I think I wrote about this earlier but
Full frame sensor 35mm wide x 24mm high w/diagonal 43.3mm
Canon 20d 2/3rd sensor is 22.5mm wide x 15mm high w/diagonal 27.06mm
full size sensor diagonal 43.3
Canon 20d diagonal is 27.06
(43.3 / 27.06 = 1.600 ratio
8mm Sigma is really is a 12.8mm lens effective 35mm = 8 x 1.6 = 12.8mm.
The largest image Canon 20 records is 3504 w x 2336h pixels
When you put your 20d on the vertical bracket your image will be oriented long side up. This way you get a full 180 degree angle of view in the up position. However, now the horizontal angle of view is truncated to 119.9 or 120 degrees
Full 180 degree angle of view covers top to bottom of the image that is 3504 pixels high.
So 3504 h / 2336 w = .66666
180 degrees x .66666 = 119.9 or 120 degrees angle of view.
120 degree angle of view x 4 images = 480 degrees with overlap
480 - 360 = 120 / 4 = 30 degrees overlap for stitching.
So it appears that 4 images taken every 90 degrees on your pan head is good enough for stitching.
Ahhhh .. now I forgot where we were going with this discussion. I will read this thread again and come back later for a conclusion.
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