Pentax DA* 11-18mm F/2.8 DC AW Review

Pentax created a new optical formula for the DA* 11-18mm F/2.8 DC AW to resolve 24 megapixels of detail that required by the K-3 II, their flagship camera of its time. The optics in this top-of-the-line lens are truly impressive. They deliver nearly perfect image-quality at all combination of focal-length and aperture. This lens has been optimized for absolute uncorrected image-capture which makes it large and heavy.

For a lens capturing such an extreme angle-of-view, geometry is rendered remarkably well. Near 11mm, there is a barely visible amount of barrel distortion, less than 1.5%. Zooming in, this slowly disappears until minimal pincushion distortion appears right at 18mm, less than 1% distortion. This is quite important for a DSLR that cannot preview any software compensation in its viewfinder.

Significant vignetting is present in images taken with the DA* 11-18mm F/2.8 DC towards shorter focal-length. At its widest and fully open, there is almost 2-stops of vignetting. Stopping down improves the situation marginally and even at F/8, there is a full-stop of vignetting remaining. The least wide side performs much better. It shows noticeable vignetting fully open, about 1-stop but this becomes imperceptible by stopping down to F/4. Vignetting is easily correctable and software with minimal image-quality loss, so this is not a huge issue.

Contrast is excellent with this lens which shows really good resistance to flare. Colors are also good at most combinations of focal-length and aperture. Oddly, there is a shift towards yellow when heavily stopped down. It happens around F/8 near the wide end and near F/11 towards the long end. This is a little unusual but most photographers will not notice since Automatic White-Balance compensates for that.

Pentax 11-18mm F/2.8

Pentax DA* 11-18mm F/2.8 DC AW Sharpness

Sharpness from this lens is really good in the center of images and softens towards extreme corners according to the aperture. The center is sharp wide open at all focal-lengths but is optimal from F/4, just one stop down from the maximum, which is really good. Diffraction sets in at F/13 when mounted on a Pentax K-3 III.

Corners start off notably soft at F/2.8. Just a half-stop down, the entire image becomes sharper yet still leaving some corner softness. At F/4 images are sharp until all edges and corners become even sharper at F/4.5. Stopping down from there gives high consistent sharpness until the diffraction limit..

What is shown below are 5 crops taken from a photograph, repeatedly captured for each combination of focal-length and aperture. The smaller pieces are cropped from the extreme corners of the image, while the middle wide crop comes from the center of the image. Select an aperture in a row for a desired focal-length to see the crops from the corresponding image. When judging quality, keep in mind that these crops come from a 26 MP image which is normally used to print an image up to 30x20". On a computer display, these may appear much larger which magnifies image defects.

Upper Left

Upper Right

Center

Lower Left

Lower Right

The above interactive crops really show the difference between extreme corners to the center and how it varies according to aperture. Stopped down at F/4, this lens produces detailed images. By closing the aperture even just a little further to F/4.5 and beyond, the output from this ultra-wide lens is solid. It does not reach the level of the DA* 16-50mm F/2.8 PLM AW yet because it is much more difficult to make a sharp ultra-wide zoom.

Autofocus Performance

Following years of struggle with SDM autofocus motors, Pentax switched many of its modern lenses to DC motors. These are proving to be immensely more reliable yet outstandingly slow. The autofocus performance of the Pentax DA* 11-18mm F/2.8 DC AW is as slow as expected. It easily takes a second for it to move focus even over a relatively small distance and its performance is equally slow on the K-5 IIs as the flagship K-3 III. This makes sense since the focus motor is inside the lens and it has to move rather large optical elements.

This lens is inadequate for following any moving subject yet, that is clearly not its intended use. Typically a lens like the DA* 11-18mm F/2.8 is chosen for architecture and astrophotography. Stars move so slowly that they appear stationary and buildings do not tend to move either, so autofocus performance is not critical here.