The Pentax? HD D FA 645 Macro 90mm F2.8 ED AW SR ($4,499.95 list) is one of three modern lenses available for the company's digital medium format system. It's the first lens to boast the latest HD coating, which replaces the venerable supermulti coating (SMC) formula that has adorned Pentax glass in one form or another for decades. It's also the first Pentax lens to boast in-lens vibration reduction?the company builds that technology into its APS-C cameras, but it's missing from the comparatively giant sensor found in the 645D.
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 weighs 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 it is fully weather-sealed, so capturing objects such as insects in their natural habitat is possible?even when the weather is less than ideal.
The lens focuses as close as 1.4 feet, but only offers 1:2 magnification. This isn't atypical, as 1:1 magnification generally doesn't kick in until you hit 100mm in the 35mm format. This lens provides a 70mm field of view when compared to a full-frame 35mm camera due to the larger sensor found in the 645D. Because the camera's sensor is larger than even those found in pro D-SLRs, the depth of field you get from this 90mm f/2.8 is shallower than a similar lens would be on even a full-frame camera; you can blur the background behind your subject with ease, and if you're working on a macro shot you may find yourself having to stop down to f/11 or smaller to get your desired depth of field.
I used Imatest to check lens sharpness when paired with the 645D. Even at f/2.8, the lens keeps up with the camera's 40-megapixel image sensor. It records 3,276 lines per picture height at the maximum aperture, nearly double the 1,800 lines we use to qualify an image as acceptably sharp. Stopping down doesn't improve performance?it stays about the same until f/16, where diffraction starts to detract from the quality of images. There is virtually no distortion, as is expected of a macro lens.
The Pentax? HD D FA 645 Macro 90mm F2.8 ED AW SR is an incredible performer that pairs well with the 645D camera. It's weather-sealed, has a fast aperture for medium format, and a shake-reduction system to further help you get sharp, blur-free images. The asking price of the lens is quite high, even though the 645D is an expensive camera in its own right. If you can afford it, you won't be disappointed with the performance. But if you need a macro lens for your 645D system and this one is out of reach, the older 120mm lens, available in manual focus and autofocus versions, sells for as little as $500 on the used market. Just understand that it's an f-stop slower, doesn't have a vibration-reduction system, uses the older SMC lens coating, and isn't weather-sealed.