Taking a closer look at the Pentax-D FA Macro 100mm f/2.8 Lens

Gary Wolstenholme examines the latest 100mm macro offering from Pentax.

Posted: 11/01/2011 - 16:41

Even with today’s close focusing zoom lenses, the situation often arises where you just can’t fill the frame with your chosen subject. Enter the macro lens, in particular the new Pentax DFA Macro 100mm f/2.8 WR, designed to explore our world in tiny detail.

Pentax currently offer three macro solutions in their standard range, one with a focal length of 50mm, and two at 100mm. The advantage of a 100mm macro lens is that you are twice as far away from your subject for a given magnification. This allows for easier composition and fewer issues with blocking out light from your subject as you get in close. Telephoto focal lengths also help to isolate your subject from its surroundings, due to the narrow field of view and shallower depth-of-field at wider apertures. As you are further away from your subject, a flatter perspective is achieved, which lends longer focal lengths to applications such as portraiture and product photography.

Of the two 100mm lenses we will look at the latest WR addition to the Pentax stable, offering a weather resistant construction, rounded diaphragm blades, quick shift focusing and a water-repellent ‘Super-Protection’ lens coating.

On Pentax digital SLRs, this lens gives an equivalent field of view of a 150mm lens on a 35mm body, and because it’s compatible with these 35mm SLRs, those who enjoy shooting film should not feel left out either.

Although the lens is not designated as a Limited Series lens, the design and build quality is reminiscent of such optics. The main lens barrel is constructed from lightweight, tough aluminium and the general fit and finish inspires confidence that it will continue to function in all kinds of conditions.

It costs around £680, which puts it in line with top-of-the-range Pentax optics, but does it deliver a top-of-the-range performance?

Overall I found using the lens a very pleasurable experience. The quick shift focusing system allows the focus to be adjusted manually once the camera has done its bit, which I found very useful. There is no need to muck around with any focus switches if an adjustment needs to be made. The focus ring can just be nudged in the appropriate direction to attain focus just where it is needed.

There is a slight drawback with this system, as the focus ring cannot be disengaged during autofocus. I did catch my fingers on it a few times as I got used to the lens, but in time you learn to keep your fingers clear, unless a focus adjustment needs to be made.

Macro lenses are notoriously slow to focus, but not in this case. In good light, the lens locks on very quickly for a macro as the camera’s focus motor powers it through the focus range very quickly. In poor light, the lens tends to hunt a little and as there is no focus limiter on the lens it will track back and forth through the whole focus range until it acquires a lock on something.

To test the optics I used a Pentax K-7 body and analysed the results with Imatest. Resolution-wise, this lens is a very good performer, producing images with good to excellent resolution depending on the aperture selected. Wide-open at f/2.8, the performance in the centre is already very good, which will lend itself well to applications such as head & shoulders portraits, where the main subject will be more-or-less central and shallow depth-of-field is required.

As the lens is stopped down, the sharpness improves as far as f/5.6, where the centre resolution is excellent and the quality is still very good towards the edges of the frame.

Beyond f/5.6, diffraction starts to reduce the resolution, but results are still very good at f/11 and acceptable at f/16. This characteristic will make this lens ideal for insect and flower photography, where the lens may have to be stopped down enough to produce adequate depth-of-field.

Chromatic aberrations can often be an issue with telephoto lenses, as the highly refracting lens elements used often disperse the colours that make up the light forming your image. This effect can lead to coloured fringes around high contrast subjects and can sometimes take the appearance of a ‘Ready-Brek’ glow of red or violet in the worst cases.

SMC PENTAX-D FA Macro 100mm f/2.8 WR Chromatic Aberration Charts

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I’m glad to say that this lens performs well when it comes to controlling these aberrations. At their worst colour fringes cover an area just exceeding three quarters of a pixel-width, which will be barely noticeable in most photographic applications.

Applications which require straight lines to be preserved, such as copying documents, are also ideally suited to this lens. Imatest recorded 0.03% pincushion distortion, which is a negligible amount. Light falloff in the corners is also kept in check, with the corners being under half a stop darker than the image centre at f/2.8.

Stopping the lens down to f/4 provides perfectly even illumination, should that be required. Control of flare and ghosting is also very good. Strong point sources of light in the frame will cause a little flare, but only in extreme circumstances. A deep plastic lens hood is supplied as part of the kit, which does an excellent job of shielding the front element from extraneous light.

The rounded diaphragm blades produce a buttery-soft quality to out-of-focus backgrounds, although specular highlights at close distances are often rendered as a bright circle, rather than a disc, which can lead to busy looking backgrounds in some cases.

Overall I have been very pleased with the results produced by the 100mm, and the results recorded during testing only serve to reinforce how good the performance is. If you intend exploring the creative possibilities a macro lens can offer with your Pentax camera, then this optic comes highly recommended.