LDA-explorer

A single‑page, in‑browser viewer for ARRI LDA Files. This tool was written after editing some .tab files with a hexadecimal editor and is not an official tool attached to the brand but a support for testing, compatibility as well as for educational purposes.

• Drag & drop one or more .tab files, or a .zip that contains them.
• Runs entirely locally in your browser (no upload to a server).
• Two modes:
  • Default: “clean” view (lens metadata + point tables + motor min/max per axis).
  • Expert: exposes raw/diagnostic fields (clusters, markers, offsets, preambles, raw bytes).

This repository/document captures what was reverse‑engineered during the session. It is not an official ARRI specification.

---

What an LDA .tab contains (high level)

An LDA .tab file stores lookup tables that map motor/encoder positions (U16, big‑endian) to ring marks (focus distance, iris, sometimes zoom). Cameras use this to display correct focus distance / T‑stop / zoom markings for a given motor position.

The file behaves like a small container:

• A binary header with constants and per‑axis headers.
• Several ASCII, null‑terminated strings embedded in the binary (lens name, manufacturer, serial, etc.).
• One or more axis blocks (focus / iris / zoom) each containing a list of points.

File sizes are variable because the number of points per axis is variable.

---

Key byte patterns and structural markers

Signature block (common pivot)

A recurring signature appears in many files:

• 02 0F 20 (signature)

Immediately after the signature, two U16 values appear (big‑endian). They match end‑stops (min/max) but their order can vary between files.

A typical focus header around the signature looks like:

02 0F 20  [U16 A] [U16 B]  00 81 [FLAG]  F0 [U16 C]  F9 [U16 INF]

Observed meanings:

• U16 A, U16 B: end‑stops (min/max, order may swap)
• 00 81: constant marker in the header
• FLAG: a per‑file flag used for clustering (see below)
• F0 [U16 C]: header key / reference value (often repeats later)
• F9 [U16 INF]: infinity motor position (the “∞” ring mark)

Records separator

Within an axis, each point record is structured as:

[ring_value (1..N bytes)]  F7  [motor_pos_u16be]

• F7 separates the “ring mark” payload from the motor position.
• Motor position is U16 big‑endian (0..65535).

Axis terminator

Each axis point list ends with:

F0 [U16 end_key]

The end_key often matches the axis header key.

Marker blob 36/37 03 0F 02

Many files contain a blob before a non‑focus axis (commonly iris in the analyzed dataset):

[U16 X] 00..00  [36 or 37] 03 0F 02  [axis_key_u16] ...

Notes:

• The 36/37 byte is treated as a variant marker.
• The bytes before it often look like U16 + zero padding. Padding length varies across files (e.g. 4 bytes vs 5 bytes preamble), suggesting reserved/padding differences between variants/firmware.
• 0F 02 appears constant in the analyzed corpus and is likely an axis/type identifier (still not fully confirmed for zoom/metric variants).

---

Cluster detection (two common families)

Across a large Leica subset, two stable “clusters” were observed using the focus header fields:

• Cluster 1: FLAG = 0x80 and u16_after_header_f0 = 0x0034 (decimal 52)
• Cluster 2: FLAG = 0x00 and u16_after_header_f0 = 0x0069 (decimal 105)

This appears to represent a format/option variant (possibly resolution/units/firmware). The exact semantic meaning is not confirmed.

---

Ring value encoding (what we can decode today)

1) Focus distance — imperial (feet / feet+inches / inches)

The reverse engineering found a compact nibble/BCD encoding rather than standard floats.

Feet (<10)

• xB (one byte) ⇒ x feet
  • Example: 8B ⇒ 8'

Feet (10..99)

• XX BD (two bytes, XX is BCD) ⇒ feet
  • Example: 30 BD ⇒ 30'

Feet (>= 100)

• 12 0B (two bytes) ⇒ 120'
  • Rule: b0 is BCD for the first two digits ("12"), and the high nibble of b1 is the third digit (0), with low nibble B.

Feet + inches

• xB yD (two bytes) ⇒ x' y"
  • Example: 3B 6D ⇒ 3'6"

Inches (<10)

• xC (one byte) ⇒ x inches
  • Example: 9C ⇒ 9"

Inches (>=10) — variant A

• XX CD (two bytes, XX is BCD) ⇒ inches
  • Example: 22 CD ⇒ 22"

Inches (>=10) — variant B (highlighted in Expert mode)

• 0B XX (two bytes, XX is BCD) ⇒ inches
  • Example: 0B 21 ⇒ 21"

This second inches encoding (0B XX) appears on some lenses below ~2 feet and seems to come from a different authoring workflow.

In charts, focus values are normalized to inches for a single numeric Y‑axis.

2) Iris (T‑stops)

Iris marks are encoded with an A/D nibble pattern and/or BCD for integers:

• xD (one byte) ⇒ integer T‑stop x

  • Example: 4D ⇒ 4

• xA yD (two bytes) ⇒ decimal x.y

  • Example: 2A 8D ⇒ 2.8

• 11 / 16 / 22 (one byte, BCD) ⇒ integer stops

In charts, iris values are numeric T‑stops.

---

Infinity mark (important fix)

The focus header contains:

• F9 [U16 INF]

This U16 is the motor position at infinity (the farthest ring mark). It must be included as the first point (displayed as ∞). It also participates in the true motor min/max for that axis.

---

What is still unknown / future work

• Metric focus encoding (meters/cm) was not conclusively identified in the provided dataset.
• Zoom axis identification and its ring encoding need more samples containing 3 axes.
• The precise semantics of:
  • FLAG and u16_after_header_f0 (clusters)
  • the 0F 02 field in the marker blob
  • the preamble/padding length variability before 36/37 03 0F 02

---

LDA explorer UI behaviour

• Default view shows:

  • extracted ASCII strings (snippets)
  • per‑axis point list: ring mark → motor position
  • per‑axis motor min/max (including infinity)
  • optional charts (enabled by default)

• Expert mode additionally shows:

  • cluster classification (flag + key)
  • marker blobs (36/37 + preamble)
  • offsets and raw hex values
  • highlight for the inches variant 0B XX

---

Files

• LDA_explorer_en.html – the English single‑page tool.

---

Disclaimer

This is a reverse‑engineered description based on a limited corpus of .tab files. It may not cover all ARRI firmware versions, all lens families, or all unit/axis configurations.