I am not a huge fan of zooming aspherics, well, ok, I hate them: in flood modus the beam is too even (I like a beam more intense in the middle, that makes sense because that is where you want to look), in throw modus they are plainly ugly and they throw away most of the light inside the body. In contrast: reflector lights have nice smooth beams with more intensity in the middle where you want it, and a spill with weaker light for an idea what else is going on (and most of the light that is produced actually does make it out of the flashlight!).
Unfortunately I own two aspheric lens zoomable flashlights. I bought them to find out what the fuss was about them. It is a 18650 xml brandless zoomie (similar to the later marketed sipik 98) and recently a xre-zooming headlight. The latter light was throwing well, but was so unusefull (even modding appeared impossible, but that is another story) with far too much throw even in flood modus, that I came up with the idea that has improved the head lamp, and is explored further in this post.
This is the light I took for the experiment; it had been (rightly so) sitting under a pile of flashlight hobby junk for months till now:
(My advice:just don't buy xml flood-to-throw lights (even not the beautifull z8), the spot brightness just does not increase considerably when zoomed in, but you do get that hideous led projection.)
First thought was to make some diffuser to help improving the beam.
But then, what the heck, I'm not married to this horrible light, why not diffuse the lens itself? And there I went. Best would be to diffuse the inside of the lens (with sanding paper), because by doing that the outer curve of the lens still does its focussing, giving more intensity towards the center of the beam. And next step was sanding the lens again with finer sand paper and later polishing with ordinary copying paper to make it back into a clear(-ish) lens, making beamshots and ceiling bounce luxreadings along the way. Here is some pictures of the abused lens in several stages:
Here are the beamshots for the six chronological stages of the process in flood modus and zoomed in (unfortunately two of the flood beam pictures were accidently deleted, but you get the picture anyway). Flashlight and camera were 1.70 meters from the wall, camera settings were all the same: iso125, f3.5, 1/40sec. The images give a reasonable idea of the beams, although you will never catch the full spectrum of light intensities with a camera.
First zoomed in: 1-before sanding, 2-sanded with 400 sanding paper, 3-sanded finer with 800 sanding paper, 4-polished with plain copying paper, 5-light sanding again with 800 sanding paper, 6-polishing again as well as possible with copying paper.
Now 4 of the six stages zoomed out:
So this went well ! , sanding the lens magically turns disgusting beams into beautiful beams. Personally I like situation 3 and 5 best, some of the zooming ability is still there, and zoomed out there is that beautifull beam, gradully dimmed out towards the edge (the pictures do not justice to how nice the beam was).
And what happened to the total light output? Everything is relative (just like real life), so when the output before the experiment zoomed-out is stated as 100, all other readings are given relative to that:
original output zoomed-in is 55 (ouch), stage2zoomed-out: 78, stage4zoomed-out: 89, stage5zoomed-out: 76, stage6zoomed-out: 84. Then I discovered that the battery wasn't full anymore (later readings should therefore be interpreted a bit higher than measured) so recharged the battery and after that stage6 was back to 100.
The readings zoomed-in were in line with the zoomed-out readings when taken relative to the initial zoomed-in reading (of 55).
The output readings summarised: you do not loose an awfull lot of light when the lens is diffused like this, if done with care you loose only 15 to 20%.
Conclusion of the experiment: if you own any one of those ugly zoom-to-throw flashlights (now I am going to have it in this BLF-world full of Sipik68-devoties ), there is hope, you can turn its beam into something really nice