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Design

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.

Compared to Tokina AT-X Pro 12-24mm F4 DX

One of the big attractions of primes is that they tend to be substantially smaller and lighter than zooms covering the same focal length. This is especially true at the wideangle end of the spectrum, with even slow zooms being distinctly heavy and bulky.

To illustrate this advantage, here's the 15mm F4 all packed up and ready to travel, alongside a fairly typical APS-C wideangle zoom, the Tokina AT-X Pro SD 12-24mm F4 (IF) DX (which is closely related to Pentax's own 12-24mm F4 design). While it's not quite as tiny as its 'Pancake' siblings (the 21mm F3.2, 40mm F2.8 and 70mm F2.4), the 15mm F4 is substantially less than half the size and weight of the zoom. This means that it takes up very little space in a camera bag, and will even fit into a coat pocket in a way a wideangle zoom most certainly won't.

On the camera

The 15mm F4 is decidedly petite, and sits comfortably on any size body from the large-ish K20D (left) to the tiny K2000 (right). It's a joy to use; the focus ring falls perfectly to hand, and the 'quick shift' system means focus can be adjusted manually even when the camera is set to autofocus mode. The sliding hood is well-designed and operates smoothly too.

Autofocus

Autofocus is driven by a screw-drive mechanism from the camera, and is therefore dependant primarily upon the capabilities of the specific body used. On the K20D we employed for testing, autofocus was generally fast, accurate and reliable, with no systematic problems. As always, it must also be noted that focus speed and accuracy is dependent upon a number of variables, including subject contrast and light levels.

Lens body elements

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 lens even features 7mm-wide machined aluminium grips for holding the lens while mounting and dismounting it.

The sum of all these little touches make this lens (like the rest of Pentax's Limited series) an absolute pleasure to use.
It's not often we talk about lens caps, but this one's different. To match the 'Limited' ethos, Pentax has provided a beautifully engineered screw-in aluminium cap, which is even lined with felt to protect the front element.

Like all screw-in caps, though, it's hopelessly impractical in actual use, especially if you change lenses frequently. Buy a 49mm clip cap instead (just make sure it fits inside the hood).

Reported aperture vs focal length

This lens allows an aperture range from F4 to F22 to be selected.

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