trying out another macro
Well, here's just one comparison between the 'new' Promaster 100mm f//3.5 and my Sigma 50mm f/2.8. Both are shot around 1:2 closeup, which is the 100mm native max (but comes with a matched closeup lens for 1:1). Both look quite nice and sharp here at f/4.5; the Sigma is a touch bluer and lower key but other shots did not show this. Another shot was more of a bokeh torture test and showed the Sigma looks a bit better in that regard - but at double the focal length so many parameters change! So often in testing a single shot will show things that will not prove repeatable, so one must learn a little then learn to let go of what one wants to believe is "always true".
This Promaster was surprising, as it looks identical to the somewhat-revered Cosina known as the "plastic fantastic". That lens has reviews in many places, and once again shows that what is "always true" isn't. The Cosina feels fragile, sounds like a meat grinder when focusing, and cannot be used in manual focus because of the grip/slip nature of turning the ring by hand.
I will let stand the plastic part, as it does feel like a less-durable plastic than most lenses - but like every screw-drive lens I own the AF is a short buzz and MF has less resistance than I'd like. I posted at pentaxforums and found another owner with the same result.. so something has changed since those Cosina reviews were released, apparently. I paid more than the average Cosina price too, but given how it exceeds the reviews I'm OK with that.
In any case, a 205-gram 100mm lens that can do 1:2 closeups with no effort sounds good to me! It could shake up the kit again around 50mm if this knocks the Sigma off its macro perch. I expect the Sigma is better overall - but one thing that is "always true" is that no macro lens is a bad lens. That can't always be true can it? :^) The Tamron 90 is an excellent lens but at 400+ grams it stayed home too often; at about half the weight for similar results, the Promaster can stick around a while.