Pentax is only just starting to venture into full-frame digital, although it has kept some lenses from the film era that were produced in order to complement the digital K-1. Pentax SMC D-FA Macro F2.8 (299.95) is one of these lenses. It delivers excellent optical performance, despite its lackluster fit and finish. The internal motor, weather sealing and focus limiter are not features that you'll find on older glass. Pentax photographers looking for a macro lens can't be faulted by its image quality and price.
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Design
Macro 50mm ($316.95 at B&H Video). The Macro 50mm is compact and lightweight, measuring just 2.6 by 2.5 inches (HD), and 9.3 ounces. The front element of the lens is small and supports a 49mm filter. However, the lens telescopes as it closes in focus, which is quite different from macros that have internal focus. Although the lens feels a little more fragile due to its extension, the included lens cover will keep it from bumping into things during normal usage.
The lens has an electronic aperture control, however, it also comes with a physical control dial that can be used on older films cameras which don't have electronic control. You can adjust the aperture in increments of three stops when using a modern camera like the K-1. However, if the ring is being used for control purposes you will be limited. You can set it at f/2.8 from f/4 to f/16, in half-stop increments or at either f/22 and f/32.
The screw-drive mechanism drives autofocus. This means that the lens will make noises when it is driving. The lack of a focus limiter switch means that it can be slow to lock focus, particularly when moving from macro to infinity distances. The lens takes approximately one second to focus on its entire range. It is possible to have a focus limiter, which allows you to do a macro search on the lens's range but it is not available.
You can make manual adjustments and quick shift focus while your camera is in autofocus mode. A focus clamp is included on the lens, which secures the focus point. This prevents accidental focus changes using either the autofocus system of the manual focus rings. This feature is not available on any macro lens currently in production but it's a great advantage for those who work from a tripod, copy stand or similar setups.
You can focus up to 5.99 inches from the image sensor of your camera. This allows you to get close to subjects. Focusing as close to the subject as possible will project them onto your image sensor in full-size at a 1:1 magnification. You don't get that with every 50mm macro design—many are limited to 1:2 reproduction.
Weather sealing is not available. Pentax has a D FA 100mm F2.8 Macro Wr macro that protects against the elements. This lens is longer in focal length and costs $550 more. The D FA 100mm has a lot more trouble. This issue is not present with the D-FA Macro-50mm.
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Image quality
The Macro 50mm was tested with the 36MP K-1 at Amazon for $1,599.00. The lens produces sharpness results at f/2.8, with 2,869 lines per image height using the Imatest center weighted evaluation. This is better than 2,250 lines that we prefer to see from the K-1. The image quality throughout the frame is excellent, with a decent edge resolution (2,327 line) but still fairly sharp.
For close-up photos, narrow apertures can be used to achieve a greater depth of field. The lens is strong at f/4 (2,805 lines), and it reaches peak resolution at f/5.6 (3,087 lines). This gives the lens a performance that's even from end to end. High resolution is maintained at f/8 (3.081 lines), and f/11 (2.973 lines).
Diffraction causes a decrease in scores at smaller apertures. Although you can still get crisp results with f/16 (2.794 lines) the drops at f/22 (2.426 lines) or f/32 (1.995 lines) are significant enough to suggest that focus stacking be used when there is a large depth of field.
The lens is free from distortion so it can copy documents and be used for architectural work. At larger apertures, there is some corner dimming. At f/2.8, the corners trail behind the center by about -2.5EV and at f/4 by around -1.3EV. The deficit drops to a negligible 1EV at f/5.6 or smaller. If you are shooting in Raw format, the K-1 can correct JPGs in camera with four levels. Adobe Lightroom can also be used to brighten dark corners.
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Conclusion
Although the Pentax SMC D-FA Macro Macro 50mm F2.8 doesn't have modern design, it delivers sharp macro images at a fraction of the cost. To better match modern Pentax SLRs, we would love to see this lens upgraded with weather protection, an inner focus motor and limiter functions. If you are more focused on image quality than build, it is worth considering, particularly at $300.