HD vs. SMC Pentax-DA 18-50mm F4-5.6

Chromatic Aberration

Chromatic aberration occurs when colors do not all have the same focal point. It results in a slight shift between the red, green and blue channels, visible mainly at the boundary between contrasted areas (black and white, for instance). It occurs to some extent with all lenses, and can be controlled by the use of achromatic elements. It can also be corrected fairly easily in post-processing. Modern cameras have an automatic correction mode that can effectively remove any pronounced chromatic aberrations.

Chromatic Aberration Test

For this test we used a well-lit, sharp transition from dark to bright, in order to demonstrate any chromatic aberration present in the image. The K-3's automatic CA correction was disabled. Various apertures were used. The lenses were set at 18mm since wider focal length usually show more CA. The image is tilted at approximately 45° of the focal plane of the camera.

We then looked at three parts of the image: the focus point, the top, and the bottom (beyond and before the focus point). You can click on the images to see larger views, and navigate each aperture image set by using the left-right arrows.

Focus Point

	18-55mm	DA L 18-50mm	HD 18-50mm
F3.5
F4
F5.6
F8
F11

Performance in the central, in-focus area are above our expectations, showing no discernible chromatic aberrations for either of the three lenses.

Top

	18-55mm	DA L 18-50mm	HD 18-50mm
F3.5
F4
F5.6
F8
F11

The top shows a strong color shift visible as a green on the black-to-white transition. None of the lenses tested differentiated from the others, though the new 18-50mm design might have a small edge. The HD coatings did nothing to improve CA control.

Bottom

	18-55mm	DA L 18-50mm	HD 18-50mm
F3.5
F4
F5.6
F8
F11

Aberrations show themselves mainly as purple fringing in the bottom of the frame. The absence of such a behavior in the top part suggests that, in this case, PF is caused by the lenses. Again none pulls ahead; the three lenses show fringing at all apertures.

Verdict

The lenses perform very well for in-focus subjects, showing no visible chromatic aberration. Things are different in the out-of-focus areas, however, with all three lenses exhibiting both chromatic aberration and purple fringing. All three versions showed about the same amount of fringing, which is moderate. It could still be visible in highly contrasted scenes. Luckily third-party softwares can remove many chromatic aberration effects, and in-camera corrections can also fix CA for JPEG files. This correction slows down the camera's operation, but we recommend using it if you do not wish to spend time post-processing your images.