EXIF focal length: which tags Metashape reads, and the no-crop-factor rule¶

Status: unverified
Applies to: Metashape Pro 2.x and Metashape Standard 2.x — same EXIF-tag handling as PhotoScan 1.x
Edition: Pro / Standard
Diátaxis: explanation
Confidence: high
Last reviewed: 2026-06-04

Confidence: high. Both the EXIF-tag set and the no-crop-factor rule are forum-attested with permalinks (Agisoft support, 2015). The fallback behaviour for missing FocalPlaneXResolution is described by Agisoft support in the same thread.

Problem¶

A common confusion among photographers new to photogrammetry:

"I have a 25mm lens on a Micro Four Thirds body — should I enter 50mm (the 35mm-equivalent) into Metashape?"
"My drone records 8.8mm in EXIF, but the manufacturer says it's a 24mm-equivalent — which value should I use?"
"My RAW converter reports a different focal length than the JPEG's EXIF — which one is right?"

The short answer: never apply a crop factor. Use the actual optical focal length (the value engraved on the lens or written in the sensor manufacturer's spec sheet). Metashape combines this with sensor pixel pitch to derive the pixel-equivalent focal length internally — and that's what the bundle uses.

Which EXIF tags Metashape reads¶

"PhotoScan reads the following tags from EXIF to estimate focal length in pixels: FocalLength, FocalPlaneXResolution, FocalPlaneYResolution" — Agisoft support, 2015-10-26, PhotoScan 1.1 (permalink)

EXIF tag	Unit	Role
`FocalLength`	millimetres	The optical focal length of the lens
`FocalPlaneXResolution`	pixels per unit	Horizontal sensor pixel density
`FocalPlaneYResolution`	pixels per unit	Vertical sensor pixel density
`FocalPlaneResolutionUnit`	enum (1/2/3/4/5)	Unit for the above (typically 2 = inches)

The conversion is straightforward:

focal_length_x_pixels = FocalLength_mm × FocalPlaneXResolution × unit_factor

where unit_factor = 25.4 if FocalPlaneResolutionUnit = 2 (inches, the most common case), or 1.0 if it's already in millimetres.

A worked example: a Sony A7R III with a 35mm lens reports FocalLength=35, FocalPlaneXResolution=798.246, FocalPlaneResolutionUnit=2 (inches). The pixel-equivalent focal length is:

35 × 798.246 / 25.4 ≈ 1100 pixels

(Note: some cameras report FocalPlaneXResolution in mm-per-pixel or pixels-per-millimetre directly; Metashape parses FocalPlaneResolutionUnit to determine which.)

The no-crop-factor rule¶

"PhotoScan uses both focal length in mm and sensor pixel size to convert focal length into pixel units. So you do not need to recalculate focal length according to the camera crop factor." — Agisoft support, 2015-10-25, PhotoScan 1.1 (permalink)

Why this matters: photographers used to thinking in "35mm equivalent" focal lengths sometimes manually input 35mm-eq values when filling in the Camera Calibration dialog. The result is a 1.5–2× overestimated focal length, which causes Align Photos to start far from the optimum and slows convergence (or in extreme cases, fails alignment entirely).

The correct value is always the physical optical focal length, as printed on the lens or in the sensor manufacturer's specification — never the crop-adjusted equivalent.

Examples by camera class¶

Camera / sensor	Lens focal length	Use this value
Full-frame (35mm) DSLR with 50mm lens	50mm	50
APS-C DSLR with 50mm lens	50mm	50
Micro Four Thirds with 25mm lens	25mm	25
Smartphone with "24mm-equivalent" main camera	physical (~5mm)	5
DJI drone with "24mm-equivalent" 1-inch sensor	physical (~8.8mm)	8.8

The drone example is a frequent source of error: DJI and similar manufacturers prominently advertise the 35mm-equivalent focal length in marketing material, but the actual optical focal length in EXIF is the physical value (~8-13mm depending on model).

When EXIF data is missing or wrong¶

Three failure modes and their fixes:

1. `FocalPlaneXResolution` missing entirely¶

Some cameras (older smartphones, some industrial cameras) don't write FocalPlaneXResolution. Metashape falls back to a default sensor size estimate that may be wrong by 20-50%.

