# What does focal length change in an image, what does camera-to-subject distance change, and where does the 'telephoto compression' idea come from?

- id: art_01M5MSVF8N5JVWF88P8VRKVP52
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T02, T03 · tags: none
- updated: 2026-10-23T16:35:07.661Z
- url: https://veyrul.com/articles/art_01M5MSVF8N5JVWF88P8VRKVP52

Two inspected manufacturer sources agree on the structure: focal length (for a given sensor) sets how much of the scene is captured; perspective is set by where the entrance pupil (centre of projection) sits relative to the subject. Panavision states that the same field of view can show different perspective when focal length and distance both change, and Zeiss states that holding subject size constant means roughly doubling distance with doubled focal length. 'Telephoto compression' is therefore, in these sources, the combined result of choosing a longer lens and standing farther back to keep framing; whether the lens alone does anything is not argued by either source in so many words (the Panavision sentence is ambiguous on this, see conflicting_evidence). Practitioner accounts use 'compression' as a look they like (attributed).

Two inspected manufacturer sources agree on the structure: focal length (for a given sensor) sets how much of the scene is captured; perspective is set by where the entrance pupil (centre of projection) sits relative to the subject. Panavision states that the same field of view can show different perspective when focal length and distance both change, and Zeiss states that holding subject size constant means roughly doubling distance with doubled focal length. 'Telephoto compression' is therefore, in these sources, the combined result of choosing a longer lens and standing farther back to keep framing; whether the lens alone does anything is not argued by either source in so many words (the Panavision sentence is ambiguous on this, see conflicting_evidence). Practitioner accounts use 'compression' as a look they like (attributed).

Curator qualifications: 6 of 6 excerpts present. Curator derivation (proposed resolution of the candidate's open CE): Zeiss states the centre of projection is the entrance pupil; under that central-projection model an object point at depth Z and height H images at h = f·H/Z, so for a fixed camera position the ratio of two objects' image sizes (H1/Z1)/(H2/Z2) does not depend on f; a longer lens from the same position is a uniform enlargement (crop), and the apparent 'compression' of background comes from the larger camera distance chosen to keep framing. Ignores lens distortion, focus breathing and entrance-pupil shift with focus, which are second-order. Panavision's caption sentence 'The longer the focal length, the closer the background will appear' is therefore a description of the combined lens-and-distance practice, not of the lens alone.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://www.panavision.com/highlights/highlights-detail/5-ways-focal-length-affects-your-subject",
      "note": "Panavision '5 ways focal length affects your subject' (live 2026-10-22; bytes differ from producer pin fafe759a, excerpts present); Zeiss technical article 'distortion' PDF (equals pin) p.2; Zeiss technical article 'depth of field and bokeh' PDF (equals pin); Leitz Cine interview Ben Richardson ASC (live; bytes differ from pin 4a6aeda8, excerpt present); central-projection derivation from Zeiss E4 (curator; not a source statement)"
    },
    {
      "url": "https://lenspire.zeiss.com/photo/app/uploads/2022/02/technical-article-distortion.pdf",
      "note": "Panavision '5 ways focal length affects your subject' (live 2026-10-22; bytes differ from producer pin fafe759a, excerpts present); Zeiss technical article 'distortion' PDF (equals pin) p.2; Zeiss technical article 'depth of field and bokeh' PDF (equals pin); Leitz Cine interview Ben Richardson ASC (live; bytes differ from pin 4a6aeda8, excerpt present); central-projection derivation from Zeiss E4 (curator; not a source statement)"
    },
    {
      "url": "https://lenspire.zeiss.com/photo/app/uploads/2022/02/technical-article-depth-of-field-and-bokeh.pdf",
      "note": "Panavision '5 ways focal length affects your subject' (live 2026-10-22; bytes differ from producer pin fafe759a, excerpts present); Zeiss technical article 'distortion' PDF (equals pin) p.2; Zeiss technical article 'depth of field and bokeh' PDF (equals pin); Leitz Cine interview Ben Richardson ASC (live; bytes differ from pin 4a6aeda8, excerpt present); central-projection derivation from Zeiss E4 (curator; not a source statement)"
    },
    {
      "url": "https://www.leitz-cine.com/interview/ben-richardson-asc",
      "note": "Panavision '5 ways focal length affects your subject' (live 2026-10-22; bytes differ from producer pin fafe759a, excerpts present); Zeiss technical article 'distortion' PDF (equals pin) p.2; Zeiss technical article 'depth of field and bokeh' PDF (equals pin); Leitz Cine interview Ben Richardson ASC (live; bytes differ from pin 4a6aeda8, excerpt present); central-projection derivation from Zeiss E4 (curator; not a source statement)"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Two inspected manufacturer sources agree on the structure: focal length (for a given sensor) sets how much of the scene is captured; perspective is set by where the entrance pupil (centre of projection) sits relative to the subject. Panavision states that the same field of view can show different perspective when focal length and distance both change, and Zeiss states that holding subject size constant means roughly doubling distance with doubled focal length. 'Telephoto compression' is therefore, in these sources, the combined result of choosing a longer lens and standing farther back to keep framing; whether the lens alone does anything is not argued by either source in so many words (the Panavision sentence is ambiguous on this, see conflicting_evidence). Practitioner accounts use 'compression' as a look they like (attributed). · locator: Panavision '5 ways focal length affects your subject' (live 2026-10-22; bytes differ from producer pin fafe759a, excerpts present); Zeiss technical article 'distortion' PDF (equals pin) p.2; Zeiss technical article 'depth of field and bokeh' PDF (equals pin); Leitz Cine interview Ben Richardson ASC (live; bytes differ from pin 4a6aeda8, excerpt present); central-projection derivation from Zeiss E4 (curator; not a source statement) · source: https://www.panavision.com/highlights/highlights-detail/5-ways-focal-length-affects-your-subject

Rights:
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