The Pentax DA 14mm and the Rokinon 14mm lenses both have some degree of distortion. That is to be expected since most ultra-wide angle lenses are prone to distortion. Both lenses are designed as rectilinear lens. They are designed to display images, such as horizontal or vertical lines in a straight pattern. The lenses should have little or no barrel or pincushion distortion. Barrel distortion would cause the lines to curve or bulge out ward. Pincushion distortion would cause the lines to curve inward. Thankfully, both types of distortion can be corrected by software, such as Lightroom or DxO Optics Pro.
For this distortion test, we found an evenly lit interesting brick wall that had regularly spaced joints. With the camera on a tripod, parallel and level with the brick wall, a series of shots were taken. We wanted to see how well the lenses were designed to eliminate distortion.
The results of the brick wall clearly indicated that the Pentax 14mm was by far the better lens. The Rokinon displayed slight barrel distortion. The big surprise was that the Rokinon is designed as a full frame lens and should have handled the distortion better when used on an APS-C sized sensor, which utilizes a smaller part of the image circle.
Vignetting by definition is a reduction of an image’s brightness compared to the image center. Vignetting can be done on purpose to draw attention to the center of an image and is used extensively in wedding and portrait photograph. Ultra-wide lens are more susceptible to optical vignetting because they cover a wider field of view.
For this vignetting test we took a series of pictures of a grey board and looked for an even spread of grey across the image. The other factor that effects the lens vignetting is the F stop. As the lens is stopped down it will reduce the effects of the optical vignetting. Problems exist when one or more of the corners are significantly darker than the center. Below is a picture of the worse of the vignetting from the Pentax DA 14mm at F2.8. The lower right and left corner is darker than the center.
The Rokinon did much better than the Pentax 14mm wide open in part because the lens is designed for a full frame camera. Below is a picture of taken by the Rokinon 14mm at F2.8.
The Pentax 14mm had visible vignetting at F2.8, but it did clear up at F4.0. The Rokinon did not have a problem. It would be unusual if it did have problems in that area since the lens is designed for full frame use and the APC-S sensor is smaller.