DA 12-24mm vs Sigma and Tamron 10-24mm Comparison
Image Quality: Bokeh
Bokeh is Japanese for blur, or the aesthetic quality of the blur, in a photograph. A lens is said to produce good bokeh if the blurred pattern is pleasing to the eye. Conversely a lens that produces bad bokeh is said to have a harsh or course blur pattern. Bokeh is produced in part by the lens design. The shape and number of blades in the aperture does not always determine if the lens produces good or bad bokeh. However, the number of blades determines the shape of the out-of-focus -in highlights. On the Pentax DA 12-24mm f/4 which has eight blades the highlights will be nearly circular. On the Sigma DG 12-24 f/4.5-5.6 which has six blades the highlights will look like rounded hexagons. On the Tamron SP AF 10-24mm f/3.5-4.5 which has seven blades the highlights look like a rounded heptagon.
For this test we could not spot a clear winner. The bokeh that the lenses produced was not bad. On the other hand, none of the lenses produced the smooth creamy quality that should be expected when we think of “good” bokeh. Of the three lenses, in this comparative review, the Sigma DG 12-24 f/4.5-5.6 did produce the best bokeh. 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, all have a very large depth of field making it difficult to get the background out of focus and still be visible. When purchasing an ultra-wide lens you are usually looking for a lens that will be used for sweeping vistas and architectural views, not for portraits. If you want a lens that consistently provides good bokeh, you might want to consider another type of lens.