Own the DFA 100 and tried out the Tamron 90 and the Sigma 105.
The DFA is better stopped down as it is a little soft wide open. Frankly, I think this Pentax macro isn't as good as compared to the 100/105mm macros from Canon/Nikon or KM/Sony. I believe the bigger and heavier FA version is better optically. The DFA barrel construction is plastic and is overall quite light. AF can have difficulty locking in focus at higher magnifications. Quick shift is useful and can be faster than focusing manually.
The Tamron is an update of an old tested design. It performs better at the wider apertures, which isn't so ideal for macros DOF wise, but good for portraits. Focusing is slow and macro is best performed manually. Probably the cheapest of the three.
The Sigma is good optically but its weakness is in its bokeh (subjective) but it has the best build quality. It is also the priciest (at least where I am). Also 105mm vs 90mm/100mm does give a little more subject isolation.
Tokina also has a 100mm macro but unfortunately it is not available in Pentax mount. Optically it is pretty good.
BTW I have a Sigma 90mm f/2.8 Macro in PK-A mount. A very compact and lightweight manual focus 1:2 macro. Optically it is very good, even better than the DFA wide open. This is a good lens to look out for if you're OK with manual focus. I rate this lens highly.
Last edited by creampuff; 02-24-2008 at 08:44 PM.
Reason: spelling