Pentax-DA 50mm F1.8

Bokeh

Bokeh is a Japanese term which refers to the quality, look and texture of the background blur. It does not relate to the depth of field but to the areas in the image which are beyond the range that is expected to be in focus.

A smoother bokeh is generally perceived as being of a higher quality. Bokeh is influenced by the lens design, with a significant role being played by the aperture blades. Their number and shape will influence the look of the bokeh, especially around highlights and light sources.

The DA 50mm uses 7 aperture blades. That number is on the low side, but they are curved (or rounded) to provide a smoother more even shape for the aperture, especially at wider values.

Bokeh Test

In order to evaluate the characteristics of the background blur, we took pictures at varying apertures, using these test parameters:

The scene provides interesting out-of-focus features, as well as a distance gradient for the highlights. The focus point was set at infinity. Click on the images to see larger versions.

F1.8
F2
F2.8
F4
F5.6
F8
F11
F16
F22

The shape of the out-of-focus highlights is very even, especially at wider apertures. The rounded aperture blades play their role well. Wide open, the bokeh is a smooth, featureless field, which is what we would expect from a F1.8 lens. Shapes start to become clearer at F4, but the outlines still remain relatively smooth. At F5.6 some highlights start to show a geometrical shape (look at the brighter spot just left of the center), showing that the rounded blades have less effect at this point.

Starbursts appear starting at F8 for the light sources farther away from the camera (thus closer to being in focus).

Real-world examples

One of the main purposes of lenses with larger apertures is to create shallow depth of field images. So we created the two following images to illustrate this, and to show the texture of the bokeh in a realistic, busy frame. The images were captured with the lens at its closest focus distance. Focus was locked manually on the central shaft of the clock. Two images were captured, at the widest setting and at f2.8, a benchmark value for many high-end lenses. Click on the thumbnails to load full-resolution JPEGs.

F1.8
F2.8

Portraiture Compared to the Kit Lens

The DA 50mm's faster aperture gives it a big advantage for shooting portraits when compared to the 18-55mm kit lens.  You can see this in the video below:

Verdict

The DA 50mm has a smooth and pleasing bokeh, in particular at wider apertures. Even when closed down, the bokeh remains devoid of harsh features. An excellent result.

Further examples of the performance of the lens can be found in the sample gallery near the end of this review.