Howdy Unknown. Good questions.
I used to use magnifying lenses, including fresnels, quite a lot. Had quite a variety of sizes and magnifications at one time. Caught deals on some credit card sized fresnels in 3x and 5x so those tend to be what are in my kits now.
The key with Solar Ignition is to think of magnifying lenses and reflectors as funnels.
The larger the the physical size of the lens, the more photons (light) it gathers. The higher the magnification, the tighter the focus. Just as when funneling a liquid, larger funnels are faster and hold more while tighter funnels are slower yet more precise. So too with lenses and sunlight.
Therefore what matters most is the size of the lens. Larger lenses will catch the most photons, and they are what does the work. A large, low power lens will produce an ember much more quickly than a small high power lens. Higher power does matter, and performs better, when the lenses are the of the same size. But only then.
On the lowest end I have gotten embers with 1.5x reading glasses. Took a looooong time. The polished bottom of a soda can was faster. With the 3" x 5" fresnels at 3x and 5x, there is only a slight difference between them. Yet with about three times the surface area of the reading glasses, they were each much faster than those.
To give some more examples, if your choice is a 1" diameter round lens at 20x vs a 3" x 5" fresnel at 5x, go with the latter. An 8-1/2 x 11 at 5x fresnel will outperform a 4" diameter 15x glass lens. Both will be fast, but the larger will be faster.
Scuffed lenses lose efficiency quickly. Think of a funnel clogged with debris. Fresnels are made of very soft plastic, so they scuff easily. Glass lenses are harder so they resist scuffing better. Try to cover any lens with a soft cloth and a dust proof case for long term carry.
For tinders we want good coal extenders (aka CEs). CEs are anything that will grow a coal. The less tending they need to do so, the better or more optimal we consider them.
Some, such as primo white rot punkwoods, chaga, the trauma and pore tube layers of Hoof and Artists fungi, etc are fantastic. Merely start an ember in one end and they will burn down to nothing but a pile of fine white ash without any effort on our part. Others, such as most shelf fungi, red rot punkwoods, etc will work, but they need a lot of babysitting. Most CEs are in between.
Color matters. Try using light colored punkwoods, plant fluffs/downs, cotton balls, white paper, etc. Then smear them with charcoal, a sharpie (black magic marker), etc. Darker is better. Even with pieces from the same object, when darkened we can see the difference in time to ignition.
Solid is better. Loose crumbles, plant fluffs, grasses, etc can be tough to work with. If you can't get a large enough piece to work with, go the opposite way and crush it. Fine powders work almost as well as solids. It's the stuff with lots of edges and gaps that give us fits.
Another tip is to pre-char an area before going for an ember.
Just as with micro-charring NUTs with F&S, or filling the notch with Friction Fire, the tiny embers with Solar Fire have a tough time growing in the less optimal tinders. We can make the tinder better on the fly by charring an area with our lens. Usually a dime to nickel sized area suffices. It will depend on the tinder, lens and our skill.
On bright summer days with large lenses and better tinders we can go straight to flame. There are vids out there of guys taking 2x4s straight to flame with large fresnels! However most of the time we will be going for an ember when working with Solar Ignition. We can make embers any time of year with Solar Ignition, but we do need strong sunlight. It needs to be more than half the value of full sunlight at noon to work well. So get your materials ready and seize the moment when the clouds pass!
As with all of the primitive skills, they seem difficult at first. As we gain knowledge and skill we figure out the little things that make it work or not work. From then on it seems easy to accomplish our tasks. So stick with it!! One day you'll look back and realize that you are an old pro and flame with Solar Ignition is as easy as with matches for you.
Happy Trails Y'all.
