Pentax D FA100mm F2.8 Macro W ($549.95), is an updated version of a classic macro lens that was part of the Pentax full-frame lens range for many years. There's a lot to like about it—it captures images that are crisp from edge to edge, features a sealed, all-metal build, and is quite compact. However, heavy chromatic aberration (in the form purple color fringing within high-contrast parts of the frame) is quite annoying and needs to be removed. Unfortunately, there are not many options. Pentax is currently unable to purchase the Tamron SP90mm f/2.8 Di Macro 1:1 VC US ($899.00)
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Design
For a 100mm lens, the D FA 100mm is extremely compact ($446.95 on Amazon).The all-metal barrel and black finish make the D FA 100mm comparable to the HD DA Limited series. A weather seal is completed at the base by an o-ring. The lens hood comes in plastic, however the front element of the camera is recessed so it can be used without one. Supported are small 49mm front filters.
The lens is 3.2x2.5 inches (HD) when focused towards infinity. However, the barrel expands as the focus moves toward the minimum distance of 11.8 inches. The image sensor projects objects at life size, with a 1:1 magnification.
Your camera's autofocus screw drives focus. It can take the lens a while before it moves from its nearest focus distance to zero. The lens doesn't have a focus limiter, and you cannot force it to hunt within its closest focus distance or beyond that range like you can with macro lenses.
A cutout is found on the barrel showing a focus scale. It can be marked in meters and feet as well as magnification. This is a great tool to use for manual macro focus. The corresponding depth-of-field scale will show you how much wiggle space you have with macro photography even at f/32.
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Image quality
To test its quality, I used the D FA100mm in combination with the full frame K-1 (1,599.00 on Amazon). It scores a sharp 3,284 lines per inch at f/2.8, which is much better than what we would expect from a 36MP sensor. Even more, the image quality is excellent from edge to end, with only 150 lines of lag behind the center.
The score rises to 3,611 lines at f/4. Diffraction occurs early. The lens drops to 3,326 lines with f/5.6 and 3,105 lines with f/8. 2,985 lines are at f/11. 2,765 lines is at f/16. These are good results but not as impressive as the ones you can get with f/4. At very narrow apertures there's a steeper drop—2,375 lines at f/22 and 1,879 lines at f/32.
For macro photography, many photographers stop at a very low level to improve depth of field. Although the drop in clarity is not significant as you go beyond f/4, it could pose a problem for those photographers who value resolution. Focus stacking is a common technique for photographing static subjects within a studio setting. You just need more images to stack with a larger aperture.
Chromic aberration is more of an issue. While lens makers have made great strides to minimize the purple-colored fringing that older lenses can produce when they are used with modern digital cameras, the optics of this lens remain unchanged.
This is why you will see thick halos with a purple- or greenish hue in areas that are high contrast. It is common for power lines and leaves to be visible in shots that are taken against bright skies. You can remove the excess using Lightroom ($9.99/Month Adobe). However, the extra 100mm may cause problems at times. This is evident in the crop below.
It is not possible to see distortion, as one would expect from a macro lens. At f/2.8, the corners fall behind the center by about 1.7 stops in lighting (-1.7EV). This is reduced to -1.1EV when you turn the lens f/4. Field conditions make this barely visible. The frame will be evenly lit at f/5.6 or smaller.
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Conclusions
The Pentax D FA 100mm F2.8 MAcro WR is unquestionable in its resolution. It can make the best use of Pentax K-1's high-resolution sensor. The camera is also reasonably priced, well-built, waterproof, and can be used in all weather conditions. It can focus close enough to take images up to life size. It does capture a lot of color, but it is not ideal. The lack of a limiter and long focus throw can slow down focus acquisition. Pentax could make improvements to this design and deliver comparable performance to modern competitors like the Tamron 90mm Macro.
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