Pentax 18-55mm f/3.5-5.6 SMC P-DA

Name: Pentax 18-55mm f/3.5-5.6 SMC P-DA
Brand: Pentax
Rating: 8.1 (10 reviews)

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Lens Review
Specifications

18-55mm	$76 average price

Lab Test Results

Blur
Chromatic Aberration
Vignetting
Geometric Distortion

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Buy the Pentax 18-55mm f/3.5-5.6 SMC P-DA

(From Pentax lens literature) The Pentax SMC P-DA 18-55mm ƒ/3.5-5.6 is recommended for use with digital cameras. Ideal for beginners, it provides a focal length covering a wide-angle to moderate telephoto range. Like all Pentax lenses, the SMC P-DA 18-55mm ƒ/3.5-5.6 zoom lens is treated with Pentax's acclaimed SMC coating for maximum light transmission, sharp definition, and high contrast while minimizing flare and ghost imaging.

Introduction
The Pentax 18-55mm ƒ/3.5-5.6 is the ''kit'' lens sold with the K10D and K100 (and previous ''*ist'') digital SLR cameras. It is designed to fit a camera with an APS-C (''subframe'') sensor, so while it will mount on Pentax film bodies, it will show obvious vignetting when set to any focal length wider than 24mm.

Small and light, the 18-55mm represents a field of view of approximately 28-80mm in 35mm terms. To economize and create a more efficient design, the lens is equipped with a variable aperture; as the zoom extends the focal length, both the smallest and the largest aperture changes. The following chart represents the largest and smallest apertures you can expect at a given focal length:

Focal length	18mm	19-34mm	35-49mm	50-55mm
Largest aperture	ƒ/3.5	ƒ/4	ƒ/4.5	ƒ/5.6
Smallest aperture	ƒ/22	ƒ/29	ƒ/32	ƒ/40

The lens ships with a petal-shaped hood, and while it comes standard with several body packages, you could probably find it on its own for less than $100.

Sharpness
Most ''kit'' lenses give a fairly respectable zoom range and above-average performance when stopped down to around ƒ/8. I'm happy to report this version of the Pentax 18-55mm ƒ/3.5-5.6 pushes the envelope to some degree.

The lens never exceeds three units on the blur chart, regardless of the focal length and aperture combination, which is fairly remarkable considering the range of focal lengths available. Wide open and wide angle (18mm at ƒ/3.5), the lens provides a fairly sharp image hovering around two blur units on the scale, though the center is sharper than the edges. With a difference of one unit between the center and the edges, I imagine this difference would be difficult to detect. Stopping down to ƒ/5.6 provides uniform sharpness at just over one unit.

Zooming out towards 35mm one can see similar performance, but at 35mm the image is remarkably sharp at the largest available aperture, ƒ/4.5. Sharpness at this aperture is almost identical to sharpness at ƒ/8, and I would suggest that either of these setting combinations would be optimal for this lens.

The lens still performs well zoomed in all the way to 55mm, with good performance at ƒ/5.6. As the aperture is stopped down you do encounter diffraction limiting at apertures greater than ƒ/22. Even there, where other lenses show wildly soft results, the Pentax 18-55mm ƒ/3.5-5.6, as mentioned previously, doesn't exceed three blur units, and the image quality is uniformly smooth across the frame.

All in all, for the money you'd pay for this lens (or didn't, if it came with the kit), this is outstanding performance for sharpness.

Chromatic Aberration
Wide-angle lenses typically have some issue with chromatic aberration, and the 18-55mm ƒ/3.5-5.6 doesn't escape this trend completely. Set to wide angle (18mm) you see the worst results, but pleasingly, only at smaller apertures. At ƒ/4, you actually get the best performance out of the lens for resisting chromatic aberration. As the aperture decreases, the chromatic aberration becomes more evident. Even here, the lens doesn't exceed 9/100ths of a percent of frame height which, while noticeable, isn't the worst we've seen, and that's with the lens set to ƒ/22. With the focal length set to anything greater than 18mm, chromatic aberration decreases significantly, until by 35mm the effect is negligible.

