O **centro ótico** (a.k.a. **entrance pupil** ou **no-parallax point**) é o single point num lens system onde incoming rays appear to converge. Numa lente zoom, este ponto **muda posição** conforme focal length muda — mas a qualquer *dado* focal length, fica fixo relative to lens barrel.
Por Que Importa
- Panorama stitching: Rodar câmara **around the entrance pupil** → zero parallax between frames → seamless stitch. Rodar around tripod socket → near/far misalignment (ghosting).
- Focus stacking: Same principle — pivot at entrance pupil prevents perspective shift between focus slices.
- Photogrammetry / 3D scanning: Accurate camera pose estimation requires known pupil location.
Como Encontrar (DIY Method)
- Montar câmara em nodal slide (Really Right Stuff, Nodal Ninja, ou DIY Arca rail).
- Set zoom to working focal length; **lock zoom ring** (tape).
- Apontar near object (1 m) com distant background.
- Pan slowly left/right; watch near/far alignment in live view.
- Slide camera fore/aft até **no relative shift** → mark position. That's entrance pupil for *that* focal length.
- Repetir para cada focal length you'll use (varies 10–50 mm along barrel).
Dados Fabricante (Raro mas Existe)
- Algumas cinema lenses (Cooke, Zeiss, Angénieux) publicam **entrance pupil distance from flange** at each focal length.
- Still-photo zooms rarely do — measure yourself.
Confusão Comum
- Nodal point ≠ entrance pupil (nodal points are angular; entrance pupil is spatial). Para panoramas, **entrance pupil** é correct pivot.
- "Optical center" é marketing-speak; technicians say **entrance pupil** ou **no-parallax point**.
Encontrar once per focal length, mark your rail, e seus panoramas / focus stacks will stitch on first try.
