Not seen a review of the Samyang 24 t/s yet, but hoping it is a fine lens. If the Samyang 85mm and 35mm are anything to go by, it should a great lens.
With that in mind, for pentaxians who do architecture it would be a must have as it is practically the ONLY T/S lens for Pentax (apart from the mega-expensive Schneider)

Good point about the DA21 Dave, I would certainly add it to the list of must haves. I love mine

I have proved my point with the FA* 85mm, you cannot get it, and those who have it will not part with their copy.....

But the Sigma 85mm F1.4 is an excellent substitute, albeit expensive. So it is allowed by the OP (ie me) as a must have lens for pentaxians.

Here you go

http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-de...

So, it is my understanding that the 2x converter works the same way as the crop factor of an APS-C sensor. Thereby the focal length of a lens is the same, but the angle of view of the lens differs taking into account the crops factor of either the sensor or converter. Therefore you have to multiply the aperture by the factor of the sensor/converter to give you the depth of field equivelent.

The only way is to confirm this is to do for example

75mm with 2x on at F4 on a full frame
100mm on APS-C at F5.6
150mm at F4
All from the same position.

Confused? I know I am! LOL

On the other hand, if your converter gives you what you want, then just use it.

Okay, we are all Pentaxians, and love the system. But, which lenses would you consider as must haves for any Pentaxian? Obviously cost would be taken into account, but if a lens that is widely available but expensive is a "must have", then include it. We shall exclude rare or very difficult to get hold of lenses, as only a small number of people have them and won't part with them (I am thinking of the FA*85mm F1.4 as a start)

For me, it would be the following:-

DA*50-135mm F2.8. The most expensive, but it is a stunner! Plust you can get them new.
55mm F1.8 - A must! does not matter which version (any Tak, or the K-55), any pentaxian has to have one to realise the sheer delight of a small well made sharp prime.
35mm F3.5 - A takumar, which proves fast does not mean perfect! This IS perfect.
50mm F1.7 - Sweeter than the F1.4 and cheaper too. Welcome to the world of the "nifty"
28mm F2.8 - again any flavour. I have the SMC-F one. ugly but sharp and stunning!
135mm F3.5 - I don't have one, but anyone who does say it is the sweetest 135mm Pentax made, either M42 or K-mount.
DA-40mm F2.8 Limited - Cheapest Limited and a proper Pancake. Sharp as anything and intro to the world of Limiteds. So small that it is a talking point too.

Your turn.

Roger, you will love it!
The K-5 is such a big advance over the K-7 it is scary.

Not if I get there first! LOL
Just kidding. Good look in your search

So, what is the rarest then?

Can I just confirm then that the AF IS better on the K-5II and IIs over the K-5 for those who use both?
I only ask as I did some portrait shots with just a Beauty dish and no other lights, and the focus was hit and miss on my K-5 with the tungsten modelling light.

That 122K with a lens for that price could be good. But having said that, I would rather have an MX or P60. Still for the price.......hmmmm.

Like you say, the Zenitar is "interesting! But at least I know what it looks like

As for the Helios, the price is a bit steep, but I suppose being rare it can command that price. I think I will stick to my M42 model (must resist buying....)