Shading (''Vignetting'')
The Pentax 18-55mm ƒ/3.5-5.6 does present some corner shading in certain situations. Set to wide angle (18mm) you'll get about 2/3EV darker in the corners when set wide open to ƒ/3.5, but this is about the worst it gets for this lens. At 18mm, corner shading never really goes away - it approaches 1/4EV at ƒ/22 - but set to other focal ranges, the effect is more tolerable. It's noticeable at any focal length below ƒ/8, but otherwise, the effect is negligible.

Happily, the lens seems to be optimized for the focal length of 35mm, so setting it here, even at ƒ/5.6, produces non-vignetted results.

Distortion
Being a wide-angle lens, distortion comes with the territory when set to wider angles, and because of a simpler design, the distortion gets somewhat complicated when zoomed in greater than 35mm.

Beginning with the wide-angle picture, distortion approaches 1% barrel in the corners when set to 18mm. This is fairly typical of lenses with this focal range, and the distortion is uniform throughout the image. As the focal range approaches 35mm the distortion approaches 0%.

Zoomed in past 35mm, some pincushion (''squeeze'') distortion results in the corners, however this is complicated with some lingering barrel distortion re-appearing in the center. This effect is significant on our chart, but may be hard to see on a print, as the effect is less than 0.25% in the corners. However, if straight lines are important to you, than shooting outside of the focal length of 35mm will be asking for a bit trickier correction in your image processing software.

Autofocus Operation
Autofocus timing depends largely on the body the lens is mounted to, as the lens is mechanically driven with a screw. On a K100 Super, the lens is surprisingly quick to focus, at less than one second to rack through the entire range. Being mechanically driven, the lens does make some noise while the lens does a full-range focus. Point to point focusing on the K100 is very quick, and comparatively silent.

Macro
The Pentax 18-55mm ƒ/3.5-5.6 isn't a dedicated macro lens, and this shows in its maximum magnification of 0.33x and close-focusing distance of 25cm (around 8 inches). Not bad, but there are better dedicated lenses.

Build Quality and Handling
At only 225 grams (7.9 oz), the 18-55mm ƒ/3.5-5.6 is very light, but its construction is very solid. The mounting plate is metal but I suspect the filter ring is plastic. It has some nice finishing touches such as a distance scale, and the ability to override autofocus and focus manually by just turning the focus ring at any time (you don't need to switch it into manual). The lens hood also gets special marks in my book for having a removable plate that lets you have access to, and turn, a mounted polarizer. The lens hood also mounts backwards for easy storage, though it's not easy to use the lens with the hood mounted this way.

Speaking of polarizers, mounted filters won't turn when the lens is zoomed or focused, which is always preferred. It's not a constant-length lens, in that as you select a different focal length by zooming the lens will change its length. It's at its smallest at 35mm, and the lens will extend out at any other focal length from there. Zoom and focus rings take a little more than a quarter turn to run through their entire range, and the rings are well textured and smooth to turn.

Alternatives

Pentax 18-55mm ƒ/3.5-5.6 AL II SMC DA ~$200
Slated to be available in March 2008, it remains to be seen whether this upgrade will be a better performer than its predecessor; according to the specifications, there is one less lens element and it magnifies a hair more (0.34x instead of 0.33x). However the lens could be completely redesigned, to give improved performance.

Pentax SMC DA* 16-50mm ƒ/2.8 ED AL IF SDM ~$750
We haven't yet tested this lens, but it's clear just from the price difference that Pentax has put a bit more technology and design behind this lens.

Pentax 18-35mm ƒ/4-5.6 AL SMC P-FA J ~$150
The build quality of the 18-35mm is a bit more economical, with a plastic lens mount, but it's a full-frame lens. Optical quality is very good, but comparable to the 18-55mm, so there's not much point unless you're mounting on a film body.

Sigma 18-50mm ƒ/3.5-5.6 DC ~$100
The Sigma comparable for the focal length and aperture range, it costs about the same, but unfortunately, I can't tell you how it compares because we haven't tested it. With a bit more reach on the Pentax, everything (especially cost) being about even, I'd go with the Pentax.

Conclusion
I was initially skeptical of the Pentax kit lens, but in the final analysis I can report that Pentax has produced a solid lens, with very nice handling and excellent optical properties. It has a few small problem areas - some trouble with chromatic aberration and an odd distortion pattern - but this is a sub-$100 lens we're talking about here. They say you get what you pay for, but in this case I think you get a little more than what you pay for.

Sample Photos

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.