1.3 The Importance of Film Back
This article is part of Level 1. An Introduction to Matchmoving in PFTrack of The Pixel Farm’s Training Courses for PFTrack. Find out more and register for the next available class.
During the Introduction to Matchmoving in PFTrack class we mention that you can enter your camera’s film back or sensor size in a Clip Input node. But why is it important to make sure PFTrack knows the correct sensor size? When solving a scene, PFTrack has to determine a plethora of information about the camera and location a clip was shot, purely from how features are moving in 2D throughout the shot. Generally speaking, the more (correct) information you can give PFTrack about the camera and scene you are trying to solve, the less PFTrack has to figure out itself and the quicker and easier it is to get a good result. One of the many camera parameters PFTrack’s solver nodes have to calculate is focal length.
01. Using a Known Focal Length
Providing the camera focal length for a clip can be relatively straightforward if a prime lens has been used and you have knowledge of which lens it was that has been used for a shot. Even for a zoom lens, knowing which lens has been used can help by providing minimum and maximum values for focal length. However, focal length is usually measured in mm, and before PFTrack can make any use of a known focal length value in mm, it needs to know the correct film back (or equivalently, sensor size) of the camera.
02. How Does the Sensor Size Influence the Focal Length?
From a provided focal length in mm, PFTrack’s solver nodes calculate the horizontal and vertical field of view (fov). The field of view for any lens, however, differs depending on whether you are using a 35mm full frame camera, or a camera with a different sensor. This is why focal length information is often also given as “35mm equivalent”, if the camera’s sensor size is different.
The following diagram illustrates this relationship. The film back (sensor) size is identical in each row, the columns have identical focal length. In each case, the field of view is different.
03. Shooting with a DSLR
When shooting a movie with a DSLR, additional care must be taken. Films shot with your DSLR may have a different aspect ratio (for example 1920x1080 with an aspect ratio of 1.7778) than still images, indicating that only a part of the sensor is being used to record movie clips. A common scenario is that pixels are being cropped from the top and bottom of the sensor to shoot movie clips.
In this case, even if you know the sensor size of your DSLR, be aware that it is not using the full height of the sensor for movies. PFTrack helps you by automatically calculating the height based on the clip’s aspect ratio when entering the film back width.
A detailed explanation of these cases can be found in the Clip Input’s reference help page, where the diagram above has been taken from. You can open the reference help page by clicking the Help button when editing the node.
04. Entering a Camera Film Back Size
The size of a camera’s film back is part of the camera preset. The Clip Input node automatically assigns a default camera preset to a clip based on its resolution. However, sensor size information normally isn’t stored in metadata embedded in the source files. As a result, the automatically assigned preset often represent a generic camera, rather than the one that has actually been used.
If the current preset doesn’t match the actual camera, you can either select the correct preset from the drop down list, or create your own by selecting Custom from the menu and entering the values yourself. Custom presets can be saved and used again later for different clips (shot with the same camera model).
A Clash of Sizes
When entering the film back, you only have to specify one of either width or height. The other value will be calculated automatically from the clip’s aspect ratio. If the resulting value isn’t what you expected, either width, height or the aspect ratio may be wrong. Also check if the sensor isn’t cropped.
05. Entering a Known Focal Length
Most nodes, like the Camera Solver and Survey Solver nodes, let you enter a known focal length prior to solving for the camera. To enter a known focal length in these nodes, change the Focal length drop down list from Unknown to Known, then enter the focal length.
Specialised nodes like the Target Track cannot automatically determine a camera’s focal length, so it is especially important to enter the correct focal length.
Entering a Focal Range
Other than only specifying the exact focal length, the solver nodes also allow you to enter a focal range to specify a minimum and maximum focal length value for zoom shots, or to help a solver to estimate the focal length. Select Initialised from the Focal length drop down to enter a focal range.
In this article, you have learned about the fundamental relationship between the size of a camera’s sensor or film back, and the lens’ focal length. It then introduced the special case of cropped sensors, before explaining how to use camera and focal length information from set in PFTrack.