Vignetting by definition is a reduction of an image’s brightness in the corners compared to the image center. Vignetting can be sought purposefully to draw attention to the center of an image, which is why it is used extensively in wedding and portrait photograph. Ultra-wide lenses 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. All lenses have some vignetting but it usually falls within acceptable tolerances. For the Pentax DA 12-24mm f/4, the Sigma DG 12-24 f/4.5-5.6, and the Tamron SP AF 10-24mm f/3.5-4.5 DI II lenses in this review, we 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. In reviewing the images, keep in mind some may be darker than others but that does not necessarily indicate a problem. Problems exist when one or more of the corners are significantly darker than the center.
Earlier we mentioned that vignetting can be done on purpose, it can also be done by accident. Because these lenses are so wide at the extreme zoom range of 12mm, it is possible to create unintended mechanical vignetting. If you use a filter and the filter ring is too thick it can darken the corners of your pictures. In the picture below of the fluffy clouds you will notice that the corners appear a little darker..
All the lenses did very well on the vignetting test. The Pentax DA 12-24mm f/4 and the Tamron SP AF 10-24mm f/3.5-4.5 DI II did have some minor vignetting in the corners at 12mm and 10mm respectively, when they were wide open. Both lenses improved dramatically when they were either stopped down or moved beyond the extreme range of the zoom. The Sigma DG 12-24 f/4.5-5.6 never had a problem, but since the lens is designed for full frame use and the APS-C sensor is smaller it would be unusual if it did.