How will i know which zoom lens fits my SLR camera?

only lens of e same brand fits? or does every lens fits all camera?

i recently bought a 2nd-hand Dynax 300Si. any reccommendations as to what lens is good?

Check your mount. Usually it says EF (Canon) or something before the "x"mm that is your mount name. Look for lens that are mounted to that name.

Not all lenses fit every camera. Canons only fit Canons, Nikons fit Nikons and so on. However, there are after market lenses that will fit name brand camera, if you go to a camera store, they can get you lined out.

You can go with your camera's brand, or one of the "aftermarket" brands: Sigma, Tokina or Tamron, with the correct mount for your camera.

No, not every lens fits every camera.

My friend , usually your brand camera will take the same lenses ONLY. There are 3rd party lenses by SIGMA, TAMRON, TOKINA , VIVITAR etc... that make lenses for your SLR too that fit and are usually 25% Cheaper as well ! Providing you own a Canon, Nikon, Minolta, Pentax...
Hope this helps !

not all lenses will fit - if it is a Nikon then you should look at nikon lenses but not all Nikon lenses will fit and some may work differently on your camera i.e a non digital lense on a digital camera can work but the "zoom" is different. If it is a canon look at canon lenses.

Another option is going to a camera store such as Henry's where you can buy new and used cameras and parts. For example i have an old manual Practika SLR and have found a variety of brands of lenses to fit and work - the threading for attaching needs to fit. take your camera with you and try the lenses.

Generaly with lenses there are a range of "quality" - a leica lense for example is great and then there lesser quality brands. Even with big names such as Nikon and Canon there are varying degrees of quality oin their lenses.

I see lenses by Konica-Minolta as well the major aftermarket manufacturers (Sigma, Tamron and Phoenix) offered at the major mail order houses in New York. You have to buy lenses with the specific mount needed by your camera. For instance, even in the Minolta line, there were different mounts used over the years. I only know that one, which is currently in use, is called the "A" mount. You would be well off to take your camera to a local camera shop and verify that a lens fits before you buy your first one. Once you know exactly which mount is required, you might be more comfortable ordering a lens from somewhere.