Focal Length Explained

Focal Length Explained

Focal length is the distance between the lens and the sensor when the lens is focused at infinity, measured in millimeters. It determines how much of a scene the camera sees and how the perspective looks. A short focal length like 18mm gives you a wide angle view with lots of visible distortion. A long focal length like 200mm gives you a narrow telephoto view that compresses the background.

Wide angle lenses are common in VFX heavy shots because they show more of the environment, but they come with challenges. The perspective distortion makes objects near the edges of the frame look stretched. This is not a lens defect; it is just geometry. A sphere near the edge of a wide angle shot will look elongated. Your 3D tracking needs to account for this distortion, or your CG elements will not line up properly.

Telephoto lenses have the opposite effect. They compress perspective, making objects at different distances look closer together than they really are. This is why car crashes in movies are often shot with long telephoto lenses. The cars look like they are inches apart when they are actually feet apart for safety. In VFX, telephoto shots are easier to track because there is less distortion, but the camera movement is often very subtle.

Zoom lenses can change focal length during a shot. This is very challenging for camera tracking because the focal length changes every frame. The tracking software has to solve for the changing focal length as well as the camera position. Some tracking software can handle this if the zoom is smooth, but it is always more difficult. Prime lenses with a fixed focal length are much easier to work with for VFX.

When setting up a 3D camera in Maya, Blender, or Houdini to match real footage, you need to enter the exact focal length used on set. If the cinematographer used a 35mm lens, you set your 3D camera to 35mm. But you also need to account for the sensor size, because the same focal length gives a different field of view on different sensor sizes. A 35mm lens on a Super 35 sensor gives a narrower view than on full frame.