Glass lens for starting fires

[ateallthepies]

I'm gradually getting my fire kit sorted but would like to add a glass lens, trying to stay traditional as possible. Been looking on ebay and basically it comes down to either a 38mm or 50mm double convex lens.

But...

They do them in different focal lengths from 10cm to 50cm.

They will all probably work but anyone know which may be more efficient, I'm guessing the larger diameter but does focal length make a difference?

Steve.

 

[Retired Member southey]

Id say (and I am only working on the under standing that the focal length would be the point at which the suns light would become a very hot spot) the 10cm one would be the better and maybe the 50mm lens would be the more controllable,

 

[drewdunnrespect]

now southey i nwould have to agree with you mate that your probably right but i wouldnt like to say for certain and i bet some one with more knowledge is going to come along in a minute and say yes or not to weather we are right or not

 

[ateallthepies]

Sounds right about it being easier to keep the spot controlled the closer it is.

Steve.

 

[Retired Member southey]

now southey i nwould have to agree with you mate that your probably right but i wouldnt like to say for certain and i bet some one with more knowledge is going to come along in a minute and say yes or not to weather we are right or not

Eh what a tosh post chap, so your saying my thoughts are not to be trusted?

 

[lou1661]

Eh what a tosh post chap, so your saying my thoughts are not to be trusted?

your right, but i will need to see that comfirmed

 

[drewdunnrespect]

Eh what a tosh post chap, so your saying my thoughts are not to be trusted?

no i am saying i agree withu but wouldnt like to have said for certain either way cos i am not sure

 

[JohnC]

A wide lens should be better, and having used a sak lens which has a short focal length,about 1.5cm and an old binocular lens with a longer focal length of about 6cm, that the longer length is much easier to handle.

 

[ateallthepies]

Sounds like the 50mm by 10 cm is favourite so far.

Steve.

 

[ged]

It's the amount of light collected that matters, not the focal length. The bigger the lens the better.

I've lit fires with the glass out of an old railway engine running light. One of the ones that used to have a paraffin lamp in them.

The optical quality doesn't really matter much but some of the really bad Chinese things aren't much cop.

I'd go for something like a big plastic Fresnel lens. I have one that's about 10" x 7". It knocks the spots off any magnifying glass and it's lighter.

Here's the sort of thing I mean:

http://www.ebay.co.uk/itm/Plastic-Fresnel-Convex-Lens-Magnifier-Sub-A4-sized-260-X-177mm-Solar-Firelighter

Mine is decades old and yellowed with age, but it still lights things very quickly.

 

[ateallthepies]

The Fresnel looks a cool bit of kit but I'm trying to avoid plastics and keep things traditional. I believe glass lenses have been around hundreds of years?

Steve.

 

[ged]

The Fresnel looks a cool bit of kit but I'm trying to avoid plastics and keep things traditional. I believe glass lenses have been around hundreds of years?

Ah, sorry, I thought 'traditional' meant 'not using a lighter.

Yes, Glass lenses have been around for thousands of years, not just hundreds.

http://en.wikipedia.org/wiki/Lens_(optics)#History

It was only relatively recently (hundreds of years) that they were combined to make things like telescopes and microscopes.

http://en.wikipedia.org/wiki/Telescope#History
http://en.wikipedia.org/wiki/Microscope#History

 

[The Ratcatcher]

I have two bi-convex lenses intended for fire lighting. The larger one is from the old RAF survival kit, and is labelled "Burning Lens, 2.5", MoD Part No. 14A/7002112". It is 63mm diameter and has a focal length of 24.1 cm. (I have no idea of its magnification power, as it isn't specified.) The other is mounted in a Hudson Bay tinderbox, and is a 30mm diameter x10 magnification lens with a focal length of 4cm. Having used both, the smaller one is easier to use because it is easier to keep the distance between the lens and the tinder constant. With the RAF lens it is surprisingly difficult to maintain the correct distance.

Alan

 

[Wayland]

£2 from Morrison's and a bit of leatherwork.

As Ged says, it's the light gathering power rather than the focal length that makes the most difference.

The bigger the diameter of the lens the better which is why the Fresnel lens mentioned above works so well.

 

[Biker]

It's the amount of light collected that matters, not the focal length. The bigger the lens the better.

I've lit fires with the glass out of an old railway engine running light. One of the ones that used to have a paraffin lamp in them.

The optical quality doesn't really matter much but some of the really bad Chinese things aren't much cop.

I'd go for something like a big plastic Fresnel lens. I have one that's about 10" x 7". It knocks the spots off any magnifying glass and it's lighter.

Here's the sort of thing I mean:

http://www.ebay.co.uk/itm/Plastic-Fresnel-Convex-Lens-Magnifier-Sub-A4-sized-260-X-177mm-Solar-Firelighter

Mine is decades old and yellowed with age, but it still lights things very quickly.

Thanks for the link ged, just bought one. Got the credit card sized ones in my wallet but that one's a beast... now all I need to have is the sun. I wonder where that's gone .

 

[ateallthepies]

Cheers Alan for your experience of lenses, large diameter with small focal length to get the most power and ease of holding it steady.

That's nice that is Wayland, very nice! Will try and make something like that for myself so thanks for the inspiration.

Steve.

 

[bilmo-p5]

Salvaged this 7" monster from an old radar set. It's a cracker for burning but a bit of a lump to hold steady at 2 lb 10 oz.

 

[ged]

Salvaged this 7" monster from an old radar set. ...

Is it flat on one side? Looks ever so much like the one I had from the railway engine. Sadly that was over fifty years ago now, I don't know what happened to it.

 

[bilmo-p5]

Yes, flat on one side & about an inch and a half thick.

 

[Biker]

If you ever get your hands on an old overhead projector they have enormous magnifiers in the base part of it. Got one stashed in a drawer in my work bench I salvaged about 9" across and 1½" thick. Convex on one side and flat on the other. So powerful it even works using the reflected light off a full moon... well sorta.