I shoot 5D II which is too heavy. I want a crop factor. Are Pentax lenses lightweight?
There are a number of Pentax k5 users who have switched from full frame because full frame lenses and bodies weighed too much.
A canon L 70-200 f2.8 lens weighs 2.9 lbs. The Pentax DA* 50-135 f2.8 (full frame equiv 75-202.5) weighs 1.5 lbs.
The Canon 100 macro weighs 625 grams, the pentax 100 macro w/ wr and image stabilization (in body) weighs 340 grams.
The canon 85 1.8 weighs 425 grams, the Pentax DA* 55 f1.4 (with weather sealing and image stabilized) weighs 375 grams.
The canon 24-70 f2.8 weighs 803 grams vs the Pentax DA* 16-50 that comes in at 565 grams (I do think the canon is a superior lens in this instance...).
Weight and size do matter and when it comes to things you carry on your back, less is more. I could switch, but I don't really like the idea of carrying more weight. The Pentax primes are great, especially their Limited series. I have several... the 43 1.9, the 77 1.8, the 35 2.8 macro and 40 2.8. I want, no make that need, the 15 f4 and the 31 1.8, but that's several paychecks down the road. But they are tiny in comparison to my uncles crop Nikon lenses. And he rides a motorcycle where space is a premium.
I'm no pro, but here's a set of my favorite shots by me:
most of the time, I don't use a tripod (again the weight thing). but maybe you can see what Pentax can do with their lenses....