my f717 (a 5MP sony digital camera) and my gl2 video camera both have 58mm threads. I'd like to get a tele extender for the f717 that will also work reasonably well with my GL2. I'm guessing the Sony extender that is designed for the f717 will be the best since it's the f717 that needs the extender the most but if anyone here has experience in using a tele designed for a camera on a video cam or visa versa, I'd appreciate hearing about it.

I have and use a Sony 1.7X tele extender.....

see here on how well it works

http://stevenunez.com/GL2frames/gl2grabs.html

Highly recommended.

Sony makes two 1.7 tele extenders. I assume you use the first one listed here. It is made for their video camera. Is that right?

The first one here is made for their camcorder. it is larger/heavier and more expensive.
VCL-HG1758 1.7X High Grade Telephoto Lens
Close in on distant objects with Sony's VCL-HG1758 High Grade Telephoto Lens. Featuring an ultra quality 58mm thread, and a 1.7X telephoto converter for the Sony DCR-VX2000 Camcorder $ 400.00

The second one is for the f717 digital camera. Smaller, less expensive.Well...looks smaller in the picture anyway.

VCL-HGD1758 Close in on distant objects with Sony's VCL-HGD1758 High Grade Telephoto Lens. Featuring an ultra quality 58mm thread, and a 1.7X telephoto converter for your DSC-F707 or F717 camera. $ 299.99

My lens says "HG 1758"

I'm guessing the numerics mean

VCL-HG1758
VCL=Video Camera Lens
HG=High Grade
1758= (1.7X & 58mm)

The D is probably denoting "Digital" for the digicam.

In any case- I have the more expensive HG and it's excellent!

I hope to have that lens at some point in the future but it'll take a while before I get over the sticker shock of the gl2 and microphone. :)