the DSC-F717 doesn't have interchangeable lens. You have to get a tele-converter. You should be able to find these at any camera shop, or maybe even Best Buy or the alike. Make sure to bring your camera in with you so you can make sure that you are gettin' the right size, i.e. fits the threads and size at the end of the lens.
If i remember right, the lens is a 58mm diameter, i could be wrong but i believe it to be correct. I personally have a DSC-F828 and it has a 58mm lens diameter.
note: make sure not to get confused with the focal length(i.e. 135mm, i think for the f717) and the lens DIAMETER.
also, you probably won't be able to get a HUGE difference with a teleconverter. Typically i see teleconverters range in 1.4X to 1.7X so, you 135mm lens will become approx. a 230mm lens.
One more thing, the lens on these cameras are fantastic, putting a teleconverter of lesser quality WILL reduce image quality.
Hopes this helps
