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.

[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.

[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.

[R.Lewis]

Largest lens for light gathering and preferred focal length would depend on where you are comfortable holding it. Most people would find a short focal length easiest as can easily judge distance and steady hands.

Big lens at 10cm be ideal.

[oldtimer]

I remember playing with magnifying glasses at school and we found the biggest diameter and the shortest focal length best to make fire. I've carried an old, chipped lens in my possibles bag for years and I know it works because I used to light my pipe with it when I lived in the tropics. Although whenever I've needed fire in a hurry it's usually wet, cold and overcast. I have a credit card sized fresnel lens in my wallet mainly in case I break my glasses, but even in ideal conditions I've never been able to make fire with it.

[Biker]

I have a credit card sized fresnel lens in my wallet mainly in case I break my glasses, but even in ideal conditions I've never been able to make fire with it.

Hate to admit it but I carry a fresnal lens in my wallet the exact same reason, I swear they're making print smaller nowadays. Must admit I haven't tried lighting a fire with the fresnal lens though. Figured I use one of the other means I might have with me if I had to.

[bilmo-p5]

I have a credit card sized fresnel lens in my wallet mainly in case I break my glasses, ...

Jolly useful in the car, too, especially if you need to consult an A-to-Z at night.

[FGYT]

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.

Ive several out of the head of the OHP approx 4" diameter but you dont see many in skips these days and a friend used to use one to burn house name signs on timber

ATB

Duncan

[rik_uk3]

Any old lens worked for frying ants and burning holes in ties when I was at School, you don't need anything special really.

If you insist on quality I've got one with a black frame so its a tactical one I'll sell for £150. I've a 'Buscrafty Tactico' model which is also black framed but in a little leather pouch for £175 or you could get this for a couple of quid
http://www.ebay.co.uk/itm/MEDIUM-MAG...#ht_778wt_1168 and just take the lens out.

[ged]

Can't help feeling the OP ought to get himself a couple of bits of thick flat glass and some jeweller's rouge.

Using equipment that's been made with CNC machinery somehow doesn't seem to me to sit well with wanting to be as traditional as possible.

[ateallthepies]

I think you have a point ged but I ordered a lens last night anyway.

Steve.

[Andy BB]

Hate to admit it but I carry a fresnal lens in my wallet the exact same reason, I swear they're making print smaller nowadays. Must admit I haven't tried lighting a fire with the fresnal lens though. Figured I use one of the other means I might have with me if I had to.

Mine burnt a hole very nicely in a piece of paper, despite both being hand-held and with a bit of a breeze blowing!

[Colin.W]

Any old lens worked for frying ants and burning holes in ties when I was at School, you don't need anything special really.

That takes me back a few years, there was also the odd burning the back of someones neck when the teachers had us sat outside when it was sunny whilst he droned on about clouds and rain cycles or something (I wasn't really listening, too busy trying to focus my lense on the neck of the person in front of me)