lightcraft/docs/focus-points.md

Focus points

Most cameras record where they focused. LightCraft reads that point from the file, draws it over the photo and zooms to it, so checking whether the subject is sharp takes one key while culling. Lightroom shows no focus points (photographers add plug-ins for it); this goes beyond parity (tracker row LR-VIEW-FOCUSPOINT).

Using it

• View ▸ Show Focus Point (⌥F; view.focusPoint {on?}) draws the camera's AF frame as green corner brackets in Detail, Compare and Survey. It follows Rotate, flips and the crop; a point cropped away is not drawn. Off by default; the choice is saved with the app's settings.
• View ▸ Zoom to Focus Point (view.zoomToFocus {on?}, on by default): Toggle Zoom (Z or Space) zooms in centred on the focus point instead of the last position. A click on the photo still zooms where you click, and a photo without a focus point zooms as before. On by default because it changes nothing for photos without a point, and for those with one the focus point is what a 1:1 check is for. In Compare the two panes share one pan, so Z centres on the active photo's point; per-pane focus zoom is a follow-up.
• Info panel: a Focus row says how the camera focused, e.g. AF-C · Wide · Animal Eye Tracking (focus mode, the AF area set on the camera, and what it tracked when the camera says).
• Agents: photo.inspect (and the MCP lightcraft://photo/active resource) returns it as meta.focus: {rect: {x0, y0, x1, y1}, mode?, area?, tracking?}, the box normalized to the upright (EXIF-oriented) photo, y down. Photos without one have no focus key. view.zoomToggle replies {focus: [x, y]} (the pan it centred on) when it zoomed to one.

The point is read when a photo is imported. Photos imported earlier get it with Photo ▸ Reload from Disk (photo.reload), which fills camera data the catalog lacks; it never replaces a point already stored. It is read-only camera data: never written to XMP, and absent from the catalog JSON when unknown, so older catalogs read unchanged (no catalog format change).

Cameras

Sony first, on bodies whose files have been checked: ILCE-1 (α1) and ILCE-7SM3 (A7S III) (lightcraft_meta::focus::SONY_MODELS). Other Sony bodies store the same tags, but a body is added only after its files are checked, so an unchecked one shows nothing rather than a misleading box. Fujifilm, Canon, Nikon, OM System and Panasonic record similar data and are later steps.

Sony layout and evidence

Tag identities and value names come from the public ExifTool tag-name documentation (Sony MakerNotes table). No third-party code was read or ported. Every field was confirmed on real files by comparing LightCraft's decoder with ExifTool's output, run as a black box: 333 ARWs sampled across four years of a photographer's library (180 α1, 153 A7S III; 12 portrait or upside-down, 54 in APS-C crop or reduced-size modes), with no mismatch in the point, frame or names.

MakerNote tag	Layout	Notes
0x2027 FocusLocation	4 × int16u: frame width, frame height, focus x, focus y	in the sensor's landscape frame (a portrait shot still says 8640 × 5760); the frame is the image size (5616 × 3744 in α1 APS-C mode)
0x2037 FocusFrameSize	6 bytes: int16u width, int16u height, then 2 bytes that are zero when no frame was shown	zero in manual focus; one AF-C α1 file had no frame (a small box is drawn: 3 % of the short side)
0x201b FocusMode	int8u: 0 Manual, 2 AF-S, 3 AF-C, 4 AF-A, 6 DMF, 7 AF-D
0x201c AFAreaModeSetting	int8u: 0 Wide, 1 Center, 3 Flexible Spot, 11 Zone, 12 Expanded Flexible Spot, 13 Custom AF Area
0x2021 AFTracking	int8u: 0 Off, 1 Face tracking, 2 Lock On AF
0x9402 bytes 0x16 / 0x17	enciphered: focus mode (low 7 bits) and the AF area mode used (0 Multi, 2 Spot, 3 Flexible Spot, 11 Zone, 14 Tracking, 15 Face Tracking, 20 Animal Eye Tracking, 21 Human Eye Tracking, 255 Manual…)	read only when its focus mode equals 0x201b's

The documentation says the 0x94xx blocks use "a simple substitution cipher". On the files each byte p < 249 is stored as p³ mod 249 (bytes 249–255 unchanged): ExifTool's decoded AFAreaMode 20 is stored as 0x20, a decoded 0x83 as 0x77. Cubing is a bijection modulo 249 (= 3 × 83), so the inverse is a table lookup.

On the α1 files the box sits on the subject: drawn on the embedded previews, it lands on the bird's eye in eye-AF shots and follows spot, zone, wide and tracking shots, in portrait and landscape.

When there is no real point

Nothing is stored (so nothing is drawn and Z zooms as before) when:

• the body is not one of the checked ones, the tags are missing or malformed, the frame size is zero, or the centre lies outside its frame;
• manual focus (0x201b = 0, or the block's area mode is Manual). The camera still records where the AF frame was parked: 83 of the 91 sampled manual-focus files carry an off-centre location that says nothing about focus;
• the location is the exact frame centre while the area used was Multi (Wide with nothing tracked; without the block, Wide with tracking off). The wide area reports the centre when it picked no particular spot; 7 of the 34 sampled autofocus Multi files did. A centred Spot, Zone or Flexible Spot frame is a choice the photographer made, and is kept (49 of the 111 sampled spot and zone files were centred, the camera's default position).

DMF (AF, then manual touch-up) keeps its point.

Code

• crates/meta/src/focus.rs: the decoder (bounded; hostile values give no point, never a panic) and its tests, including an opt-in test over real files (LIGHTCRAFT_FOCUS_SAMPLES=<photo folder>).
• crates/ui-egui/src/panels/detail.rs (focus_box, focus_pan, draw_focus_point): mapping through the user's rotation and crop, the overlay, and the zoom target; crates/ui-egui/src/tests_focus_point.rs.