
As I already explained in last year’s blog on filming with Super 8, light meters measure the intensity of light. Enter the ISO or film speed of the film stock you are working with along with your shutter speed into your device, measure the incoming light of the area you want to film and the light meter will give you a recommended f-stop to use for your shot, along with a tendency of whether it is closely bordering to a higher f-stop that might be more favourable for your shot.
As you may see, film requires you to know your laws of exposure and to be able to manually control or manipulate it to match your vision and style. Obviously, if you know your trade well enough, you can experiment with the exposure of your film stock and decide against the recommendation of a light meter by underexposing or overexposing your footage by a couple of stops, depending on the desired outcome.
If you seek more information and details in your shadows, you deliberately overexpose the image and correct the overexposed elements in post and if you want more details or information in the light, you underexpose the image and correct the underexposed parts in post. With film, you can often get away with deviating one to two stops from your light meter recommendation, whilst three stops or more would be pushing your luck. But hey! That’s what experiments are for! You can also play around and use daylight film stock for tungsten lighting and vice versa to create various effects.
Now, the use of film stock is not only slightly more complicated than digital film, it is also much more expensive. With roughly £95 per 400f-can of 16mm material (and almost double the price for the same in 35mm), you need to make every available second count. This does not only entail blocking and timing your shots before you actually film a single second of it, it also requires you to calculate how much time you have for every can of film, depending on the shutter speed you decide to film in.
Kodak, thankfully, offers the Film Calculator App for free that allows you to calculate how much time you have depending on the film stock and the length you ordered, as well as the shutter speed you selected. With one 400f-can of 16mm film stock shot at 25 fps, you get a total time of 10 min and 40 sec per can.
I have downloaded the app onto my iPhone together with the Depth of Field Calculator that Kodak offers. But more about that with follow in a bit.
Furthermore, with Kodak, every single frame in every film can is coded and can thus be traced back to its production batch. This is important as it allows the manufacturers to pull a batch or even several cans from the market if an error has been reported with one or several cans. Film stock, in and of itself, is nothing else than a chemical solution that is based on a combination of different compounds. The exact composition of these chemicals cannot be recreated twice and will thus slightly vary between batches, which, in turn, can affect the visual outcome of the footage if mixing two or more different batches. This is why cinematographers need to make sure they order their film stock from the same batch.
The chemical compounds - before they are being exposed and processed - further need to be chilled to prolong the longevity of the film stock. This, in turn, also needs to be factored in when preparing a film shoot, as you will not only need to chill the film stock, you also have to remember to take the film stock out of the fridge 24h prior to filming to ensure that the material had enough time warming up to room temperature again. For going below the 24h mark might entail that the chemicals on the film strip will react differently, affecting the visual outcome as well.
With 16mm film, you will have to record sound on a separate device, either digitally or on tape as well. Here, with physical film it is important that you record the sound at the same speed as the film itself as both tapes won’t match up in length otherwise. To ensure that sound and visuals sync up in post production, a clapper board (or ‘slate’ or ‘marker’) is used to give both a visual as well as an acoustic point of reference.
Furthermore, pulling focus will be of the essence when filming on real film. This becomes even more important the smaller the size of the film stock is and the greater the desired blow-up will be. If you film on 35mm and blow it up to the size of a cinema screen, your margin of error is greater than if you want to blow up 16mm to the same cinema screen. This is due to the phenomenon that is called Circles of Confusion.
Due to the circular shape of the spherical lens, the actual film image is made up of tiny little circles. The smaller these circles are, the sharper and more detailed the image gets, and the bigger, more disk-like they are, the blurrier the final image becomes. This depends on the position of the focal point, the point in which all the light rays coming through the lens converge. If the focal point lies exactly on the ‘focal plane’ (the actual exposed film surface), the circles become tiny and become what is called Circles of Confusion. If the focal point lies before or after the focal plane, the cone of light that is entering the lens is either ‘cut off’ or already spreading again, leading to the circles becoming bigger the further your focal point is off the mark. The following video demonstrates this in a visually compelling way that is easy to understand:
Finally, I am coming back to the Depth of Field Calculator that Kodak offers. Here, you can calculate your total depth of field if you enter the film format, the focal length in mm, the subject distance in m, an the f-stop you are filming with. It will also churn out the size of the circle of confusion and the Hyperfocal Distance Point, which is an important feature to calculate when an object will be in sharp focus. In earlier times, the equation was rather difficult, so I am happy that modern technology has taken that yoke from me. The equation is as follows:
H = F² ⁄ c × f
H = Hyper Focal Distance
F = Focal Length of lens
c = Circle of Confusion
f = F-stop
The most important issue to notice though is the fact that the DoF-Calculator does not support imperial but metric measurements, which should be kept in mind on international productions to avoid misunderstandings and unnecessary loss of footage.
With that in mind, I am looking forward to soon work with real film stock. It’s going to be exciting!
References:
Kodak.com (2018) Camera Films [online] https://www.kodak.com/GB/en/motion/Products/Production/default.htm [Accessed on 5 October 2018]
Kodak.com (2018) Film Calculator [online] Image taken from: https://www.kodak.com/GB/en/motion/tools/film_calculator/default.htm [Accessed on 5 October 2018]
Kodak.com (2018) Kodak Cinema Tools [online] https://www.kodak.com/gb/en/motion/tools/mobile/default.htm [Accessed on 5 October 2018]
Kodak.com (2018) VISION3 250D Color Negative Film 5207/7207 [online] https://www.kodak.com/GB/en/motion/Products/Production/5207/default.htm [Accessed on 5 October 2018]
Kodak.com (2018) VISION3 50D Color Negative Film 5203/7203 [online] https://www.kodak.com/GB/en/motion/Products/Production/5203/default.htm [Accessed on 5 October 2018]
Kodak.com (2018) VISION3 500T Color Negative Film 5219/7219 [online] https://www.kodak.com/GB/en/motion/Products/Production/5219/default.htm [Accessed on 5 October 2018]
thekineticimage (2012) Circle of Confusion Explained and Illustrated [online] https://www.youtube.com/watch?v=Pdq65lEYFOM&frags=pl%2Cwn [Accessed on 5 October 2018]