Three Ways to Turn Light Into Numbers

04.2 §1The Array Pixel Never Sees

The Bayer colour filter array is the dominant approach in digital photography, used in effectively every consumer interchangeable-lens camera from the entry-level to the professional tier. Each photosite on the sensor is covered by one of three coloured filters — red, green or blue — arranged in a mosaic first patented by Bryce Bayer at Kodak in 1976. Green covers half the total pixels; red and blue split the remainder equally. The reasoning is physiological: human visual sensitivity peaks in the green portion of the spectrum, and the mosaic attempts to weight luminance resolution accordingly.

The cost is that no single pixel ever captures full colour. A demosaicing algorithm interpolates the missing colour channels at each site by sampling its neighbours — an inference, not a measurement. The quality of that interpolation determines fine-detail rendering and the character of false-colour artefacts at high-frequency edges. Sensor makers and camera makers both invest heavily in the demosaicing pipeline, since the raw photosite count is only the starting point. The K-3 Mark III carries a 25.7 MP Sony-sourced APS-C Bayer sensor; the 25.7 MP figure describes the photosite grid before demosaicing introduces its structural assumptions.

04.2 §2Depth Instead of Area

Sigma's Foveon sensor takes a different path. Rather than assigning colour by position across the sensor plane, it exploits the fact that silicon absorbs light at different depths depending on wavelength. Blue photons are captured close to the surface; green photons penetrate further; red photons deeper still. Foveon sensors stack three photodiode layers at each pixel location, recording all three colour channels simultaneously at every point on the grid.

The consequence is that full colour data exists at every pixel without interpolation. The spatial resolution available to each colour channel is equal, and the characteristic artefacts of Bayer demosaicing — colour fringing, moiré at certain frequencies — are structurally absent. The tradeoff is in sensitivity. Stacking photodiodes means each layer receives only a fraction of the incoming light, and noise at high ISO values rises faster than in a comparably sized Bayer sensor. Foveon sensor development passed to Sigma when the company acquired Foveon Inc. in 2008. Sigma's roadmap toward a full-frame Foveon implementation has moved slowly; no full-frame Foveon body has shipped as of mid-2025.

04.2 §3The Filter Removed Entirely

The third approach discards colour capture altogether. A monochrome sensor is a Bayer sensor — in the case of the K-3 III Monochrome, the same Sony APS-C silicon as the colour body — from which the colour filter array has been removed or, in fabrication terms, replaced with a clear filter that passes all visible wavelengths to every photosite. Every pixel now records luminance across the full visible spectrum.

The engineering consequences follow directly. Without the filter array blocking two-thirds of the incoming light at any given site, each photosite is more light-efficient. At equivalent ISO settings and pixel counts, a monochrome sensor accumulates more photons per site than its Bayer counterpart. Resolution of fine luminance detail is higher, because there is no interpolation step and no filter-induced spatial compromise. The tradeoff is absolute: the K-3 III Monochrome produces files with no native colour information, and adding colour requires physical external filters over the lens — a workflow borrowed from large-format film practice.

04.2 §4What the Three Share

All three approaches transduce photons into electrical charge; the differences lie entirely in what spatial and spectral discrimination is applied before the analogue-to-digital conversion stage. According to CIPA's published shipment data, interchangeable-lens camera volumes have contracted sharply from their 2012 peak, which concentrates engineering resources among fewer manufacturers. Bayer remains the industry default because the interpolation penalty is manageable and the manufacturing ecosystem is mature. Foveon offers a structural answer to Bayer's interpolation problem but carries a sensitivity cost that limits market reach. The bare monochrome sensor maximises luminance resolution at the cost of the colour information most photographers consider non-negotiable. Each is a genuine engineering position; none is free.