Fresnel lenses - a play in the sun

[Andy BB]

Working in Izmir at the moment, and as its a nice sunny day, thought I'd have a quick play at lunch with a fresnel lens I keep in my wallet but had never used (get them on ebay - credit card size - for peanuts).

Bit of a breeze, and temps in the low 20cs, but clear sky. holding lens in one hand and piece of paper in the other, managed to burn holes straight through it with little problem (other than trying to keep both lens and paper stationary in order to focus the lens - wind didn't help!)

So - lots of heat produced. No fire though - just burned straight through, leaving hole with charred edges. Seems a worthwhile backup to me!

 

[ged]

Working in Izmir at the moment ...

Some people get all the luck!

... a quick play at lunch with a fresnel lens ... managed to burn holes straight through it ... No fire though - just burned straight through, leaving hole with charred edges. ...

I find the larger ones, about the size of a paperback book, are better. Although the spot tends to be bigger because they're difficult to keep perfectly flat, they give you a lot more power and the bigger spot just means you get more char forming.

The trick I find with these things is to make a kind of cozy nest for the fire to start in. Imagine putting three dice in an L shape on a flat board. Then focus the spot in the inside corner. That's about the optimum for minimizing the wasted heat, as when the heated surfaces re-radiate their heat a lot of that heat goes back into the area you're heating. If that makes sense.

 

[Zingmo]

I had never thought of using one to roast peanuts! I used to have an A4 size one which could ignite a piece of wood. For the amount of space and weight they are a good standby. No good in foul weather though and you have to remember to light the fire before it gets dark!

Z

 

[Andy BB]

Some people get all the luck!



I find the larger ones, about the size of a paperback book, are better. Although the spot tends to be bigger because they're difficult to keep perfectly flat, they give you a lot more power and the bigger spot just means you get more char forming.

The trick I find with these things is to make a kind of cozy nest for the fire to start in. Imagine putting three dice in an L shape on a flat board. Then focus the spot in the inside corner. That's about the optimum for minimizing the wasted heat, as when the heated surfaces re-radiate their heat a lot of that heat goes back into the area you're heating. If that makes sense.

I was kinda wondering how effective a credit-card size one would be, as they are just so light and fit in a wallet just fine. Pleased to say that it worked a lot better than I was expecting, And it's something I'll have on me at all times, unlike 99% of my kit!

 

[ex-member BareThrills]

I use the credit card ones all the time. Work great with cramp balls or any other ember holding tinder.

 

[Stringmaker]

I use the credit card ones all the time. Work great with cramp balls or any other ember holding tinder.

+1 to that

 

[bikething]

If you're using newspaper - aim for the black print

 

[Caboose]

used a credit card sized one also with great sucess. I've personally made fire with just some dry grass/wood "paper" shavings mixed in and packed down densely, then kept the lense still for a fair time so it gets a nice coal going & then blown that to flame. One of the easiest most relaxing ways to get a fire going. I preffer it to flint & steel, but obviously keep that in the pack as well!