Pentax 40mm f/2.8 Limited SMC P-DA

Name: Pentax 40mm f/2.8 Limited SMC P-DA
Brand: Pentax
Rating: 9.8 (8 reviews)

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Lens Review
Specifications

40mm	$380 average price

Lab Test Results

Blur
Chromatic Aberration
Vignetting
Geometric Distortion

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(From Pentax lens literature) The SMC P-DA 40mm ƒ/2.8 Limited lens is designed for use with digital cameras. At only 15mm thick it is exceedingly thin. This exclusive "Limited Lens" assures exceptional image quality, with high contrast, high resolution, great sense of depth, and minimal aberrations. Sophisticated and portable it offers a natural perspective close to the naked eye. The SMC P-DA 40mm ƒ/2.8 Limited is an ideal standard lens that performs superbly. Like all Pentax lenses, this lens is treated with the acclaimed SMC coating for maximum light transmission, sharp definition, and high contrast images.

Introduction
The 40mm ƒ/2.8 SMC P-DA limited edition is a fixed-focal length ''pancake'' lens, one of the smallest lenses to bear that moniker, and probably the smallest that Pentax produces. A pancake lens is typically constructed with the fewest number of lens elements possible, and in this case the 40mm is no exception, with just 5 lens elements in four groups.

Pentax 40mm f/2.8: lens cap, filter-mount shroud and lens.

The lens weighs just 90gm (3.2 oz), and is described by Pentax as being designed for use with digital cameras. We mounted the 40mm ƒ/2.8 on a Pentax film body, and could find no evidence of vignetting. However, our film body doesn't have aperture control, which would make it hard to use effectively as the 40mm ƒ/2.8 isn't equipped with an aperture ring. The lens takes both 30.5mm and 49mm filters in an interesting lens cap/hood design, and is available now for around $250.

Sharpness
Our review sample of the 40mm ƒ/2.8 seemed to be slightly sharper on the left side than the right, but only marginally so. It would probably be hard to distinguish this difference, as the difference is about one blur unit.

Overall, the 40mm ƒ/2.8 is a very sharp lens, never exceeding three blur units and averaging around 1.5. Set to apertures less than ƒ/8, some corner softness is visible, surprisingly most at ƒ/5.6, where the right side of the frame approaches three blur units, but the center stays sharp as sharp as we can detect at one unit. The lens performs better at ƒ/2.8 and ƒ/4, where is corner softness is visible, but less significant. At ƒ/8 the lens is sharp but for a very slight blur on the right side, and by ƒ/11 it's tack-sharp across the entire frame. At ƒ/16, the effects of diffraction limiting set in slightly - in the form of 1.5 blur units across the entire frame, and hitting 2 units by ƒ/22. With an exceptional observation that is almost perceptually insignificant at ƒ/5.6, we see excellent performance for sharpness with this lens.

Chromatic Aberration
There is virtually no chromatic aberration visible in testing the 40mm ƒ/2.8. Chromatic aberration is almost non-existent with the lens set to its widest aperture, and increases a negligible amount as the aperture is stopped down. We're talking less than 2/100ths of a percent of frame height on average, and not exceeding 3/100ths in the corners - this is outstanding performance.

Shading (''Vignetting'')
The 40mm ƒ/2.8 does vignette very slightly at ƒ/2.8, showing almost a half-stop of shading in the corners. This improves at ƒ/4, and reaches its average and imperceptible level of a quarter-stop of corner shading at ƒ/5.6 and smaller.

Distortion
Being a prime lens with a fixed focal length, distortion is also fixed. In the case of the 40mm ƒ/2.8, this distortion is fixed at an imperceptible 0.1% barrel distortion on average, and 0.25% in the extreme corner regions.

Autofocus Operation

Pentax 40mm f/2.8 with standard AA battery for scale.

