Originally posted by stevebrot Strangely, this tends to be true despite attractive (may I suggest better?) options from third-party vendors at similar price points. I won't give examples except to say that I own one.
I'm a fan of third party lenses, but I know numerous folks that will only buy OEM.
Honestly, I haven't had great luck with earlier Tamron models (all bought brand new) so far as quality control and calibration tolerances are concerned... Enough to suggest (to me, at least) that its earlier production methods weren't as precise and consistent as OEM - possibly because film photographers weren't pixel-peeping like we digital folk. But once I've had them sorted by the service centre, they've been great. My K-mount 28-75 f/2.8 is probably my all-round favourite and most-used lenses on APS-C, for its combination of constant aperture, decent performance, nice rendering, light weight and compact dimensions. And on A-mount, I really like the Tamron 90 f/2.8 Macro. The 150-600 (1st gen) is extremely good considering the price... and my 70-200 f/2.8 USM (also 1st gen) is simply outstanding in every respect - a phenomenal lens. Tamron has upped its game hugely in recent years.
With Sigma, I've had no such problems... my K-mount 17-50 f/2.8 is optically excellent and serves me well, while the somewhat-maligned 30 f/1.4 Art has become possibly my favourite K-mount APS-C prime in terms of rendering character. The AF motors in both are fantastic. Same with the 18-300, which is - IMHO - far better optically than the Pentax 18-270 I previously owned. Even the cheap and cheerful A-mount 28-80mm F3.5-5.6 Aspherical I recently picked up new for pocket money is surprisingly decent optically. But I do believe Sigma's AF performance on K-mount to be a little compromised due to the reverse engineered AF protocols. I'm not unhappy with the performance, but I find it less consistent on the first AF attempt.
Even OEM lenses aren't perfect, though. I had problems with two new HD DA16-85's in succession (which led to me buying the Sigma 17-50 f/2.8), a DA*16-50 that was outside the +/-10 AF adjustment of my K-3, and a few kit-level zooms (new and used) that were de-centered. But, most of the Pentax lenses I've bought have been perfect...