Charles,
Be aware that there are multiple versions of the Sigma 18-50 f/2.8 constant zoom. The original one has a 67mm filter size. The revised version has a 72mm filter size and is labelled "macro" because it can focus slightly closer. In addition, many users claim that the macro version is sharper than the original, because of a different optical formula used. Nevertheless, the original version is the lightest and most compact of the Sigma 17/18-50mm f/2.8 constant zooms.
I've been happy with my non-macro Sigma 18-50 but it's definitely not that sharp wide-open, and from what I've read, trails the Tamron 17-50. But stop it down to f/4 and it really starts to shine. I use it more outdoors than indoors but can use it indoors in a pinch, especially with a flash. For utmost sharpness I slap on a prime lens but for convenience snap shooting, the Sigma 18-50 has been a nice and inexpensive workhorse.
According to this Member at this post:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/92006-if-no...tml#post937697 Quote: I still have the old (pre-macro, pre-HSM) Sigma 18-50/2.8 and will keep it. Not only because my own DA 16-50 also fell apart a while ago and I want a backup at hand, but also, because it is much smaller and has less distortion. In terms of sharpness and contrast, the Sigma can easily hold its own, compared with the DA.
So wow, he has compared his copies of the DA* 16-50 and the first version Sigma 18-50, and thinks Sigma is its equal in terms of sharpness and contrast! That's reassuring to hear.
Here are some more helpful threads on this lens:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/131837-foll...macro-hsm.html https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/6177-sigma-...2-8-macro.html https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/29276-sigma...ro-anyone.html
Good luck!