Fix: Set the calibration manually. Tools → Camera Calibration; input the actual sensor pixel size (from the manufacturer's data sheet) and the image dimensions. Metashape then derives the pixel-equivalent focal length from the FocalLength EXIF tag and your pixel size.

2. EXIF reports a "computational" focal length¶

Modern smartphones with multi-camera systems sometimes report a synthetic focal length that doesn't match any physical lens (e.g., a 0.5× ultrawide module reporting 13mm-equivalent). The RAW frame may have been digitally cropped or fused from multiple sensors.

Fix: Use a calibration target to determine the real focal length empirically — Lens calibration (using chessboard pattern) in Metashape (Agisoft KB) walks through the on-screen-chessboard Tools → Camera → Calibrate Lens procedure (or use an AprilTag grid). Load the result via sensor.user_calib (see sensor.calibration vs sensor.user_calib).

3. Pre-undistorted images¶

DJI Phantom 4 RTK, Mavic 3 Cine, and some cinema cameras internally undistort fisheye / wide imagery before writing JPEG output. The EXIF still reflects the original optical focal length, but the image content has been digitally rectified to a narrower equivalent.

Fix: Treat the camera as a Frame sensor type with a calibration solved from the rectified output. Don't try to recover the original fisheye parameters — they don't apply to the rectified pixels.

EXIF focal length is only a starting estimate¶

Demo verified: ✗ — pending Tier 3 reproduction on a real Metashape install.

import Metashape

chunk = Metashape.app.document.chunk

# After Align Photos / Optimize Cameras, the bundle has refined
# the focal length away from the EXIF starting value.
for sensor in chunk.sensors:
    init = sensor.user_calib  # may be None if not user-set
    refined = sensor.calibration  # the bundle-adjusted value

    print(f"{sensor.label}")
    if init:
        print(f"  initial f (mm): {init.f}")
    print(f"  refined f (px):  {refined.f}")
    print(f"  refined cx, cy:  ({refined.cx}, {refined.cy})")

The bundle adjustment refines focal length to fit the tie-point cloud. The EXIF value's accuracy mostly affects:

Alignment convergence speed. A wildly wrong starting focal length (e.g., 100mm when the truth is 35mm) can take many bundle iterations to converge or fail entirely.
Failure modes for low-overlap sets. With ≥80% image overlap and good distribution, the bundle converges to the correct focal regardless of EXIF accuracy. With <60% overlap, a wrong starting focal can produce a locally-optimal but globally-wrong solution.

For high-overlap aerial datasets with well-distributed cameras, EXIF accuracy is usually not critical. For close-range, single-orbit, or fisheye captures, EXIF accuracy matters more.

Caveats¶

FocalLength35efl (the 35mm-equivalent EXIF tag) is written by some cameras as a convenience field; Metashape does NOT use this tag. It reads the physical FocalLength only.
LensInfo and Lens model strings in EXIF can hint at the lens but Metashape does not parse them. Identical lens serial numbers across multiple datasets do not automatically share calibrations — each chunk's calibration is solved independently unless you explicitly link sensors.
Variable-focal-length lenses (zoom lenses, pancake retractable lenses): each photo's FocalLength is stored in EXIF. Metashape treats each unique focal length as a separate sensor by default; to merge them, group cameras manually in the Camera Calibration dialog. See Declaring a fixed-geometry multi-camera rig in Python for the programmatic equivalent.
Fisheye / spherical sensors read FocalLength but the pixel-equivalent calculation differs because the projection is not pinhole. Set the sensor type to Fisheye or Spherical before alignment for those cameras; see Fisheye and spherical sensors: the five projection types.

References¶

Metashape Pro User Manual (2.3), ch. 4 Camera calibration parameters → Initial calibration.
EXIF specification 2.32, §4.6.5 — FocalLength, FocalPlaneXResolution, FocalPlaneYResolution, FocalPlaneResolutionUnit definitions.
Metashape Python API Reference (2.3.1): Sensor.calibration, Sensor.user_calib, Calibration.f.
Forum thread, Focal length entry — should I apply crop factor?, 2015 — the canonical clarification of the crop-factor confusion.
Lens calibration (using chessboard pattern) in Metashape (Agisoft KB) — the on-screen-chessboard empirical lens-calibration procedure (Tools → Camera → Calibrate Lens) for when EXIF focal length is missing or synthetic.