I thought I'd do a comparison of two of my current lenses, both designed for the smaller crop sensor cameras. One is Canon's EF-S 10-18mm f/4.5-5.6 IS STM and the other is Sigma's 10mm f/2.8 EX DC HSM Fisheye.
The Canon is a rectilinear lens. Wikipedia explains that "a rectilinear lens is a photographic lens that yields images where straight features, such as the walls of buildings, appear with straight lines, as opposed to being curved. In other words, it is a lens with little or no barrel or pincushion distortion. At particularly wide angles, however, the rectilinear perspective will cause objects to appear increasingly stretched and enlarged as they near the edge of the frame."
The Sigma is a diagonal, or full frame, fisheye lens. Wikipedia's explains that "as fisheye lenses gained popularity in general photography, camera companies began manufacturing fisheye lenses that enlarged the image circle to cover the entire rectangular frame, called a 'full-frame fisheye'. The picture angle produced by these lenses only measures 180 degrees when measured from corner to corner: these have a 180 degree diagonal angle of view..."
Basically, although the focal length is the same, the field of view is quite different and while the lines remain straight in a rectilinear lens they do tilt.
Here are two versions of the identical scene with both lenses. First, unaltered by software straightening and then with distortion correction applied in Photoshop Elements. Both shots are at 10mm, f/8, 1/500 second and ISO 100.
|Canon EF-S 10-18mm at 10mm pre correction|
|Canon EF-S 10-18mm at 10mm post correction|
|Sigma 10mm diagonal fisheye pre correction|
|Sigma 10mm diagonal fisheye post correction|