Tamron 90mm Macro vs Pentax 100mm WR: Review
Pentax 100mm Focusing
The focus ring is 2cm long, and made of metal like the rest of the lens body. For the most part, it is ribbed for easy operation. It is well positioned at the front of the lens and easy to locate by touch. The ring moves when the AF motor is engaged. Thanks to the Pentax quick-shift system, MF operation is possible at all times by simply moving the focus ring in any mode. The movement of the ring is extremely smooth and superbly damped, like one would expect from a purely manual lens.
The focus ring has a throw of 270°, with roughly 200° for the macro range (magnification factors below 1:3), the rest for the normal range. The pivotal point is for a focus distance of about 50cm. Unlike several of its predecessors, the lens has no focus limiter, probably in part because of the weather resistant design. The lack of the limiter is no hindrance for normal use, but can be annoying in some situations in macro mode, especially near the closest focus range. When used properly, quick-shift is a viable alternative, however. It takes a short time getting used to the process but becomes second nature after a while.
The lens has no window displaying focus range and magnification, but borrows from the Limited style (and from older designs from the K, M and A lines). The distance and magnification scales are actually a part of the focus ring, and a cut-out on the main barrel displays the numbers. The writing is engraved and painted white, with a blue touch for values in feet. A depth of field preview scale is present, but with f16 and f32 being the only apertures labeled, this scale is mostly present for decorative reasons.
AF is driven by a screw : the lens does not offer silent, SDM-like focusing. Given that most recent Pentax lenses are SDM-driven, the use of a screw-drive is a surprising choice. The AF gears generate a low level of medium-pitched noise, not worse than most other screw-driven lens. Refer to the comparison section for speed figures. The front element does not rotate when focusing, allowing easy use of polarizing filters.