Lighting Ratio¶
A lighting ratio compares illumination on a subject's bright and shadow sides under a declared key-and-fill metering convention.
Core Idea¶
A lighting ratio compares the illumination of a subject's bright and shadow sides under a stated convention. If key reaches only the bright side and fill reaches both, the side ratio is ((K+F):F). A key of 200 units and fill of 100 gives 300:100, or 3:1 on the face. This differs from the isolated-source (K:F) ratio, which is 2:1. Both conventions occur in lighting instruction, so the compared readings must be named.[ref-69244e3101f4][ref-f902334902f6]
Scope of Application¶
Portrait and film crews use the ratio to plan contour and shadow detail. Kodak's TRI-X 7266 sheet recommends 2:1 or 3:1 for key-plus-fill to fill but permits higher contrast for a chosen look; it is a film-specific recommendation. Real bounce, spill and ambient illumination require measuring the subject, not simply adding fixture specifications.[^ref-aaa51b61caed]
Clarity¶
One stop between isolated key and fill makes (K=2F), so the ideal side ratio is 3:1. One stop between the combined bright-side and shadow-side readings means 2:1; Kodak's gray-card table places 3:1 near one and two-thirds stops for that comparison. The stop statements are not contradictory once the numerator is specified.[ref-69244e3101f4][ref-8efef65c804e]
Manages Complexity¶
The ratio condenses a multiple-light setup to one comparable pair of readings at the subject. It helps guide adjustments but does not capture shadow edge, color, reflectance, background separation or final tone mapping. A soft and a hard key may give the same ratio yet different images.
Abstract Reasoning¶
In the ideal model (R=(K+F)/F=1+K/F), so raising fill at fixed key lowers face contrast. If ambient light (A) reaches both sides, the delivered ratio becomes ((K+F+A)/(F+A)), below the naive two-source figure. An isolated main:fill meter reading identifies a fixture balance; it is not automatically the delivered face ratio.[^ref-f902334902f6]
Knowledge Transfer¶
The measured relation transfers from still portraits to cinema when the same side readings are used and the medium is considered. The live Ratio prime is the strict genus for the ordered light-level comparison; photographic light geometry, metering and aesthetic judgment make the named entry domain-specific. A generic digital contrast adjustment is not a lighting ratio.
[^ref-69244e3101f4]: Hurter, portrait lighting ratios, worked 2:1 and 3:1 cases. [^ref-f902334902f6]: Sekonic L-398A manual, §6 isolated source readings. [^ref-aaa51b61caed]: Kodak TRI-X 7266 technical sheet, Lighting Contrast. [^ref-8efef65c804e]: Kodak Gray Card R-27 brochure, stop-versus-side-ratio table.
Relationships to Other Abstractions¶
Current abstraction Lighting Ratio Domain-specific
Parents (1) — more general patterns this builds on
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Lighting Ratio is a kind of Ratio Prime
Lighting ratio is a domain-specific measured ratio.
Hierarchy path (1) — routes to 1 parentless root
- Lighting Ratio → Ratio → Comparison → Self Checking
Neighborhood in Abstraction Space¶
Lighting Ratio sits in a sparse region of the domain-specific corpus (74th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Visual & Cinematic Composition Techniques (24 abstractions)
Nearest neighbors
- Lighting — 0.85
- Light Curve — 0.84
- Schlieren Imaging — 0.84
- Eclipse — 0.83
- Whiteness (colorimetry) — 0.82
Computed from structural-signature embeddings · 2026-10-08