The best APS-C DSLR "digital" standard lens performer for its money.

Preamble: This review is intended for advanced amateurs to professional photographers. Some of the terms used will not be explained in details, however, I would try to explain them in brief within brackets. If you still do not know exactly what the bold terms or some other terms exactly mean, do a web search and find some other articles for more details.

Introduction:
Marketed as the first digital (optimised for the characteristics of CCD digital sensor and the smaller size of image circle as well) lens for the Pentax DSLR (Digital Single Lens Reflex) family, this is a standard zoom lens with useful 135 film equivalent range of 24-70mm and with a constant maximum aperture of f/4. I shall explain the pros and cons about this lens in the following paragraphs, for each of the major aspects in assessing the goodness of a lens.

- Operation and build quality:
The build quality is excellent, especially when considering the prosumer price tag (I got mine at $350, bought separately from the body). It has metal lens barrel outside with dark coating. The focusing ring, when used for manual focus (MF), is very smooth and is equipped with a "Quick Focus Shift" system for instantaneous MF adjustment, without setting the AF/MF level. The manual zoom ring is also tight but smooth. The soft plastic materials on the rings further enhance the handling. What I don't like is only about the zoom direction of this lens, that is, it will zoom to the longest at the widest setting. This means that a dark circular shadow will be created in central bottom of the pictures if built-in flash is used and the lens is zoomed beyond 28mm (towards 16mm). This is especially obvious for near subjects too.

- Resolution (the lens’ ability to resolve image):
Of extremely high resolution even at far corners for almost all apertures, including wide opened at f/4. Needless to say, the resolution at the image center is extremely high.

- Chromatic Aberration (usually called the CA, it refers to the positional errors in reproducing lights of different colors, especially for the red and blue lights):
Quite noticeable at corners for high contrast situations, e.g. detailed objects against a very strong backlight. The CA often becomes a little bit more noticeable when stopped down from f/5.6 and onwards.

- Contrast (the difference between dark and bright objects):
The lens is quite "contrasty" but I don't like that personally. Pictures will usually look sharper when the contrast is increased, no matter whether it is the "true" contrast produced by the lens or something artificially created in-camera. But there is no free lunch here, with the relatively weak ability in reproducing both highlights and shadows for all current digital sensors, the high contrast means the loss of either highlight or shadow details or both!

- Light intensity reproduction and 3d feel (linearity of greys + the ability in reproducing different levels (depth) of intensity and colors):
I think this lens is unable to reproduce detailed description comparatively to expensive primes, especially under high contrast lighting situations. The images and scenes recorded often look rather flat and without good 3d feelings.

- Bokeh (the attractiveness of out-of-focus area):
To be acceptable for the smoothness and consistence but still unmatched to Pentax primes. Mechanically, this lens has 8 aperture blades but not round in shape when closed down. Theoretically, the bokeh will be the best for perfect round shaped aperture. Of course, the optical formula will also contribute to the quality of bokeh.

- Color rendition and saturation:
Very rich and accurate but can still be considered somehow a little bit exaggerated. But again this maybe a digital problem instead afterall.

- Distortion:
Slightly noticeable (lines blended outwards) near the wide end but the distortion has been quite well controlled for a lens of its class, especially by considering that it is a zoom. It is even better than the Canon 17-40/4L at the widest side (peoples usually put this L lens on the Canon 10D DSLR) which nearly double the DA 16-45's price but shorter in zoom range.

- Flare resistance:
Outstanding! No matter how I point the lens directly to a bright sun or sky, no fogging of the lens nor any obvious flare can ever be seen. This really represents the true power of Pentax's renowned ghostless SMC (Super Multi-Coatings)

- Vignetting (light fall-off at the corners which results in darker picture corners):
Not much noticeable even at wide opened for the whole zoom range, i.e, f/4.

- Accessories:
Bundled with dedicated flower shaped plastic lens hood. There is a small removable window under the hood for the turning of polarizer when in use and it is a unique thoughtful design from Pentax.

- Size and weight:
Longer and heavier when compared with a similar offer from Nikon, the DX 18-70/3.5-4.5, which is also designed for APS-C DSLR. But it is smaller and lighter than the luxury Canon 17-40/4 L but with matching optical quality.

Conclusion:
There is not much choice currently on the market for such a digital standard lens for APS-C DSLR. Of course, one can use older lenses of the same system for the DSLR. But with the 1.5X multiplying factor for APS-C DSLR, new standard lenses will be required to cover the new "standard" range. Moreover, I have compared quite some expensive glasses with this digital lens and found that the "digital optimized" claims are not simply a marketing hype which quite some ppls would believe. But instead, it is quite true in certain extends the digital lens can yield better results and that an excellent lens for film SLR will not be necessary be an excelelnt lens on a DSLR.

By weighing the optical performance, price and handling characteristics, I would conclude that this would be one of the best (or even the best) digital standard lens(es) currently on the market and I would feel very comfortable to recommend it to everyone who wants to find such a lens for their DSLR but still undecided.

P. S. I have got this lens for months and have used it to shoot near two thousands of photos already.