Hi Daniel and Others,
Here are my 2 cents worth. I suppose that for starters, Dollars are going to be a consideration. (When aren't they?) Whilst I can't speak with any authority in respect of Pentax * ist DS cameras, if this is the same, or has the same type of configuration as the K 100D, then I would suggest trying extension tubes as a starter. They are cheap, (around $25) contain no glass, so can safely be bought second hand, and even if they are not automatic, it doesn't really matter that much, IMHO. An even cheaper alternative is to buy a reversing ring (less than $15), which will enable you to use your existing lens "back to front", and this will give you macro ability. A reversing ring has a male thread of the same diameter as the filter ring on the front of the lens, and a standard Pentax body fitting, the same as your lens bayonet. The lens is then reversed on the camera, by screwing the adaptor to the front of the lens, reversing it, and fixing to the camera with the bayonet fitting. It won't be automatic, but that doesn't matter at this stage, it has only cost a few bucks.
You might be able to buy automatic extension tubes for your camera, which could cost up to $200 (or more) they certainly can be bought for the Canon EOS system.
Another alternative (at least for the EOS) is to buy an adaptor for your camera that will enable you to use non Pentax lenses with your Pentax body. With the EOS (I keep mentioning this for 2 reasons, a; I have an EOS 300D, and b; many other Forum readers also have EOS cameras, so it would be particularly relevant to other readers) you can use Pentax or Nikon lenses with a cheap adaptor purchased for less than $20, and couple it with a dedicated 25+year old Macro lens. These can be picked up for anything from $25 to $100, depending on the manufacture and spec of the lens. Again, it will be manual only.
Now a word or two on manual set up. All, or nearly all, the macro shots I take are done by switching
off the auto focus, setting the shutter to 1/200 sec, and the lens to f11, and using diffused flash. The diffuser by the way is home made using kitchen paper, and a plastic butter tub, lined with aluminum foil. VERY hi-tech!!, albeit reasonably priced.
I don't use a tripod either, as it takes too long to achieve focus when taking shots of insects on leaves, blowing in the breeze. However, I do use a pole to support the camera, and lean in and out to achieve focus. It takes some practice, but in a few sessions you can become quite adept.
Using an f-stop higher than say 14 or 16 tends to produce soft images, (as Rally says, you reach the diffraction limit), so I try to limit the aperture to f 14 or lower.
I'm not knocking any of the matters raised by Rally, but adding to what he suggests by saying what I have found workable, in my experience. As I have said, I have an EOS 300D, together with a Canon 100mm macro lens, and auto extension tubes (plus lots of other lenses and gizmos). I also have an old Nikon 105mm manual Micro lens and an adaptor for the EOS, and I have got some superb shots with this set up, but a fair amount of trial and error is required initially. Keep in mind that not all cameras will allow retro fitting of other lenses, the EOS system is very forgiving in this respect, but I do not know whether the Pentax will accomodate Nikon, Canon, Minolta etc etc lenses.
My prices may or may not be current. Check on EBay, in the Trading Post, at Camera Fairs and look for the Photographic Trader in your newsagent. I'm sure you will find something to suit.
If you want any further info, shots of my set-up etc, just PM me. Happy to help.
