Tamron 90mm Macro vs Pentax 100mm WR: Review
Tamron 90mm Focusing
The focus ring is large and well rubberized, in addition to being well positioned on the barrel, making it easy to locate by touch. In AF mode, the ring moves freely on the barrel, with next to no friction, being decoupled with the focus gears. In manual mode, the ring movement is smooth, but with little damping. Manual focus is easy but nowhere near what a purely MF lens would offer. The ring has a 270° throw, 180° of which used for macro and the rest for the "normal" range, the separation being near 45 cm. There is also a focus limiter, whose stop point is also near 45 cm. The limiter is a simple switch, well positioned on the left of the barrel. The switch operates smoothly and locks in place securely.
The process of switching from AF to MF deserves some explanation. The lens has a clutch that allows, in theory, fast transitions. Simply pull the focus ring towards the camera body and you're in manual, and push it back to return to AF. In real life, this has two limitations. First (and worse), once the lens is set to MF, you still need to set the camera body to MF, since the gears are still engaged. This means switching to MF is a two-steps process, actually taking longer than for a lens without a clutch. Second, since the focus ring needs to be well aligned with the focus gears when the clutch is pulled, the mechanism will sometimes have trouble engaging and the user will need to swivel it a bit to get it to lock. All in all, the MF clutch seems like more of a hindrance than an advantage, but it probably made the AF system lighter and easier to move. A blue ring near the front of the lens appears when the clutch is set to MF. It must be noted that the Canon and Nikon versions do not have the two-steps limitation.
It is possible to leave the lens on the MF position at all times, in which case the focus ring will rotate with AF (like on most lenses). Given that Tamron took the time to implement a clutch, however, we cannot recommend leaving the lens in the MF position while using AF, since this could hypothetically stress the AF mechanism.
The lens sports a window showing the focus distance and the magnification, with pale lettering on a dark background. The readings do not change when the focus ring is swivelled in AF mode, since the clutch is de-coupled. The numbers are easy to read, except for the magnification numbers which can be shadowed by the window's edge at some angles. Below the window, the lens sports the common but almost unusable depth of field preview in relation to aperture (only f16 and f32 marked).
Around the window, the lens' credentials are engraved in gold lettering, creating a very elegant look. The writing looks durable, which cannot be said of the numbers printed on the aperture ring : those look like they could possibly fade over time.
AF is driven by a screw : the lens does not offer silent, SDM-like focusing. The AF gears generate a low level of high-pitched noise, not worse than any other screw-driven lens. Refer to the comparison section for AF speed figures. The front element does not rotate when focusing, allowing easy use of polarizing filters.
