Pentax 28 105mm f 3 5 5 6 vs tamron 28 75mm f 2 8?

Here is what the site says about that.

From Pentax HD D FA 28-105mm f/3.5-5.6 ED DC WR: Pentax HD D FA 28-105mm f/3.5-5.6 ED DC WR: A zoom lens by Pentax for Full frame cameras, 28-105mm f/3.5-5.6. A compact, weather-sealed full-frame kit zoom.

From Pentax HD D FA 28-105mm f/3.5-5.6 ED DC WR: What it frames like: - On full frame (Pentax K): as marked, 28-105mm. - On APS-C (Pentax K): like 42-158mm on full frame; f/3.5-5.6 gives about the depth of field of f/5.2-8.4 there.

Quoted from this site's pages.