How the frame is drawn.
focalcomp is deliberately simple: one fixed scene, real projection geometry, no renders. Here is exactly what the frame means — and what it doesn't.
N/01The reference plate
Every build renders on Reference Plate 01, a fixed test scene drawn as if photographed on a 36×24 full-frame sensor with a 12mm lens — a 112.6° horizontal, 90.0° vertical view. Nothing about the plate ever changes. That constancy is the whole point: when the frame tightens, it's your build doing it, not the picture.
N/02The chain
A build resolves in order: glass → adapter → sensor.
A dumb adapter multiplies by 1.0 and only solves the mount. A speed booster multiplies by less than 1 (0.64×, 0.71×), widening the view and brightening the image. Then the sensor crops whatever image circle it receives:
Anamorphic squeeze multiplies horizontal coverage only — a 2× anamorphic sees twice as wide as its spherical focal suggests, at the same height.
N/03Mapping to the plate
The overlay is a projection-plane ratio, not a linear focal ratio — the honest way to compare angles of view:
Plate mode draws that rectangle over the full scene. Sensor view scales the plate so the rectangle fills the stage — what the sensor actually sees, letterboxed to its display aspect after desqueeze.
Any plate can drive the overlay — the illustrated Plate 01 (12mm reference), a photo plate, or your own upload. A photo only reads correctly if focalcomp knows the focal it was shot at, so each non-reference plate carries a reference focal (full-frame equivalent). Set it wrong and every crop scales wrong; that field is the one calibration a photo plate needs.
N/04What the numbers mean
- Effective focal — physical focal × adapter multiplier. The lens the sensor "feels".
- FF equivalent — effective focal × diagonal crop factor. A familiarity anchor, quoted diagonally and spherical-only.
- Crop factor — 43.27mm ÷ sensor diagonal.
- Display aspect — (sensor width × squeeze) ÷ sensor height, after desqueeze.
N/05Verifying the sensor data
Sensor size is the only camera-side number the framing math depends on, so it's the one worth checking. Every body's crop factor is derived from its width and height (43.27 ÷ diagonal), then cross-checked against the manufacturer's published crop factor. A mistyped dimension surfaces immediately as a crop factor that doesn't match reality — across the registry the derived and published figures agree. Absolute dimensions are confirmed against spec sheets brand by brand; where a body has several readout modes, focalcomp uses the widest, so a multi-aspect sensor reads a touch wider than its 4:3-only crop.
N/06Honest limitations
- Field of view only. No distortion, breathing, vignetting, or image-circle coverage — a 2× anamorphic will not actually look rectilinear at the edges.
- Sensor dimensions are full-width photosite areas; some bodies crop further in certain recording modes.
- One adapter per build for now. Stacked adapters (C/Y → EF → booster) are on the roadmap.
- Compare is scaffolded as pin-a-build (B-frame); full side-by-side A/B is next.
N/07Why a reference plate
Lens tests are only meaningful against a constant. Real-estate photos, portraits, landscapes — they all lie differently. A fixed plate with known geometry (subject at 4m, chart at 4m, marks at 2/4/8m) makes every comparison legible, repeatable, and boring in the best possible way.
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