Pentax 14mm F2.8 vs Rokinon 14mm F2.8

Chromatic Aberration and Bokeh

Chromatic aberration occurs because the lens is does not focus all the different wavelengths onto the image sensor at the same point. The results are usually seen as a purple fringe on the image. The likelihood of chromatic aberration increases in high contrast situations and wide open aperture. To decrease the chances of chromatic aberration avoid high contrast situations if possible; if not, then stop down your lens.

In the picture below you will notice the right side of the palm is exhibiting chromatic aberration. Just like distortion, chromatic aberration can be corrected by software, such as Lightroom or DxO Optics Pro.

Chromatic Aberration Verdict

The Rokinon was the clearly better in this area. Not only did it have very limited chromatic aberration wide open, but it was completely clear by F4.0. The Pentax DA 14mm, on the other hand, clearly displayed chromatic aberration at F4.0 but it did improved significantly beyond F5.6.

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.

Both the Pentax 14mm F2.8 and the Rokinon have six blades so the highlights will look like rounded hexagons. The two lenses are classified as ultra-wide angle lens and therefore 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.

Below is a sample of how difficult it is to achieve bokeh with an ultra-wide lens. The picture was taken with the aperture wide open but because of the depth of field everything is in focus. The alternative is to focus very close to the object but then you might create distortion.