I am looking for a wide angle zoom lens for my Pentax K5

[trick10]

Hello, I am new to this Forum and not sure how things work yet, however, I have a new K5 that I just purchased without a lens as I believe that while the Kit lens (18-55) is ok there are a lot better lenses available around this size. I already have a Pentax-F SMC 1:4-5.6 35-80mm and also a Pentax-F SMC 1:4.5-5.6 100-300mm (Both autofocus lenses) from my Mz5-n Film camera. These lenses seem to work fine on the K5.
I understand that these lens need to be multiplied by 1.5 to indicate their trye size when fitted to my K5.
Therefore, am I on the right track to think I require something like a 17-70 Sigma or Tamron to fill the void of the wider angle?
Which of these is better?
I was also thinking I really need a wider aperture and am considering the 2.8 in one of these choices.
I do not have a lot of experience in the Photography world and would appreciate any comments (positive or negative) about my equipment and/or ideas.

[Kym]

I have the Sigma 10-20 for my landscape work, http://www.sigmaphoto.com/shop/10-20mm-f4-56-ex-dc-hsm-sigma
You can go wrong with that for landscape as many will attest to here on AP.

I use a Tamron 28-75 f/2.8 for may 'walk around'.

For prime options...
I have an older Pentax 50mm f/1.7 SMC manual) which can be had from Ebay for $100.
I also have a Pentax 35mm f/2.4 http://www.dpreview.com/news/1009/10090908pentax35mm.asp brilliant for the price.

Yes, the crop factor is 1.5. The focal length remains the same, but the effective FL is 1.5 greater.
See: http://www.ausphotography.net.au/forum/showlibrary.php?title=New_To_Photography:Focal_Length_vs_Crop_Factor for an explanation

[PH005]

:th3: on the Sigma 10-20. Must be one of the best in this class. Go and check one out.

[ricktas]

Yep,the sigma 10-20 is a wonderful bit of kit for a cropped sensor camera.

[kiwi]

Tokina 12-24 aint shabby

Truth is almost all UWA are good, it's their primary use

[crf529]

Depends how wide you want to go? 10mm is much wider than 12mm, and 8mm is even wider again than 10mm...

The 8-16mm and 10-20mm Sigma are both great lenses for our pentax crop sensors. I personally can't split them, so at this point in time I have a bunch of cash but neither lens haha.

[trick10]

Thank you for your feedback. I have now purchased a 17-70 Sigma and so far it appears fantastic. Mind you anything new in photography is exciting to me.:)
I'm planning trip to central australia in september/october and hope to digitally bring central australia home with me. I am hoping the 17-70 Sigma is the correct choice for my best option fr this adventur.
Again, thank you, I think this site is bloy fabulous, well done Ricktas.