sometimes we take the kind of pictures of our loved ones that we wish they would take of us.

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https://www.amazon.com/11-75-Fresnel-Magnifier-Projection-PREMIUM/dp/B00IITFX02/ref=sr_1_3?keywords=fresnel+lens&qid=1562905587&s=gateway&sr=8-3

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go for it.

Love me some Terry Gilliam!

They use fresnel lenses in the real film I think, not very expensive if you wanted to take it to the next level.

http://amzn.com/B00IITFX02

It doesn't get soft at all, it gets more focused so you can shoot it further out. Birds at 300ft, architecture from way back and such. I think it would actually be bad for portraits because you could damage your subjects eyes. I got myself with a speedlight at close range once and it hurt, having a Fresnel lens on it could have done permanent damage.

And that's all this is, a fresnel lens and some plastic arms to hold it in front of the flash, if anyone wants to make one. You can find the lenses at the dollar store too.

a laser, like a laser pointer, is a very bright light source, it will easily trigger a phototransistor. See here for example circuit There's no need to have more than one detector, so what you might want is to have a large light collector, such as a fresnel lens which in this case is not for its usual application (collecing a large amount of light and focussing it) but rather for collecting a fixed amount of light (the laser beam) and making sure it gets focussed onto the detector. Actually, as I think about it, the laser is probably bright enough that if you made like a box with the phototransistor at the back and a large frosted translucent sheet over the front, I would expect it to be sensitive enough to detect the light scattered into the sensor.

Now, because you also specified the timing accuracy /u/suprjami is certainly right, the rpi itself won't be accurate enough. This is because it will be checking the status of the voltage on a pin in a loop that will take several milliseconds to run. The arduino can run this loop much faster, and since you're looking for a time interval you could simply have the arduino count the time interval and output that to the rpi over serial and display it.

try one of these

Cheap and reliable is best for fire.

• Fire been sorted out for hundreds of thousands of years.
  
  Even more important when you need fire, sometimes you need it NOW! and delays, dead batteries etc are not okay.
  
  
• Get a combination of reusable fire starters like flint and steel or Fresnel lens (only daylight) and anytime fire (matches.)
  
  
• Get fire helpers like Magic biscuits, or saw dust and wax, or accelerates (like ether, gasoline, charcoal lighter fluid)
  
  
• Get an 8'5" x 11" frenzel lens. (for starting with sun light.) https://www.amazon.com/dp/B00IITFX02/
  
  
• Matches
  
  
• birthday candles for kids / make your own fire starters from make-up remover pads and parifin wax. These are the best fire starter helpers you can have. (you can use crayon wax if you don't buy paraffin) https://www.youtube.com/watch?v=uCni8yLXR3w
  
  The only thing the above video fails to do is use a double boiler so what you want is to make a "cup" or the like with some tinfoil, put the wax in that, then set the cup into a pot of boiling water. In this way clean up is a breeze, whatever wax you don't use you can just wait to cool and harden and throw away with the tin foil.
  
  These "magic biscuits" style fire starters will stay dry forever, and you can "crack" them in half to expose cotton fibers for things like flint and steel starters or the focus point of your Fresnel lens, or a match. etc.
  
  

  Make use of the small fire you do start.
  

  
  

• Getting a flame is just the start. One of the VERY BEST things I've ever bought cost about 10 bucks. It was a collapsible tube used like a very long straw to blow air at a fire without having to have your face close enough to suck in a lot of smoke when you take a breath.
  
  Made in america version for $12 from amazon.
  
  Absolute cheapest version $4
  
  

  Skills
  

  
  Fire is great. You don't need to go out int he pouring rain to start a fire, but doing so once in a while will help. Same with the wind. Mostly you need to know how to build a stack of wood so that stuff burns. There are many ways to do this.
  
  

• most of the time building your fire (kindling and med sized wood.) before hand and then covering it with a tarp or plastic. is the best practice. Building on a slight hill or digging a trench around the fire area is also a good "prep" to do before hand.
  
  
• If you are going camping or want a few kits making easy to use fire starter kits from pine cones, dryer lint, wax, pop-sickle sticks, cardboard etc will help. Old pine wood scrap from two by fours or lumber mills or woodworker shops works well too. These are things you can make in your spare time, when it's dry and comfortable.
  
  

  Fire cups!
  

  
  

• Fast food cups that are the wax paper cups (down south and in warmer places use Styrofoam cups) are great for a kit. You can put Popsicle sticks, some matches, some magic biscuits (waxed cotton pads), some black paper (to better start with Fresnel lens / sunlight). Then you put the plastic lid on with a bit of tape over the straw hole and you have a "water proof" cup you can set out, and use when you see fit to start a fire.
  
  Edit corrected spelling.