1 The idea
The angle of view depends on two things: focal length and sensor width. The same lens on a smaller sensor sees a narrower view. When matching a CG camera, set both the focal length and the sensor (film back) to the real camera.
This plate was shot on Super 35 (about 24.9 mm wide; it varies a little by camera) at 35 mm. Our CG camera is full frame, 36 mm wide. Which focal length gives the same view?
hFOV = 2 · atan(sensor width / (2 · focal))2 Hands-on
Get the hFOV number equal to the plate’s
3 Deep dive
Two numbers, one angle
A focal length alone doesn’t tell you the view. You also need the sensor width: hFOV = 2·atan(sensor width / 2f). The same 35 mm lens gives very different views on different sensors.
The table: a 35 mm lens on three sensor sizes, and the focal length that gives the same view on full frame (real sensor widths vary a little by camera model).
| Sensor | Width (mm) | hFOV at 35 mm | Full-frame equivalent |
|---|---|---|---|
| Full frame | 36 | 54.4° | 35 mm |
| Super 35 | ≈ 24.89 | 39.1° | ≈ 50.6 mm |
| Micro Four Thirds | ≈ 17.3 | 27.8° | ≈ 72.8 mm |
The film back is the area actually recorded
Many cameras record in several modes, and some use only the middle of the sensor (windowed). The width you need is the width for the mode that was actually used, not the full-sensor number from the spec sheet.
If the plate is resized as a whole, the film back stays the same. If it is cropped horizontally (say, a punch-in to half the width), shrink the film back by the same ratio.
Always ask: which camera, which mode, which resolution. Then look up the sensor width.
What to collect on set
A camera report for every setup: camera body and sensor mode, lens focal length (for a zoom, note whether it moved during the take), focus distance, f-stop, camera height and tilt, fps and shutter. A quick photo of the lens and the slate helps a lot.
- 1Camera report
- 2Sensor mode
- 3Film back (mm)
- 4Nuke Camera
- 5Check hFOV vs plate
When the solver can’t find the focal length
On a flat green or blue screen with only a few markers, there isn’t enough information for a solver to separate the focal length from the camera move. Different solvers can return focal lengths several times apart, all with similarly low error.
The fix is not to let the solver guess: enter the focal length and film back from the camera report and solve the rest. This is why set data is worth so much.
4 In real work
In Nuke
- Camera: set focal length (mm) and horizontal aperture = the sensor width actually used (mm)
- CameraTracker (may need NukeX): enter the Film Back Size in mm, and set Focal Length to Known when you have a camera report
- Cropped plate: reduce the horizontal aperture by the crop ratio
- Check: place a real-size Card or Cube in the scene, ScanlineRender it and Merge it over the plate; size and converging lines should agree
- Blender: set Sensor Fit to Horizontal and type the same sensor width
Common mistakes → the fix
- ✗ Using the full sensor width from the spec when the camera shot in a windowed mode✓ Use the width of the mode that was actually recorded
- ✗ Typing the "full-frame equivalent" focal (50.6) while keeping the S35 film back✓ Enter the real focal with the real film back; an equivalent only works with a 36 mm film back
- ✗ The plate was cropped in edit but the film back stayed the same✓ Shrink the film back by the crop ratio
- ✗ Letting the solver guess the focal length when a camera report exists✓ Lock the focal from the report and solve the rest
Before you send the shot
- Camera body, sensor mode and recording resolution are known
- Film back (mm) matches the recorded area and any crop
- Focal length matches the report (zoom: check whether it changed during the take)
- The CG camera’s hFOV equals the plate’s
- A real-size object in the scene reads at the right size over the plate
5 Check yourself
Right focal length, wrong sensor size in the CG camera. What happens?
🔒 Answer the question above and this card opens
1. The camera recorded in a mode that uses only 2/3 of the sensor width. What film back do you use?
2. Same 35 mm lens. Which setup sees a wider view?
Where the numbers come from: Lens maths is the standard thin-lens model (FOV, CoC, hyperfocal), using CoC = sensor diagonal ÷ 1500, a common convention rather than a fixed rule. Distortion uses the Brown–Conrady model (k1, k2). The real DOF images are Blender Cycles renders.