On the 11th April, a presentation at The Photographers’ Gallery pulled back the curtain on a part of photography most people never see: the meticulous, highly technical world of cultural heritage digitisation. Led by John Wallace, the session challenged a common assumption - that digitisation is simply about taking a good photo - and replaced it with something far more rigorous.
Heritage digitisation differs fundamentally from artistic photography: it prioritises measurable accuracy over creative interpretation, using calibrated equipment, controlled lighting, and quantified colour standards like Delta E (ΔE) to ensure a digital image is a faithful, evidential copy of the original object. This piece summarises key technical principles from a presentation by John Wallace at The Photographers' Gallery on 11th April.
Reflected materials, such as books and paintings, are captured using light bounced off the surface, typically with two lights at 45° to manage glare and texture. Transmissive materials, such as negatives and slides, are captured with light passing through the object, requiring higher resolution and careful handling of shadow and highlight detail.
A key concept explored during the session was the difference between reflected and transmissive materials- something that fundamentally changes how you approach capture.
It’s not just a technical difference; it’s a completely different mindset.
If there was one moment where the room collectively leaned in, it was during the discussion of colour accuracy.
Colour accuracy in heritage digitisation is measured using Delta E (ΔE): a value under 2 is invisible to the human eye, while anything above 5 is clearly inaccurate. Every capture device has its own colour tendencies, which is why calibration is treated as foundational rather than optional.
Colour in digitisation isn’t subjective - it’s measured. Using ΔE (Delta E), practitioners can quantify how close a captured colour is to a known reference. A value under 2 is invisible to the human eye, while anything above 5 becomes clearly inaccurate.
This ties directly into calibration. Every device, camera, monitor, scanner, has its own “opinion” about colour. Without calibration, those opinions clash. The presentation made it clear: calibration isn’t optional; it’s foundational.
Resolution requirements vary by object: a 35mm negative typically requires around 21 megapixels for archival standards, while large-format materials can demand hundreds of megapixels or stitched captures. Bit depth matters as much as pixel count - moving from 8-bit to 16-bit capture significantly increases the colour and tonal information recorded.
Another myth the talk dismantled was the idea that higher megapixels automatically mean better results.
The reality is far more nuanced. Resolution requirements depend on the object. A 35mm negative might require around 21 megapixels for archival standards, while large-format materials can demand hundreds of megapixels or even stitched captures.
Then there’s bit depth - something rarely discussed outside professional workflows. Moving from 8-bit to 16-bit capture dramatically increases the amount of colour and tonal information recorded, enabling smoother gradients and more accurate reproductions.
In short, it’s not just how many pixels you have, it’s how much information each pixel carries.
The images shown in the presentation (particularly the equipment sections) reinforced how engineered these setups are. Every component exists to remove variables and increase repeatability.
Beyond capture, the talk also touched on file formats and long-term preservation. Open formats like DNG were highlighted as future-proof solutions, ensuring compatibility and preserving metadata over time.
And then there are the everyday challenges: dust, movement, lighting inconsistencies, and data management. Digitisation isn’t a one-click process - it’s a discipline.
What made the presentation resonate wasn’t just the technical depth; it was the purpose behind it.
Digitisation is about access, preservation, and trust. When done correctly, it allows fragile, rare, or inaccessible objects to be studied and shared without risking damage to the original. But that only works if the digital version is a faithful surrogate.
That’s the real takeaway from the session: Heritage digitisation isn’t about creating images - it’s about creating evidence.