The Pentax SMC DA 645 25mm F4 AL (IF) SDM AW ($4,999.95 direct) delivers an impressively wide field of view to owners of the Pentax 645D medium format digital camera. The lens is equivalent to a 19.5mm optic on a full-frame image sensor, and is a top choice for capturing wide landscapes. It's sealed against inclement weather conditions, just like the 645D, so you can get your shot in harsh elements without worry. You'll have to pay dearly to get such a wide field of view on the 645D's big image sensor, and even though the lens is extremely sharp, it's not perfect.
This is the second version of the lens released since the launch of Pentax's digital 645 system. The first, which had a D FA designation, had an image circle large enough to allow for use on film bodies. This version is intended for use with the 645D only, a camera that has an image sensor that isn't quite the same size as a 645 film frame.
Like many medium format optics, the 25mm is a big, heavy lens. It measures 5.9 by 3.5 (HD) inches and is downright hefty at 2.3 pounds. The front element is curved outwards, so using a traditional lens cap is out of the question. Pentax includes a metal cap that slides over the built-in hood in order to protect it when not in use. It can focus on objects as close as 1.3 feet, which can be helpful when working very close to your subject.
There is a drop-in filter system that uses relatively small, inexpensive 40.5mm filters?a clear filter and a circular polarizing filter are included. You can turn a wheel located on the lens barrel to rotate the filter and change the polarization, and if you need to switch filters there's a simple knob to twist and remove the carrier. The lens features an internal focus motor, which is quite quick to adjust the focus setting, and you get a dedicated switch on the barrel to change between manual and automatic focus.
I used Imatest to check image quality from the lens when paired with the 40-megapixel 645D. Sharpness is excellent, exceeding the 1,800 lines per picture height we use to qualify an image as sharp at every tested aperture. At f/4 the score is 2,251 lines, and that increases steadily as you stop down, peaking at 2,944 lines at f/11. At f/16 diffraction starts to set in, reducing the score a bit?the lens can be stopped down as far as f/32.
You will get a little bit of distortion, 1.2 percent of the barrel variety, which will make straight lines appear to curve slightly outward. It's modest, and can be easily rectified in Lightroom. As with all wide lenses, keeping your camera straight, level, and parallel with your subject is a good idea. The troubling bit is the propensity to exhibit lateral chromatic aberration in the form of purple and green color fringes. We noticed this at the edges of our resolution test chart at every aperture, and it also showed up when shooting with the lens out in the field. Even on an overcast day there was a noticeable amount of purple fringing around tree branches against the gray sky. A few clicks in Lightroom banished it, but it was necessary to adjust for both the purple and green, and to adjust the hue away from the default range to get rid of it all. It's something you're not likely to notice when viewing photos at screen resolution, but you'll see it when viewing photos at 100 percent on your LCD or when making large prints; and if you aren't making large prints, why shoot with a 40-megapixel camera?
If you need to go really, really wide with your 645D, this is your option. You can attempt to hunt down the previous version of the lens, but if you aren't also using a 645 film system that effort is for naught. If you don't need to go quite as wide, or simply can't afford a $5,000 lens, you do have the option of picking up an older 35mm lens for the system. It will provide a field of view that is similar to a 28mm lens on full-frame 35mm camera; the autofocus version sells for around $1,000, while you can have one for under $500 if you opt for manual focus. The DA 645 25mm still delivers impressive sharpness and surprisingly little barrel distortion, but it would be easier to justify the price tag without the color fringing.
Source: http://feedproxy.google.com/~r/ziffdavis/pcmag/~3/3rW7Cboe9QE/0,2817,2416622,00.asp
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