When we're suggesting lenses to you, we'd also need to know what brand of camera you're using. In some cases it's possible to adapt other brand lenses for use on your camera even if the mount wasn't originally the same, but you'll lose a lot of functions.
Case in point is some beat-up Leica lens I'm trying on my Canon camera of late. It works on the camera with the help of an adapter. But focus is entirely manual. Aperture control is manual as well.
My understanding (and with limited experience) is that faster aperture (and thus more expensive) lenses are likely to produce better bokeh. For example, a 400mm f/2.8 will produce better bokeh than a 400mm f/5.6. This is both the result of a larger aperture even at the same focal length, and also a difference in optical quality.
Nevertheless, with any lens, if you apply the techniques to increase distance between your subject and background, use a longer focal length lens and reduce the subject to camera distance, you are likely to see more developed bokeh. Choice of background also affects what your bokeh looks like. Put lots of backlit leaves in the background and lots of other specular highlights, and you're likely to get very distracting bokeh even if all the elements are well out of focus.
Very few lenses are going to produce convincing bokeh with a maximum aperture of f/5.6 or even f/6.3. That said, if you start buying into the long telephoto lenses, you can get nice bokeh. I've seen nice results with say, a Canon 100-400mm f/4.5-5.6, or a Canon 400mm f/5.6. The 300mm f/4 also produces good bokeh. All three lenses are in the range of $1700-2000+ on the second-hand market here.
If you want a shorter telephoto lens to do things such as half-body portraits then the 85mm f/1.8 is said to be very good value for money at around the high $500 mark. Going even longer there's the legendary 135mm f/2L that costs around $1100 that is pin-sharp wide open with tremendous bokeh. Or going back to the wide angles and standard lenses...35mm f/1.4L is also excellent, but at a price of $1700 or so.
If these are not for you, look at the 50mm f/1.8 at just $100. It's easy to produce obvious bokeh with this lens because of the wide aperture, but it is not the smoothest I've seen. Also, if you're using it on a camera with a smaller than full-frame sensor then the lens seems 'longer', which also limits its purposes in my book, unless you're strictly after portraits. The 50mm f/1.4 would be the next step up. Better quality bokeh, and at $400 more.
It's a very wide topic and not easy to talk about in sufficient detail. When it comes to tripods, I can personally recommend a Manfrotto 055PRO (or Bogen 3021) and 488RC2 ballhead. Very heavy, but also extends fairly high and goes very low. Can't remember how much I paid for it, but surely less than $300.
I think that there's no such thing as a sum of money that's considered 'reasonable' for a student to spend. It's up to the individual. If you're spending your own money (ie. not stretching out your hand and expecting parents to give you the dough) and spending within your means, that's okay. In a way I'm very much like you...when it comes to budget, I'll slog away at work to get the money if I really want something. So if someone wants to blow 6 grand on a brand-new state of the art camera, or 12K on a lens, but is doing so within their means and with their own savings, that's really okay.
Photography is extremely expensive, but if you ask me, much healthier than drinking, smoking, gambling and other undesireable activities.
Do make sure that the tripod can take the weight of your camera and lens and still stand steady Tripod do have a max load weight for it to perform at its optimum
Canon 30D, G11, 50 f1.8II, 10-22 f3.5-4.5, 24-70 f2.8L, 70-200 f2.8L IS, EX580II
bokeh depends on the design of the lens... a well designed lens at f4 will still have better bokeh than a not so ideally designed lens at f2.8...
We're photographers, not physicists! And some of us are measurebators.
Anyway, the Sony system does boast some Carl Zeiss lenses, but I have never tried any of them and hence cannot comment. It's also incorrect to generalize that German lenses have better bokeh than Japanese lenses, or vice versa. The best thing to do is to hunt down reviews of lenses you are interested in, and then look for sample pictures, either on Pbase or Flickr...
Personally I believe it's better to first decide what focal length you need, and then focus on the lenses that you believe have the best bokeh in that focal length range. For example, I currently use the 35mm focal length on a Canon system. The options available to me would be things such as a Canon 35mm f/2, the 35mm f/1.4L, all manner of Leica and Carl Zeiss lenses etc. Then, bearing in mind the price, compare the characteristics and see which one you like best.
Bokeh is really subjective. I'll bet my life that if you showed me a photo from a lens that supposedly produces 'bad bokeh' according to a lot of reviewers, I may still say otherwise. I think it's difficult to judge bokeh on its own. If it adds to the rest of the image, it's good bokeh. If it detracts, it's bad bokeh. This in itself means the photographer needs to take care of the choice of background, avoiding bright, distracting specular highlights, or gaudy colors (if it detracts from the idea of the image). This refers to the quality of bokeh.
Okay, thought to add some examples here...both were taken with the exact same lens and same aperture. Of course the subject to camera distance and distance from subject to the background is different in each photo...but notice how much more pleasing it seems to be in one photo compared to the next. Choice of background is important:
Fairly bad streaky bokeh (especially top left):
Bokeh appears smoother:
But the degree of bokeh can be controlled by the photographer to a large extent, irregardless of the lens in use. If you want a greater degree of bokeh, then:
-increase the focal length of your lens while maintaining the same subject distance (thereby increasing magnification...remember that depth of field decreases when magnification is increased)
-decrease the subject to camera distance (thereby also increasing magnification)
-use a larger aperture setting (ie. smaller aperture number such as f/2, not f/16)
-increase the distance between the subject and the background (the further away the background is from your zone of depth of field, the more blurred it will become)