Lenses in the real world are laden with imperfections such as barrel distortion, chromatic aberration and vignetting. However, virtual cameras are flawless. Mitigating that difference when introducing CG elements into live action photography requires distortion management in all stages of production before reintroducing the irregularity back on our composites.
The benefits of properly managing distortion are:
Increased precision of 2D/3D tracking solutions
Accurate 3D reconstruction of photographed objects
Full control recreating lens irregularities during composite
Lens distortion management begins on set immediately following principal photography. The lineup procedure described below is key to accuracy in your lens distortion workflow.
Photograph a Lens distortion Chart with one of the following patterns: grid, checkerboard or linear dot array. Large format charts garner higher degrees of accuracy, limiting depth of field while maximizing image sharpness. The chart should include a clear Optical Axis Point.
This is the lens grid we designed at MARZ VFX
Note: The need for large format charts becomes most evident at conditional extremes, e.g. using the widest focal length to photograph a small chart will force your camera’s position to be inches away from the chart sacrificing peripheral sharpness while introducing DOF.
There are two methods of capturing lens charts:
CLASSIC CHARTING
MODERN CHARTING
Prime Lens Distortion Data Acquisition
Prime lens’ are fixed so classic charting is suggested
Choose a spacious room with a level foundation and a flat wall at 90°
Hang the large checkerboard chart on the wall, ensure it’s level
Position your camera perpendicular to the checkerboard. Translate your camera to a position which fills the field of view with squares, ensure your squares are equally visible.
Line up the optical axis of your lens with the optical axis of your checkerboard
Be sure to eliminate any yaw, pitch, roll
Use a 3-axis spirit bubble adapter on your camera to ensure all angles are right angles
Light your lens chart evenly
Shoot Manual at ƒ8-ƒ22 at slower shutter speeds
Slate your data, WHO, WHAT, WHERE, WHEN: date, technician’s name, set location, camera distance from chart measured from nodal point of lens, lens brand, focal length, serial no.
Record 3 seconds of video, including your lens slate
Repeat steps for every fixed lens required
Zoom Lens Distortion Workflow
The distortion parameters of a zoom lens at minimum focal length compared to maximum focal length will vary.
Accounting for these varying characteristics requires gathering dynamic lens distortion data compiled from several lens distortion charts, captured at intervals, throughout a lens’ focal range.
Canon EF 24-70mm: 24mm> chart 35mm> chart 50mm> chart 60mm> chart 70mm> chart
Capture your chart at each interval, in our case above, we would provide our matchmove department with 5 lens chart images to map the Canon EF 24-70 lens.
The matchmove artist will compile the charts to construct a virtual lens by plotting the intervals on a timeline which represents the focal range of the lens. Each interval is key-framed marking its position on the scale, the keyframe tangents are smoothed out and we are left with a distortion blend-shape, which is animated by the cameras solve and output for use in composite.
Plotted intervals of zoom lens on timeline in 3DEqualizer for virtual lens build
Zoom Lens Distortion Data Acquisition
Choose your focal range intervals
Go through the steps above outlined in Prime Lens Distortion Data Acquisition
Capture each interval beginning at widest focal length
At each focal length interval, record focus distance intervals (see the spreadsheet below)
Organize your data so each interval is listed in order of each interval
AnAmorphic lens breath
Anamorphic lenses tend to breath a lot during focus pulls (see the gif below to witness the vertical/horizontal stretch of the image!) That’s very important phenomenon to replicate in compositing when finishing the shot. If data has been recorded properly and your Tracking Artist has compiled it correctly, you should have a virtual lens identical to lens it is replicating.
Anamorphic Lens Breath. Mapping by Kenny Yong using 3DE
Anamorphic Lens Breath Removed by Kenny Yong using 3DE
Note: Every lens is unique which means identical lens models will provide varying distortion data. Each lens utilized during principal photography should be measured and slated by their respective serial numbers.
Recording set data
Printing the Grid
The last printed grid I used for production was 96” x 48” print on 1/2" White/Black/Black Ultra Board. It’s a light weight and rigid material ideal for c-stand and flat wall suspensions. Here’s what it looked like at the printer!
Conclusion
When analyzing lens distortion accuracy is key. The more accurate your needs the more complex your methods will become, it is important to understand that perfection is impossible. That being said, it’s important to know what the output quality expectation is for your project, more specifically your shot, to avoid being overwhelmed by costly precision tactics.
Leading visual effects firms utilize the methods above because their clients expect it. An indie film maker, however, can shoot hand-held grids onset and garner respectable results by dictating what lines within footage should be considered to be straight in Nuke. This speaks volumes about the undistort algorithms found in softwares like 3DEqualizer, PFTrack, Syntheyes and Nuke.
Glossary
Barrel distortion, physical phenomenon bowing straight lines at the peripherals of an image
Classic charting, this chart is shot completely flat. Gathers distortion of a fixed lens
Dynamic Lens Distortion Data, compiled from several lens distortion charts, each captured at intervals throughout a lens’ focal range
Lens distortion Chart, a tangible chart to photograph
Modern charting, this chart is shot on an angle showing the vanishing point of a chart’s perspective. This data provides both lens distortion and focal length information
Optical Axis Point, the center point of your lens and lens distortion checkerboard chart
Ultra Board, light weight foam core material which is low flex and sturdy
References
Zwerman,S. & Okun, J.A. (2015) The VES Handbook of Visual Effects: Industry Standard VFX Practices and Procedures 2nd Edition. Burlington, MA: Focal Press
http://www.leova.com/vfx_library/Heckman_Set_Documentation_Dec_2006.html#Set
https://24liespersecond.com/VFX-Lens-Distortion-Grids
https://www.ssontech.com/docs/CameraCalibration.html
https://vimeo.com/109911859 PFtrack build lens
https://www.youtube.com/watch?v=G3bRWargARY&list=PLOC5FE-nMyONNM7pXWnz-z_9oBYjBhrX-&index=2&t=11s 3de dynamic Zoom distortion
https://www.youtube.com/watch?v=khMwtfmynac&t=447s Export lens distortion to nuke for 3de
UPDATE: Ultimate guide here:
https://www.youtube.com/watch?v=nVXqJNFzpoo