About the long debate between Pentax 12-24 and Sigma 10-20...

1-The Pentax seems better, but having a 17-70, how much value I'd get from only 5 mm extra lenght? So maybe the 10-20 would be better.

2-About the Sigma: there are opposite opinion, also between good review sites: dpreview : Sigma 10-20mm F4-5.6 EX DC HSM Comparison to other wideangle zooms we've tested recently suggests the Sigma's class-leading days may be numbered. On APS-C, it simply can't match the biting central sharpness of the Tamron 10-24mm F3.5-4.5 Di-II in the 10-18mm range (for which, we suspect, users will mainly be buying this type of lens), and has more problematic distortion characteristics. However in a typical case of swings and roundabouts, the Sigma shows superior sharpness when shot wide open, and is better in the corners of the frame at all settings; it also has lower chromatic aberration, is better built and uses a superior focusing system."

while on photozone "The Sigma AF 10-20mm f/4-5.6 EX DC showed an pretty impressive performance with a few bugs. The resolution characteristic is generally very good to excellent except in the extreme corners at 10mm which are a bit soft at large aperture settings. A weak aspect of the Sigma is very high vignetting at large apertures - to be fair this is relatively normal regarding the extreme ultra-wide focal-length range. The CA characteristic (color shadows at the image borders) is generally pretty good except in the extreme corners at 10mm. The build quality of the lens is very high and leaves nothing to be desired. All in all a good alternative to the native Pentax lens"

3-what about a fixed focal lens? The only one I found is the 14mm (I would have liked wider) that has the same price of the 12-24 at the moment. Any opinion?

what I found is: "that the differences between the 14 and the 12-24 in terms of quality are very small, and therefore they prefer the latter for the flexibility"
"The DA14 focuses a lot closer than the 12-24"
"the 14mm is faster at 2.8 and has no vignetting problems at all. A great lens"

Cheers
Max

Guys, any further comment on the topic?

Thank you everyone for your advices!
and for the trick hoe to use the help of the camera in MF!

I'm still not decided if go for AF or MF.

To sum up:
- AF 1.4 is the best one, only more expensive (actually I can spend £155), even if dpreview tells about a limit:

"However this excellent showing when stopped down does come at the cost of an unconvincing performance at large apertures; the lens is very soft indeed at F1.4, and while we described the Canon 50mm F1.4 as 'dreamy', the Pentax renders scenes more as though you're looking through a fine veil of mist. This is accompanied by a distinctly problematic rendition of out-of-focus backgrounds, to which it imparts a jarringly hard-edged appearance and substantial levels of green/magenta chromatic aberration. This in turn slightly limits the lens's usefulness for shallow depth of field portrait use, an application for which the (much more expensive) Sigma 50mm F1.4 EX DG HSM is rather better suited. Indeed it will be interesting to see how Pentax have chosen to design the recently-announced DA* 55mm F1.4 SDM, which is described as being particularly suitable for portraiture."

then MF, in order of quality (that seems given by the aperture value?):
-f 1.2
-f 1.4
-f 1.7
-f 2.0

Where f1.2 is the best one but more expensive, while 1.4 and 1.7 are good enough.

As I said, I can spend £155 but on the other hand I would like the idea to own an old, manual focus.

Is there any other usual (commomly bought) focal lenght that is cheap and good quality in the MF version?

For a few weeks I've been following some lenses proces
Pentax 55-300 £195 at SRS
Pentax 50 FA £155

and less attention on Sigma 10-20 and Pentax 12-24.

What I noted is that prices are very stable and usually there are no discount.
Is it like that,maybe only for some lenses, or can I hope in any promotion (at SRS or somewhere else?)

Thank you

Thank you for your clear explanations and software suggestions!

definetely!
was it that complicated to have a 100% view in the k20d viewfinder?!

I always have to bet how much larger the picture will be!

good ones!

Thanks Stefan
The Velbon looks what I'm after, but too much expensive.

I would need something similar or heavier than the Manfrotto 785b (cost about £50) (height 150.5cm/43.5cm, Weight 0.98 kg)but capable to hold something like 1,5 kg.

All I found are more than 2kilos as weight and holding weight. And much more expensive.

Thank you guys

The fact is that there are so many things to learn and would prefer to avoid to add the raw matter at the moment.

Anyway, which software are you using and can suggest?

As to the settings, I'll try the ones suggested: I like sharper images, but let's see how it is
(by the way I confused the center Focusing with the centre spot metering, so I was shooting with the last one: result was that often they were under exposed for my liking)
(by the way2=OT2 : I'm struggling to understand the _practical_ usefulness of the AF and AE-L lock: can't I have the same result pressing half-way the shutter button?)

I read good opinions about the manfrotto 785b, but it is until 1kg.

k20d + da55-300 is 1,2 kg. Probably I could risk, but prefer to avoid.

Do you know any similar product, lowweight as the 785b but suitable for some grams more?

Could this weigh 2885 grams?