Pentax has cut the cost of entry into medium format photography with its two digital cameras, the 40-megapixel 645D and the more modern 50-megapixel 645Z. Lens prices for the system run the gamut, from the $700 FA 75mm prime all the way to the $5,800 FA* 300mm. The HD D FA 645 Macro 90mm F2.8 ED AW SR ($4,499.95) is at the high end of the scale, but it's an excellent performer. It includes image stabilization, a rarity in the line, and its weather-resistant design is essential for photographers working in inclement conditions. It's a solid performer, and worth its high price. If you can live without stabilization or weather resistance, Pentax offers the FA 120mm F4 Macro ($1,599.95) as a more affordable alternative.

Design

The 645 Macro 90mm is bigger than most 35mm SLR lenses, but isn't out of line for medium format, measuring 2.5 by 1.5 inches (HD) and 2.3 pounds. It supports 67mm lens filters and ships with a petal lens hood that can be reversed when not in use. Like the 645D and 645Z, it is fully sealed against the weather, so capturing objects such as insects in their natural habitat is possible—even in inclement conditions.

There are a few toggle switches on the lens barrel. An AF/MF switch changes the focus mode, and there's a toggle to turn the stabilization system on or off. The outer barrel is composite, but the inner barrel that extends as you focus closer and the manual focus ring are metal. The focus control occupies a good portion of the lens, and supports the Pentax Quick Shift focus system, which allows you to manually override autofocus after it's been locked, without the need to change the focus mode.

If there's anything to complain about, it's that the motor that drives lens elements during focus is on the slow side. There is a focus limiter switch, a useful tool to prevent the lens from hunting for focus across its entire range. The switch limits focus from about 2 feet (0.6-meter) to infinity, or from the minimum focus distance to 2 feet, depending on where the focus point is set when it's enabled.

The minimum focus distance is 1.4 feet (41cm). When working that close the lens projects objects onto the sensor at one-half (1:2 magnification). The FA 120mm F4 Macro focuses closer and a has a longer focal length, making it the only 1:1 magnification option in the Pentax medium format system. It's not surprising that the 90mm is a 1:2 lens—its field of view on the 645Z matches a 70mm lens on a full-frame camera, and, with some exceptions, 1:1 macro lenses typically have tighter fields of view.

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Image Quality
I used Imatest to check the sharpness of the lens when paired with the 50-megapixel 645Z. At f/2.8, it well exceeds the 2,200 lines per picture height we look for in photos from high-resolution cameras. It scores 3,679 lines on a center-weighted test at f/2.8, with even performance across the frame. I had previously tested the 90mm on the 40-megapixel 645D; it showed 3,276 lines at f/2.8 on that body. It's expected to see crisper results from a higher resolution sensor.

Back to the more modern 645Z, at f/4 the 90mm is sharper, showing 3,415 lines. The upward trend continues at f/5.6 (3,808 lines), before peaking at f/8 (4,361 lines). There's a small loss of image fidelity at f/11 due to diffraction, but the 90mm still resolves 4,263 lines there. Performance remains strong at f/16 (3,970 lines). Sharpness takes a more significant hit at f/22 (3,405 lines).

There's no distortion to speak of. Illumination is also fairly even across the frame. I used an ExpoDisc and Imatest's Uniformity analysis tool to evaluate brightness from corner to corner. At f/2.8, the corners lag behind the center by about 1.2EV, which is just outside of the 1EV differential we hope to see in an image. At f/4, the corners have pulled to within 0.3EV of the center, a result that is negligible in real-world photography.

Conclusions
The Pentax HD D FA 645 Macro 90mm F2.8 ED AW SR is a standout performer that pairs well with a 645D or 645Z. It's has a weather-sealed design, a wide maximum aperture, and an image stabilization system to further help you get sharp, blur-free images. The asking price is quite high, but if you can afford it, you won't be disappointed by the performance. If you're on a stricter budget, consider instead the FA 120mm Macro. We haven't tested it yet, but it is significantly less expensive and focuses a bit closer for 1:1 magnification, though it omits weather resistance and image stabilization.

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