I’ve tried a number of small fresnel lenses in lights and have compared the results to same-size aspheric.
The results:
- Cheap ($8) 20mm optical acrylic aspheric fresnel lens - blurry image of emitter. Approximately half the throw of an aspheric.
- More expensive optical acrylic aspheric fresnel lens with finer lines between more numerous fresnel elements ($30) from Edmund Optics - much sharper image of the emitter, though still not as sharp as an aspheric. Approximately 3/4 of the throw of an aspheric.
- Optical acrylic Aspheric lens - cheapest of all, but also gives by far the best throw.
Net result is fresnels throw worse than a comparable aspheric. I think the problem is that at the transition of each peak and valley on the fresnel some light is wasted and not focused. This loss is much greater than the amount absorbed by the extra thickness of the aspheric.
If you want max throw in a flashlight aspheric is the way to go. Fresnels are more suited for when you want the absolute thinnest and lightest possible light. I agree with previous posters that fresnels make more sense in large lens flashlights where the weight and cost of an aspheric may become prohibitive.