[pabio]

shortly I will became ,a pentax owner (k-5 18-135 kit to be precise), and looking for second lens. Portrait lens that bring my attention is Sigma 24-70mm f/2.8 IF EX DG HSM. There anybody can help me with this decision?

[hcc]

Try the user's reviews:
Sigma 24-70mm F2.8 IF EX DG HSM Lens Reviews - Current Zoom Lenses - Pentax Third-Party Lens Review Database

You may also read:
Sigma AF 24-70mm f/2.8 EX DG HSM (DX) - Review / Test Report


However the 24-70mm overlaps your 18-135mm. Instead I would recommend you to consider a prime lens. That is single focal length lens, typically within about 50-85mm focal length range for portrait. Some like the 50mm, other prefer about 70mm, and some love the 85mm. I have myself both 58mm and 85 mm and each lens has its own purpose.

You may consider a portrait lens with large aperture (f1.4 or f1.8) to complement your 18-135mm. A fast prime (ie lens with large aperture) will be very useful in low light conditions where the 18-135mm does not excel. I know that the 24-70mm is f2.8 but this is not as fast, nor as good in low lights, as f1.4.

Hope that the comment will help.

[eaglem]

Just a thought, the Sigma 105mm F2.8 DG Macro is not only a very sharp Macro lens but doubles as a very good and very sharp portrait lens so you'd get double your value buying one of these. Here's a sample.

[adpo]

couple questions:
whats your budget?
how much working space do you want?
do you mind manual focusing?
do you understand the basics of manual exposure?

budget is fairly self explanatory, the more money you have, the nicer the lens you can get.
working space determines focal length. any lens can be used for portraits, it just so happens that a lot of portraits happen to be taken in the short-medium tele range
manual focusing opens up opportunities to go after high quality glass at lower prices, such as older fast 50mm lenses and whatnot
understanding exposure should be the number 1 priority. The 18-135 you're getting can take really nice portraits if you understand exposure. lenses don't magically make your photos better, you have to learn how to make good photos in the first place

with that being said, i recommend a fast 50mm lens such as the FA 50mm f/1.4 or an older manual version if you're comfortable with manually focusing and whatnot. have fun with your new k-5!