Well, it turns out that Daniel is right, Lens Fire Starting (or Optical Fire Making) can be very tricky and tinder is critical.  I would recommend to anyone that they get the biggest lens they can find.  A couple inches across is good.  Smaller than 1 inch across is very hard to do, but it can work.  Over the past years, I've heard all kinds of improbable lens fire stories.  You may have heard the fire from ice balls and ice lenses urban legend.  The TV show Mythbusters even did a fire from ice episode, with questionable results.  A drop of water held in a looped blade of grass or slender wire loop has been in some optical fire starting claims.  A clear plastic bottle with just the right amount of water and held at just the right angle is supposed to work.  Like I said, I won't believe it until I see it firsthand.  The strong summer sun works some wonders, don't get me wrong.  I'm a big believer in possibilities.  But I want to see before I believe completely.  Speaking of the summer sun, you are much better off making optical fire that time of year than with the far off winter sun.  I have rarely gotten lens fires in the winter.  The closer distance to the sun and greater ambient air temperatures really seem be of benefit during the hot months. 

So to get started, find a sunny spot to try your tinder and lens.  You could set up your fire place and camp anywhere, and transport the fire if needed, should you not have a beam of light shooting down into your woodland camp.  Lens fire making is best attempted during the midday.  Morning and afternoon will work, but the noon sun is strongest.  Select the fluffiest tinder you can.  See my tinder article on this web site for material ideas.  Try matting down your tinder in the middle to make a flat spot so your burning can be focused all in one place.  Use a really fluffy, fine fibered tinder for the flat center of the bundle.  Tulip poplar or PawPaw inner bark, rotted and pounded with a rock should be good if you live around the Mid-Atlantic.  Use Jute twine from the store pulled apart and fluffed up, if you can't get to the woods.  Do your fire starting in full sun out in the open.  Focus the light on the flat spot in your tinder bundle until the dot of light is as small as you can make it.  Manipulate the lens at different angles and at different distances from the tinder until you have the perfect "dot" and the smoke should start immediately.  Blow gently across the tinder while you are burning it.  This can be the make or break trick when doing this method.  Your extra oxygen and air movement cause any fibers that have reached glowing ignition to spread their ignited red glow into neighboring fibers.  This is basically the birth and expansion of your tiny coal.  We go from a speck of light and tinder, into a tinder born fire.  So when the first dot of light causes the fibers to smoke well, then blow harder when you see the smoke level increase.  Blow consistently (and even constantly, if you can control your breath) until the tinder flames up.  It's just that simple, and just that complex at the same time.