Okay...let me try to answer a few things here...
1) For Breny
- you need to look for AFS lenses in Nikkor/Nikon and HSM in Sigma...I can't recall the notation for lens motor in Tamron - can anyone else? In a nutshell...your camera (the D5000) does not have a lens focus motor on/in the camera body...therefore...you need to make sure you buy lenses that have their OWN focus motors IN the lens itself...that is...IF you want that lens to Auto Focus. *IF* you are okay w/ manual focus (and frankly, I just don't want to do it 98% of the time
)...then you can buy whatever...but I'm guessing you *want* auto focus...?
2) Prime lenses --- different than zooms. An 18-55mm set at "55m" would not be the same as a 50mm Prime. I'm not very good at explaining this stuff...it's tough to write out and describe...so let's see. First, I find you have more control over the aperture w/ prime lenses, and I really like that as I pretty much always shoot in aperture priority mode.
Honestly...instead of me trying to explain it...which probably won't be very good, here are some pretty good articles
:
Why a prime lens is better than a zoom Prime lenses, and why you need one :: Photocritic photography blog Prime VS Zoom - How They Fare Against Each Other - DigitalRev.com Article about Aperture / f-stops