What projection does the Sigma 8mm f/3.5 EX DG circular fisheye use?
Asked 3/25/2026
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I want to use a Sigma 8mm f/3.5 EX DG circular fisheye to photograph a forest canopy for analysis software, which asks for the lens projection/distortion model. I haven’t found clear specs online. Is the projection type documented for this lens, and if not, how can I determine the fisheye mapping function myself?
Originally by Kate. Source · Licensed CC BY-SA 4.0
Kate
2mo ago
2 Answers
3
From the 2nd sentence of the lens's manual,
This lens is an Equisolidangle Projection type fisheye lens which provides a 180 degree angle of view in any direction against the film plane.
Thus, the mapping function is $$ r = 2f\sin{\theta \over 2}. $$
Originally by scottbb. Source · Licensed CC BY-SA 4.0
scottbb
2mo ago
0
Generated from our catalog & community — verify before relying on it.
Yes. According to the lens manual, the Sigma 8mm f/3.5 EX DG is an equisolid-angle projection fisheye. Its mapping function is:
r = 2f sin(θ/2)
where r is the image radius, f is focal length, and θ is the angle from the optical axis.
If you ever need to verify a fisheye’s projection yourself, photograph a long ruler placed exactly parallel to the sensor plane and passing through the image center. Measure how far each ruler mark appears from the image center, then compare those measurements to common fisheye mapping models (such as equidistant, equisolid-angle, stereographic, and orthographic) to see which fits best.
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AI2mo ago
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