What Every Matchmove Artist Needs to Know About Lenses

…but is afraid to ask

Thomas Angarano
Jun 20, 2018 · 7 min read
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A Cooke lens on a RED Scarlet camera

What is lens distortion and how does it affect me?

When taking a picture or filming, the job of the lens is to direct beams of light onto the film or image sensor. In reality, lenses are not perfect at performing this job and photons from a straight line object often end up in a curved line, which results in a distorted image. This is called lens distortion. The most straightforward types of lens distortion are barrel distortion, where straight lines curve outwards, and pincushion distortion, where straight lines curve inwards. Lens distortion is usually more pronounced towards the edges of the frame.

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Image showing the differences between Barrel and Pin Cushion distortions.
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Result of a camera track in PFTrack with lens distortion and without. The highlighted areas match the straight wall in the corrected version better.

Types of lenses

There are (at least) two ways of classifying lenses, prime (or fixed focal length) versus zoom, which can be further complicated by being spherical or anamorphic.

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Comparison between a spherical and anamorphic lens

Focal length in matchmoving

Focal length is the most prominent property of a lens. It is often the first thing mentioned in any listing of lenses to distinguish them, and also what differentiates prime and zoom lenses. The focal length, usually denoted in millimetres (mm), defines, for a given camera, the extent of the scene that is captured through the lens. This is also referred to as the (angular) field of view (FOV).

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The relationship between sensor size, focal length (f) and angular field of view (FOV). Note how for the same focal length f, the field of view (FOV) differs for both sensor sizes.
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Cropped imaging area (36 x 20.25mm) vs full sensor size (36 x 24mm)

Conclusion

The camera’s lens has huge impact throughout the VFX pipeline and it is the matchmove artist’s job to mitigate most of this impact. The Pixel Farm’s matchmoving application PFTrack has a wide range of tools to use available information about the lens and camera, as well as handle situations where no such information is at hand. It also provides the necessary tools to manage all aspects of lens distortion.

References

VFX Camera Database

PFTrack Blog

User success stories, technical how to's and other…

Thanks to Hannah Karim

Thomas Angarano

Written by

PFTrack Blog

User success stories, technical how to's and other information related to matchmoving, camera tracking and photogrammetry with The Pixel Farm's PFTrack.

Thomas Angarano

Written by

PFTrack Blog

User success stories, technical how to's and other information related to matchmoving, camera tracking and photogrammetry with The Pixel Farm's PFTrack.

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