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Converting diagonal fisheye images to rectilinear:
To get sufficient wide angle coverage with current Canon digital SLRs one may have to resort to using a diagonal fisheye lens, like the Canon EF 15/2.8. Due to the "cropping effect" this lens gives horizontal angles of view of 116° and 94° on the EOS 1D and EOS D30, respectively. On the D30 this corresponds to the coverage of a 16.5 mm rectilinear lens.
The resulting images will of cause not be rectilinear. However, it is possible to remap the fisheye images to rectilinear using a free PhotoShop plugin called Panorama Tools developed by Helmut Dersch. See his example of a rectilinearized picture.
Once Panorama Tools have been installed, basically just one parameter is needed in order to convert the fisheye image to rectilinear - the horizontal angle of view. This has to be entered according to landscape or portrait shooting mode. In landscape the value is theoretically 116° and 94° on the EOS 1D and EOS D30, respectively, as indicated above. In portrait mode the values are 76° and 64° on the EOS 1D and EOS D30, respectively. However, the D30 images below have been converted using values of either 90° and 60°.
Additionally, values for Yaw (Left/Right), Pitch (Up/down), and Roll may be entered. The Yaw and Pitch provides the ability to correct perspective, similarly to a (tilt and) shift lens. The Roll provides the ability to correct the horizon.
One advantage of using Panorama Tools is that all the remapping filters can be applied to the image in ONE process, reducing the image degrading calculations to a minimum.
Below some examples of the possibilities with Panorama Tools, all shot with Canon D30 and Canon EF 15/2.8.
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Shopping
area, Copenhagen Kastrup Airport |
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Natural
History Museum, London |
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Extinct
Giant Armadillo, Glyptodon clavipes |
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Since the rectilinearization essentially involves magnification at the perimeter and condensation in the centre of the image, the resolution of the original image will be unevenly distributed in the resulting image. It is possible to define the size of the resulting image, and if a very large image (relative to the original) is produced most of the original detail can be preserved. If a rather small image is produced, much detail may be lost. In any case, some pixels will be remapped to outside the resulting rectangular crop, leading to a reduction in the utilized number of pixels.
Below are examples of the Central Hall in the Natural History Museum in London. A 2160 x 1440 pixel TIFF image was used for the rectilinearization using a HFOV of 60 (portrait). The resulting image was of the same dimensions. No Yaw, Pitch or Roll was applied. The images below represent crops of the original image located either near the centre or near the edge. The crops have been magnified 300% without interpolation. No other post-processing have been applied.
Original Rectilinearized Centre Edge
