I have been contemplating on getting some new lenses soon. I have grown to realise that the 18-55mm lens that I am currently using with my camera, is rather limiting. So, I was thinking of either getting a telephoto or a prime lens. However, after checking them out, I think that I’ll get a telephoto lens first, and maybe a prime at a later time.
The telephoto lens that I have decided on getting is the Pentax SMC 50-200mm (f4-5.6) zoom lens. From the different reviews that I have perused, it seems to be a very decent lens. I have also found a site that sells it for under £100 even though the SRP is almost £200. This makes it cheaper than the equivalent third party lens from Sigma, which goes for about £140.
What is good about this lens?
It is supposed to produce exceptional images at f/8 throughout the range from 50-200mm. While my 18-55mm was suitable for taking wide-angle landscape shots and portraits, the 50-200mm would be suitable for taking portraits and to zoom in for long distance shots, such as for events and parties where I may not be able to locate myself at the front row (e.g like at some friend’s weddings).
One amazing feature about this particular lens is it’s size. Most telephoto zooms are rather huge, in the range of about 12cm long closed, but this lens is only 8cm long (the image is the lense fully extended), which is the same size as my existing 18-55mm. This means that I can conveniently carry it in my existing small camera bag. This lens would be all I’d need to bring with me for outdoor holiday shots. The 18-55mm is better suited for indoor shots.
And for a prime lens, I am presently considering the 40mm (f2.8) lens, which would be very suitable for low light indoor party shots. The first party lens is only 1.5cm thick, which makes it convenient enough to store in the same small camera bag, right under the camera. However, this lens starts at around £220 and up. So, I will not be able to afford it at this time. I guess that these 3 lenses are all that I’ll need for a while. They would cover the wide-angle through to telephoto zoom range and a prime lens for indoor party shots.
One thing about getting into serious photography is the cost of the lenses. Once I invest in these lenses, I am unlikely to change camera brands for the future. The good thing is that Pentax is one of the only 2 camera manufacturers who have compability in their lenses since 1975. Yes, it is possible to use 30 year old Pentax lenses in my present camera. Presumably, this would stay the same for the next 30 years.
Sybreon Ones Nought 
I am not sure about the Pentax system but do they have a version of the 50mm f/1.8? I know Canon and Nikkon have theirs. The Canon EF 50mm f/1.8 is selling for £60-70 (cheaper in asia). It is probably the best deal you can find in terms of price to quality ratio.
The low f number is really fun to play with. like you said, it’s useful for taking pics of pretty girls. hahaha
Having said that, i think 40mm is ‘just nice’ for a crop sensor camera since i often find that I have to step two steps back to frame more into the picture.
Pentax has both a 43mm f/1.9 and 31mm f/1.8 lens. But they’re all a little pricey for me at the moment. I’ve just dropped £100 on my 50-200mm this afternoon. I should receive it by the end of this week or early next week.