DSLR – Pentax Body

The smc DA 15mm F4 ED AL Limited is Pentax’s latest lens, which was shown in prototype form at Photokina 2008 and formally introduced in March 2009. It’s a rare beast indeed: a wide angle fixed focal length (‘prime’) lens that is purpose-designed for APS-C format digital SLRs. The 15mm focal length gives an angle of view equivalent to a 22.5mm lens on the 35mm format; in essence, this lens is the modern equivalent of old favourites such as the 24mm F2.8 or the 21mm F3.5. The ‘Limited’ tag denotes premium construction, with the lens barrel made entirely from high-grade aluminium, which comes as a refreshing change from the engineering plastics used almost universally by other manufacturers.

As a fixed focal length lens, the 15mm is much smaller and lighter than zooms with a similar angle of view, and requires rather simpler optics as well. The formula uses 8 elements in 6 groups, including one hybrid aspherical element and one extra-low dispersion (ED) glass element, which combine to minimize distortion and chromatic aberrations. Other key attractions of the design are the ‘quick shift’ focus system, which allows manual adjustment even when the camera is set to autofocus mode, and a clever built-in sliding petal-shaped hood.

In a market dominated by identikit zooms, it’s certainly refreshing to see Pentax offering a genuine alternative, and catering to users who value the advantages of prime lenses over the undoubted flexibility of zooms. However the rapid advance in computer-aided lens design, coupled with manufacturing technologies such as glass-moulding for the inexpensive mass-production of aspheric elements, means that the once-yawning gulf in image quality between primes and zooms has narrowed considerably over the past decade. So the question is, does the 15mm F4 still have enough on offer to make any practical sense?

Headline features

• 15mm wideangle focal length, F4 maximum aperture
• ‘Quick Shift’ system allows manual adjustment of focus in AF mode
• Built-in sliding petal-shaped lens hood
• High quality aluminium construction

The 15mm F4 is a member of Pentax’s family of ‘Limited’ primes, which are unique amongst mainstream autofocus lenses in having a barrel sculpted entirely from high grade aluminium. Build quality is, in a word, superb, with fine engineering in every detail, complemented by markings which are engraved into the barrel as opposed to simply painted on. With its jewel-like construction, this is a lens which will certainly appeal to traditionalist photographers brought up on good old-fashioned manual focus SLRs and their all-metal primes.

The most unusual feature of the design is the built-in petal type hood, which retracts by sliding back into the lens body. While sliding hoods aren’t anything new, they are generally seen on larger lenses (normally telephotos) and tend to be simple cylinders in shape; managing to squeeze one into a body this small is an impressive feat of design and engineering.

	The lens mount is Pentax’s standard KAF type, using a mixture of electronic and mechanical connections to interface with the camera.  To mount the lens, align the red dot with that on the camera body, and twist clockwise. The black metal lever controls the aperture, and autofocus is driven from the camera body using a screw coupler; three and a half turns are required to travel from infinity to closest focus.
	The filter thread is 49mm and does not rotate on autofocus, which is good for filter users. In this view that unique petal-shaped sliding lens hood is shown fully retracted. However it still protrudes past the filter thread, which may make use of square filter systems (such as Cokin or Lee) rather problematic. Unscrewing polarizers can also be quite difficult, as there’s limited space to grip them.
	Here’s that very clever lens hood fully extended. It has slight ridges at the front that act as grips, making it easy to slide in and out. The movement is smooth, adding to the overall impression of precision engineering, and the inside is lined with black felt to minimize the reflection of stray light into the lens.
	The lens can be used with normal thickness (as opposed to slim) polarizers without vignetting, making for a far more economical solution than the slimline 77mm filters required with most wideangle zooms. As can be seen here, though, you have no chance of adjusting the filter with the lens hood extended. Get used to retracting the hood, rotating the filter, then extending the hood again.
	The focus ring has a 5mm wide grip, machined directly into the aluminium. It rotates 80 degrees anticlockwise from infinity to 0.18m, with a super-smooth action. Pentax’s ‘quick shift’ manual focus system allows you to tweak focus manually even which the camera is set to AF.
	Pentax has provided a proper distance and depth of field scale, with the markings engraved into the lens barrel rather than simply painted on. This (in principle at least) allows you to use hyperfocal or zone focus techniques very simply, great for street or landscape photography, for example.

The Pentax 15mm F4 performs quite competently in our studio tests. However like the Sigma 10-20mm F4-5.6 we tested recently, it shows distinct curvature of field, which causes problems with our (flat-field) chart tests at the shooting distance used (approx 2m). As with the Sigma, we have chosen to present data that is technically slightly rear-focused, and therefore gives a better impressions of the edge and corner sharpness at the expense of the centre (this data is also most representative of the results obtained when using the camera’s autofocus). Our test sample was also slightly decentered, giving marginally softer results on the right side of frame.

Compared to the Sigma 10-20mm F4-5.6 EX DC HSM (at 14mm), the 15mm F4 comes out on top; it’s a hair sharper, and has lower distortion and falloff. Unfortunately it doesn’t come out so well against the excellent Tokina AT-X PRO SD 12-24mm F4 (IF) DX, which we’d expect to perform near-identically to Pentax’s own smc DA 12-24mm F4 ED AL (IF) – in this case the zoom is sharper, and has lower falloff and equally low distortion, although it does show more chromatic aberration.

