Magnifying glass in pyrography--variables?

I stumbled across the work of late British artist Roger Ackling, who used a handheld magnifying glass to create pyrography on found wood. My question is: Does the shape, thickness, type of glass, or other variables affect the heat of the ray as it leaves the magnifying glass and contacts the wood? In practical terms, if one wished to add a magnifying glass as an art material to one’s everyday carry (EDC), what characteristics would give the most satisfaction?

Ideally, you’d want a large aperture lens (to collect as much light as possible) with short focal length (so you can use it at a comfortable distance) which is also lightweight, i.e. thin, so that it can easily be held steady. However, these are somewhat mutually exclusive requirements, so you would have to settle for some compromise.
I believe most magnifying glasses have spherical lenses. While parabolical (and achromatic) lenses give sharper images, they are more expensive and probably not a lot better for your purpose.

Wait, so when I was putzing around with a magnifying glass as a kid, I was creating art? I thought I was just burning stuff, ants especially. And to think my mother never thought it would amount to anything. Well, it didn’t but my point is that it could have.

Thank you, Ignotus. Dewey, yeah, me, too. I’m a little worried about all that bad ant karma I built up.

Ignotus: So basically, I should just get the largest circumference handheld magnifying glass that will be light enough (thin enough) to hold in one position for several minutes? (I’ve heard he built up to doing this for seven hours a day, but not how long it took him to build up to that.)

Another reason that you’ll want a short focal length is that the size of the spot you make will be proportional to the focal length. A shorter focal length will thus allow you to burn finer detail.

Couldn’t you use a fairly big lens that did weigh say 5 or 10 pounds and simply mount it on some sort of Adjustable arm like a lamp so you can adjust the angle relative to the sun as the minutes or hours pass, and simply pick up and manipulate the small piece of wood your working on under the still focused beam rather than moving the whole lens? (Kind of like working a piece of wood through a big bandsaw rather than using a jigsaw on a stationary piece of wood)

Flat Fresnel lenses are nice for this sort of work. The plastic ones are light and you can just tack them to a wood frame so the don’t bend. You can get an 8X10" lens w a reasonable focal length for less than $10; far cheaper than a regular glass lens. They burn patterns in wood just fine.

Thanks, Chronos, fine detail is important. Very logical, Mmmiiikkkeee, move the wood instead of the lens! Thanks, Squink, I had not realized Fresnel lenses could do this. I looked it up and here is a link to a Fresnel lens not only burning wood, but melting a penny, and for about $4, plus tax and shipping:

Pennies are now just copper plated zinc. You can melt them easily on your stove.
The copper floats and can be skimmed off.
The zinc is nice for casting figures. Beware smoke from burning zinc though. It’s quite toxic.

Do you just care about getting the highest amount of heat in the middle of the spot? If so, a large Fresnel lens is probably best, just because you can get a much bigger one for the amount of money you are willing to spend, and/or the weight you’re prepared to carry.

However, Fresnel lenses aren’t very good quality, so the spot will be very diffuse. If you are creating artwork, I’d imagine you want a small, well-defined spot. For that, I think your best bet is a camera lens. You can probably get a manual 50mm F/1.4 lens pretty cheap on eBay, and it would create a nice tiny spot.

Not very diffuse. I’ve made nice signs using an 8X10" Fresnel. The sun is not a point source after all.
If you want awesome detail, a nice CO2 laser on an X-Y plotter or 3Dprinter body is the way to go. You can buy them readymade, but they get pricey.

OK. I’m a physicist, not a pyrographer (?), so I have no idea what is a desirable spot size. For what it’s worth, a 50mm camera lens should produce a nearly perfect circular spot about 0.5mm (0.02 inch) in diameter.

That’d be great. The trouble comes in when you try to hold the lens steady. Blows your spot size all to heck, unless you’ve got some sort of elaborate stage or goose-necky type thing. Plus of course wood conducts heat, so that 0.5mm spot at 5700K is going to warm the wood right next to it up above pyrolysis temps darn quick.
A decent soldering iron, with changeable tips and temp control, is another way to go.

Alternately, there might be someone who has one already who’s willing to let you use it. There are at least two maker spaces in the Cleveland area that have such devices available to the general public for only the cost of materials.