A Deep Dive Into The Redscale Technique

My recent experiments with converting rolls of colour film to allow the redscale technique to be used has been an unexpectedly enjoyable one. I have taken rolls of colour from my stock of film (all of these rolls have been donated by my friends) and created some really interesting photographs. Redscale is a new palette that allows a creative photographer an extra tool in their arsenal. It also presents questions about other colour film options, but more of that later (possibly in a future blog post).

I was, however, a bit high level in my description of how the redscale technique works by reversing the film in the camera and exposing the colour layers to light, red layer first.  This blog post will correct that and attempt a more "engineering" approach to explaining the process.

Understanding Colour Film

It is probably worth having a quick look at this to understand how RGB works to create a colour picture.

The RGB Color Model 

Most peoples understanding of colour film is that there are a number of layers of emulsion, with each layer contributing to our Red Green and Blue (RGB) view of colour theory.  In film we are dealing with light and therefore things are slightly different.

Figure 1

Looking at figure 1, above, we are able to see that light has to traverse the three layers, each layer being exposed to the light associated with it, starting with blue, then green and finally red. While figure 1 is broadly true there are issues with rendering colour scenes so that they resemble how we experience reality. There are also light physics to contend with.

Figure 2

Figure 2 shows how the emulsion for a colour film is engineered.  Firstly, the ultraviolet part of the light spectrum can be filtered out as it is largely invisible to human perception. Next we have the blue layer, followed by a yellow filter. Yellow filter, what the heck is that for? It turns out that the green layer and red layer are also affected by exposure to blue light. In other words the green layer is blue-green sensitive and the red layer is actually blue-red sensitive. The green layer isn't affected by the red part of the light spectrum and the red layer isn't affected by the green area of the spectrum. So to ensure that the green layer is only exposed to the green part of the spectrum, the blue part of the spectrum must be filtered out using a yellow dye (remember in colour theory a complimentary coloured filter is used to stop light of a particular colour, in this case yellow will stop blue). The principle applies to the red layer.

This is all very technical and I'm sure at this point your brain will be the same as mine

Take a break for a cup of tea and a biscuit, other snacks are available 😊

Back to the Redscale Technique

Looking back at figure 2, we can see that if we are to reverse the film, we also reverse the order in which light exposes the emulsion. Firstly, the light has to penetrate the anti halation layer (ordinarily this stops light bouncing around), the base gelatin before exposing the Red layer. As we've discussed the Red sensitive layer is Blue and Red sensitive. Also there is no yellow filter to stop blue light from affecting the red layer. The light will then carry on to the Green sensitive layer, again no yellow filter to stop blue light from affecting this layer.  The next layer is the yellow filter and so no Blue light can access the blue sensitive layer. 

Why is all this important? Perhaps a quote from Google will help

All photographic film emulsions use silver halides that are inherently sensitive to high-energy blue and violet light, even when sensitized for other colors. If blue light isn't blocked from reaching the green and red layers, it causes "crosstalk", creating overlapping dye exposures. This corrupts color fidelity, producing unnatural, highly shifted colors and muddy hues instead of a clean, accurate image

In Redscale photography we want to shift the colours to be hues of Red, so the crosstalk won't really matter.

It's worth noting that while the main Redscale effect is primarily a function of shooting through the film layers in reverse order, the base film colour can have an effect. With a colour film that utilises an orange film base, firstly the film base will block light more and so a 2 or 3 stop compensation must be used, e,g, if you are using an IOS 400 film then shooting at ISO 100 is recommended. Secondly the film base being orange in colour will enhance the red hues of the final photograph. A colour film stock with a clear film base, e.g. Harman Phoenix, will only require a 0.5-1 stop compensation and will cause the resultig photograph to be buttery yellow in hues. You can find out this information by looking at the datasheet for your film.

So thats a more in depth explanation of how Redscale works, if you ever wondered.