Why do circular fisheye lenses produce a round image?
Asked 7/13/2014
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I’m trying to understand the optics behind circular fisheye lenses. My assumption is that they create a circular image because the lens projects an image circle that is smaller than the sensor area, so the full projected image fits inside the frame instead of filling it. Is that correct? Why do circular fisheyes cast a round image, and how is that different from a diagonal fisheye?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
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All lenses create a circular image, it's just that most of them have an image circle large enough that it covers the entire sensor. Vignetting at wide apertures is a manifestation of the image circle encroaching on the corners of the sensor as the circle edge is not as sharp as it would be with a narrower aperture.
With a fish-eye lens, the image circle is restricted by the field of view of the lens, which tops out at about 180°. Once you get wide enough it's not possible to make it larger than the sensor - to do so would mean parts of the image are from behind the camera, which is physically impossible.
Originally by user456. Source · Licensed CC BY-SA 4.0
user456
12y ago
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Yes—your basic idea is correct. All camera lenses project a circular image circle. Most lenses are designed so that this circle is large enough to cover the whole sensor or film frame, so you don’t notice its round shape.
A circular fisheye is different because it is designed to capture an extremely wide field of view, typically up to 180° in every direction. To fit that full view onto the sensor, the projected image circle is intentionally made small enough that the entire circle fits within the frame. That’s why you see a round image with black around it.
A diagonal fisheye also has fisheye distortion, but its image circle is large enough to fill the frame corner to corner, so the image uses more of the sensor area.
If a lens tried to show more than 180° across a straight-through image, it would imply imaging parts of the scene from behind the camera, which isn’t possible in this design. So circular fisheyes use a round image to record the full 180° field efficiently.
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