Introduction

A huge thanks to the brilliant team at Matchmove Machine for sharing this guide on crop factor.

Matchmove machine is a team of technical experts in 3D tracking and matchmoving. Matchmove Machine provide 3D tracking services for Feature Films, Music Videos, Commercials, TV Shows and are ready to be with you at all stages of production to ensure a truly impressive and high-quality result, while remaining invisible to the viewer.

What is crop factor?

The crop factor is a ratio that shows how the size of a camera sensor differs from a standard full-frame sensor.

A full-frame sensor measures 36x24mm, which corresponds to the frame of classic 35mm film. If the camera is equipped with a smaller sensor, the image produced with the same lens will appear “cropped” — cut off at the edges.

Why is this important?

When shooting with a crop sensor, lenses behave slightly differently.

For example, if you use a 50mm lens on a camera with a crop factor of 1.5, the equivalent focal length will be 75mm. This means the field of view will be narrower, as if you’re shooting with a telephoto lens.

Thus, we now know that the combination of the lens and sensor determines the angle of view (AoV) of our camera, and the angle of view forms the field of view (FoV) — the visible area, or rectangle, of our image.

Application in matchmove

By understanding this principle, we can conclude that the same angle of view can be achieved with different combinations of focal length and sensor size. A common mistake made by beginner matchmove artists is setting the correct focal length while leaving the sensor size at a default value, which results in an incorrect AoV and leads to errors in the solve.

A properly set AoV is also essential for accurately integrating CG objects into a scene, as it directly affects perspective.

What to do if one of the two variables (sensor size or focal length) is unknown?

Adjust! All 3D tracking programs allow you to calculate the unknown variable and do so very accurately.

What to do if both variables are unknown?

Adjust both at the same time! Don’t worry about not getting exact values with two variables in the equation — as you understood earlier, any combination of values that gives the correct AoV will work, and 3D tracking programs handle this task quite well.