The 40mm ƒ/2.8 autofocuses very quickly. It's mechanically driven, so it does make a noisy whine when focusing. On a K100D Super, it racks through the entire focus range in approximately one second. Point-to-point focusing is very quick, probably because there are very few lens elements to push around, and is slightly less noisy.

Macro
The lens isn't rated for macro, and this shows in its macro specifications: 0.13x maximum magnification, with a close-focus distance of 40cm (just over fifteen inches). There are better macro choices.

Build Quality and Handling
Let's get this out of the way right up front - the 40mm ƒ/2.8 is small. When attached to our Pentax K100D Super, the front of the lens was flush with the pop-up flash and only a half-inch longer than the grip. This camera and lens combination is ultimately portable - it easily fit into my coat pocket, and given the lens' 40mm focal length, it was a very usable, walk-around choice. With its black finish and all-metal construction, the 40mm is tougher than it looks.

Did we mention it's small?

So, it's small. So small, in fact, that when mounted on a camera, you can't easily see its only feature - a distance scale, etched onto the focus ring - without moving either yourself or the camera. Pentax has used its ''quick-shift'' focus system in the 40mm pancake, so you can override autofocus and focus manually any time you like by just turning the focus ring. The ring has good texture, and the focus range is stops solidly at either end. The full focus range is a forty-five degree turn of the focus ring. In practice, I found the lens to be very quick and responsive, and it produced very nice images.

However, if there was a ''gotcha'' with this lens, it has to be its choice of combination lens hood and cover. Either one is a bit of pain to remove, as they need to be screwed on or off. Given its miniscule size, the lens doesn't need much of a lens hood, so Pentax has designed a combination hood/cover, which is more of a shroud than a hood. The useful part of this front cover is that you can either remove the small circular insert and use the lens with the shroud, or remove the whole lens hood and attach 49mm filters. The front of the lens doesn't rotate during focusing, as you would expect, so polarizing filters will stay where you need them to. The small insert is 30.5mm in diameter, and yes, you can use 30.5mm filters on this opening. If I used this lens more often, I could see losing the small cover in my couch cushions.

It just so happens that the lens cap for the 21mm ƒ/3.2, a slide-on model with black felt lining, fits properly on the 40mm ƒ/2.8. It's much easier to attach and remove.

Alternatives

Pentax 43mm ƒ/1.9 Limited SMC P-FA ~$470
A slightly more up-market and exotic lens, both in style and focal length, the 43mm ƒ/1.9 showed slightly more chromatic aberration and wasn't as sharp, but still showed remarkable results in our testing. It's a full stop faster, but not as small as the 40mm; it has a much better lens hood, and is more backward-compatible with older film bodies, being equipped with an aperture ring.

Pentax 50mm ƒ/1.4 SMC P-FA ~$200
We haven't tested the Pentax 50mm. It's a full frame lens with an aperture ring which takes 49mm filters. It weighs more, costs less, and is a full two stops faster.

Pentax 35mm ƒ/2.8 Macro Limited SMC DA ~$440
We haven't tested the Pentax 35mm. Its specifications list it as a reduced-frame lens, designed for macro use. It's larger, weighs more, but has a slightly wider field of view, and the same aperture settings.

Sigma 50mm ƒ/2.8 EX DG Macro ~$230
Slightly more telephoto but just as fast, the Sigma 50mm ƒ/2.8 macro produces consistently sharper results, but they have slightly more evidence of chromatic aberration. With virtually no evidence of vignetting, 0% distortion, and macro capability to boot, this lens is an excellent alternative to the pancake; of course, it's nowhere near as small.

Conclusion
I quite liked using the 40mm ƒ/2.8 Limited edition, and its optical performance is one of the better examples of Pentax's declarations of quality they apply to its limited edition of lenses. Once I found I could use the lens hood from the 21mm ƒ/3.2 prime, I found it much more convenient to use as a walkaround lens; having to screw on and off the hood was a real frustration. So with that one caveat, the 40mm ƒ/2.8 is an excellent lens, and a worth setting aside the very small amount of space it will require in your camera bag.

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.