Converting a Photo to just lines

You should check out Andromeda's Artistic Screening Tools ( www.adromeda.com I think ).  If I read you correctly, you're interested in line art conversion using different types of line screens.  Illustrator will convert using automated vector conversions, but not line screens.  There are hatch effects that you would eventually get to via Illustrator, but I doubt that's what you're after.

You mean something like this?

This was a photograph of some trees that I traced and then changed to strokes instead of fills.

At the top of the tracing panel there are 6 buttons, the last one traces your image to outlines, doesn't work well with every image but give it a try. I would suggest filter gallery effects in Photoshop and if you need it in vector just trace your result in Illustrator.

Did an automatic trace the usual way, taking care to check Ignore White.

When expanded, this gave a single set of paths.

Then just coloured with a stroke and no fill. Chose a stroke weight and style that suited.

See also what naz0ua says. My version doesn’t have those 6 buttons though.

I have circled the button, what version are you using? This is what it looks like depending on the resolution of each image. hope that helps

In my version it’s called Live Trace and there are check buttons for Fill or Stroke. I prefer to use Fill all the time and change the paths to stroke afterwards, after I have maybe edited them.