A few months ago I wrote a short article on astrophotography with camera lenses. Part of that article described what's required for solar photography. If you have a very dark ND filter, now is a good time to photograph the sun since there's currently (5/12/12) a fairly large sunspot group on display. We're approaching a peak in solar activity, so thought this year we should be seeing significant sunspot activity. If you're interested in doing solar photography a good solar filter will only cost you around $50 (see the link at the end of this article). First here's a shot I took using a handheld Tamron 70-300/4-5.6 Di VC USD lens with a solar filter attached:
Even with a very dense solar filter, the exposure was still very short. Don't even attempt this without a filter designed for solar photography. If the sun is very low in the sky (sunset) you might be able to get away with something like a Hoya NXx400 filter, but that filter is not dense enough if the sun is high in the sky.
Here's a closeup of the main sunspot group taken with a 500mm lens and 2x TC (effectively a 1000mm lens)
For more information on what type of filters are required, please see my previous article on solar photography, http://www.bobatkins.com/photography/tutorials/astrophotography2.html
There's a link there to Thousand Oaks who sell screw in solar filters for camera lenses from 37mm to 82mm for around $50. You can also checkout solar filters at Orion Telescopes, though they mostly stock telescope rather than camera lens filters.
Final Caution - Don't try solar photography on the full sun unless you have a proper solar filter over the FRONT of the lens.. You could damage your eye, your lens and your camera.