I have started fires in very marginal conditions by use of a large Fresnel lens, the smaller credit card sized ones only work reliably in ideal conditions IME.
As has already been posted, a fire is most needed in conditions when a lens is useless. Still worth carrying though IMHO, each fire started thus is one more match left for less favourable conditions.
A large Fresnel lens, say A4 sized will start fires in unfavourable conditions and is worth stocking in a long term TEOTWAWKI kit, but is unlikely to worth carry in a backpack.
In good sunlight an A4 Fresnel lens will light a fire of large sticks without any need to procure tinder.
Very true. I do have some A4 Frensel lenses around but I don't carry them as part of my survival gear. As you say, probably good as long term preparedness items but to me a little bulk and impractical to carry vs the likelihood of use. When I most need a fire there often isn't any sun to begin with.
I will say though that fire making is one of the areas where I have the most gear on me. When I'm out on hikes and camping trips I will generally have at least two firesteels, a minimum of two lighters (one peanut-style and one Bic, and I'll note that I don't smoke), and UCO Stormproof matches. Very often I'll also have a Spark-lite and I always have a variety of tinder/fuel items (including Hexamine tablets, fatwood and Tinder-Quik tabs).
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