# A Fresnel lens keeps the angles and throws away the glass > A Fresnel lens is a heavy convex lens sliced into rings and pressed flat. The bending angle at each ring survives; the glass between them doesn't, which is why it weighs a fraction of what it should. August 19, 2026 · https://justog.club/stories/fresnel-lens-rings Concentric rings, dead centre, each one a hard step rather than a smooth curve. Cream on charcoal, spacing even from the middle out to the rim. Every one of those steps is where a piece of glass used to be. ## What is a Fresnel lens actually doing? Bending light the way a thick convex lens would, using a fraction of the glass. Take an ordinary convex lens — thick in the middle, curved all the way through — and notice that what actually bends the light is the angle of the surface at each point, not the glass sitting behind it. Slice that lens into rings, keep each ring's own surface angle, and flatten the whole thing so every ring sits at the same depth. The bending survives. Most of the weight doesn't. ## Why do the rings look stepped instead of smooth? Because each ring is a separate small lens angled to do one job, not one continuous surface trying to do all of it at once. A smooth lens changes its angle gradually, point by point, across its whole face. A Fresnel lens gives up on that continuity on purpose — the second ring doesn't care what the first is doing, it only has to bend the light passing through its own narrow band correctly. The step between them is the seam where one small lens ends and the next begins, visible precisely because nothing was done to hide it. ## Why does a lantern need a lens this size at all? Because a bare lamp throws light in every direction, and a stage needs it going one way. Without a lens in front of it, a lamp lights the inside of its own housing about as well as it lights the stage. The lens turns a point radiating outward into a controlled, adjustable beam — wider or narrower depending on how far the lens sits from the lamp — and that's a different mechanism entirely from [an iris closing down to a narrower aperture](/stories/followspot-iris-circle), which shapes a beam that's already been formed. ## Who invented this, and can that name go on a shirt? The stepped-ring principle is two centuries old and belongs to nobody currently trading. It's named for Augustin-Jean Fresnel, who built one for a lighthouse in the 1820s to get more light further out to sea from less glass — a lighthouse and a stage lantern solve nearly the same problem, throwing a controlled beam a long way on a limited budget of weight and heat. What a specific lantern maker can claim today is their own housing, their own model name, their own badge — not the optics, which have been public since before the light bulb existed. ## Does the pattern only make sense as a lens, or does it work as a shape on its own? It reads as a shape a long time before anyone reads it as optics, which is most of the appeal. Rings held perfectly even, the way they are here, read as a target or a ripple before anybody clocks that they're looking at a technical drawing of a real object — a different kind of reading entirely from [a gobo's cut pattern, which is the negative space itself rather than the lens](/stories/gobo-pattern-disc). Somebody who's never rigged a lantern in their life can still want this on a shirt for the geometry alone. ## Does haze change how the beam looks once it's through the lens? That's a different part of the same rig, and it's worth keeping separate. The lens decides how the beam is shaped the moment it leaves the lantern. What happens to that beam afterwards — whether it stays invisible in clean air or becomes [a set of hard-edged wedges crossing the stage](/stories/haze-beam-in-air) — depends on the air it's travelling through, not on the glass it came out of. ## Does it print well? Yes, and the steps are what makes it print well rather than what makes it hard. Hard-edged concentric rings are exactly the kind of flat, high-contrast shape that survives fabric without gradients going grey and muddy in the weave. Cream on charcoal keeps it to one ink beyond the ground colour, and the rings should stay genuinely stepped rather than softened into a smooth gradient, which would defeat the point of the drawing. ## How do I get one made? Describe your own lens at [JustOG](/) — how many rings, how tight the spacing, and which two colours. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped. Designs other people have published are in the [shop](/shop). Concentric rings, and every step between them used to be glass.