Resolution	Sharpness results are decent but not outstanding (however it’s important to understand that curvature of field effects are making the lens look less sharp than it appears in practice). Central sharpness is quite high wide open but corners are soft, however the lens improves considerably on stopping down. Optimum results are achieved in the region of F6.3-F9; stopping down further results in progressive softening due to diffraction, with apertures smaller than F16 best avoided (and rarely necessary in practice anyway).
Chromatic Aberration	Lateral chromatic aberration is visible although not excessive, and has a relatively linear profile which should make it straightforward to correct in software if desired. Fringing is predominantly red/cyan in color, and is essentially invariant with aperture.
Falloff	We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. We see just 1.3 stops wide open, which decreases progressively on stopping down; at F6.3 and smaller it’s effectively disappeared altogether. Overall nothing to worry about.
Distortion	Distortion is extremely low, with just a very slight barrel effect (0.3%). To all intents and purposes this will be invisible in actual use.

Chromatic aberration

Lateral chromatic aberration is visible in many real-world situations, taking the form of red/cyan fringing which is most visible on high contrast edges towards the corners of the frame. The samples below show how this appears in real-life, and how effectively it can be removed in post-processing. In this case, we’ve used Adobe Camera RAW v4.6, with a value of -20 applied to the ‘Fix red/cyan fringe’ slider in the ‘Lens Corrections’ tab. This has essentially eliminated the red component of the CA, leaving just moderate (and visually less disturbing) blue/yellow fringing in the extreme corners of the frame.

• Compact and lightweight
• High quality aluminium construction
• Quick shift manual focus system
• Built-in sliding hood (but see below)
• Good flare resistance
• Almost no distortion
• Small 49mm thread, doesn’t require slim filters

• Rather soft at F4 – F5.6
• Hood design causes problems with filter use (particularly square systems)

Overall conclusion

The Pentax smc DA 15mm F4 ED AL Limited is a unique lens in the current market, a small, lightweight wideangle prime for APS-C cameras. And let’s make it clear from the outset that Pentax should be applauded for producing such a lens, to provide a genuine alternative to the wideangle zooms which are the only option in other manufacturers’ lineups. This is indeed typical of the company’s attitude towards primes in general; it makes arguably the best APS-C optimized set currently on the market, which counts as something of a selling point for the K system as a whole.

The 15mm is not a bad performer, either. Chromatic aberration is reasonably under control (and easy to correct if necessary), falloff is low at just 1.3 stops wide open, and rectilinear distortion is essentially absent. Also important in real-world shooting is the fact that the lens is very resistant to flare when shooting into the light, an especially desirable characteristic for a wideangle optic. Unfortunately though it’s rather soft wide open, and even when stopped down is never breathtakingly sharp in the fashion we’d hope for from a premium prime. This is a little disappointing, especially considering the price and relatively slow maximum aperture; but then again you rarely have to shoot a lens this wide at F4 anyway (especially with the in-body stabilization in all current and recent Pentax DSLRs).

One positive aspect not to be overlooked though is Pentax’s signature ‘Limited’ build quality, which is excellent. The machined aluminium barrel, engraved markings and smooth focus ring may have no effect on the final image, but they certainly make the lens a more desirable object in its own right, and one which is a pleasure to use. The sliding lens hood, of course, will have a genuinely positive impact on image quality, and the fact that it’s built-in makes it much more likely to be used as a matter of course. Thankfully the metal construction doesn’t result in excess weight, and the lens is eminently portable, making it ideal if you don’t want to deal with the bulk of a wideangle zoom.

There are some issues with filter usage, which is something of a double-edged sword. On the plus side, polarizer fans can use standard 49mm filters, rather than the 77mm slimline types required by most wideangle zooms. This can easily amount to $100 difference in price, which is not to be overlooked. However filters can have something of a tendency to bind in place on the lens (presumably due to its aluminium thread), at which point removing them can become a frustrating battle, as the retracting hood impedes your grip. If you do use a polarizer you’ll also find yourself spending a lot of time sliding the hood to and fro, to gain access to the filter and adjust its angle. The other problem is that the hood protrudes past the thread when fully retracted, which will cause problems with square filter systems and therefore neutral density gradients – the other most useful type when shooting with wideangle lenses.

The real problem for the 15mm F4, though, lies in the competition, and specifically Pentax’s own 12-24mm F4 zoom. This has much in common optically with the Tokina AT-X PRO SD 12-24mm F4 (IF) ED, which we tested recently and found to be a superior performer in almost all regards. It’s simply much sharper than the 15mm F4, especially wide open, and exhibits lower falloff and equally low distortion (although it does suffer more from chromatic aberration). It’s not often a zoom lens can outgun a prime in terms of image quality, but that certainly seems to be the case here, and by a surprising margin too. At current prices at least you also pay relatively little extra for all the flexibility of the zoom (but do bear in mind the 12-24mm is a well-established product, and the 15mm brand new, so its price is likely to fall over time).

So overall we have a lens which is difficult to categorize. Its very uniqueness makes it desirable – if you want something a bit wider than the 18-55mm kit lens, but without the bulk of a wideangle zoom, it’s your only choice. Like all of Pentax’s Limited series primes, it’s also a finely crafted piece of kit which is a delight to use – a somewhat frivolous reason to choose one, perhaps, although still perfectly valid in its own way. Shot carefully stopped-down to its sweet spot, it also delivers very good but not outstanding image quality. So if your priorities lie in keeping your kit small, light and discreet, it’s a great option, but for most users we can’t help but feel that a wideangle zoom still makes more sense. Ultimately when compared to the alternatives, the 15mm F4 falls just short of offering enough of the expected advantages of a prime (smaller, faster, sharper, and cheaper) to earn our highest recommendation.

Detail	Rating (out of 10)
Build quality	9.0
Ergonomics & handling	8.5
Features	8.0
Image quality	8.0
Value	7.5

Recommended

The full review can be found at www.DPReview.com.

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Kamil Wawrzyszko

The power of PENTAX!!!