OK, beware, there is a fair amount of trigonometry in this post! Based on the info here: Fisheye Projection - PanoTools.org Wiki first we need to work out exactly the focal length of this lens, because 7.5mm is bound to be a rounding. The Samyang is meant to be an equisolid (equal-area) fisheye lens, so the lens equation is: R = 2f * sin (Θ/2) Θ (theta) is 90 degree in this case, the angle from the corners to the centre, R is 10.83333mm, half the sensor diagonal. What we get for f, the focal length, is 7.66mm. This is the smallest focal length without vignetting. Now, if we defish this to rectilinear, just how wide is it, and what focal length would we need to get an rectilinear lens that is as wide? This is a little more tricky. If you've run any fisheye images through tools to defish, you will have noticed that the result becomes a much wider aspect ratio than the 4:3 you started with. Horizontal angle of view is R = 8.666, f = 7.66 as we just found out, gives us 2Θ (the full aov from left to right) of 138 degrees. Do it again for the vertical angle of view, R = 6.5, and you get an angle of view of 100 degrees. Just for comparison, the 7-14mm at 7mm gives a horizontal angle of view of 102 degrees, so a fisheye is crazy wide in comparison! Now we need to change equation, to the rectilinear one. That page on the Panotools wiki handily gives this too: R = f * tan (Θ) So we know the focal length (7.66) and the angles, so how large a "pretend" sensor do we get when we defish? Horziontally we get about 40mm, and vertically we get 18.3mm. So the aspect ratio is about 2.2:1 which seems about right. How about if we had an m43 rectilinear lens, that we shot at 4:3 ratio then cropped to 2.2:1, what focal length would it have? R = 8.666, Θ = 69... and the focal length is 3.3mm! Very, very VERY wide.