That does look like a great flashlight, except turbo is only 500 lumens and a modded driver does not exist.
Search for “mc10”
That is not variable, the dial has 4 positions…
4 modes, Strobe-Low-Mid-High
Wasn’t there a groupbuy for an imalent with this function?
Why can’t you just use a potentiometer?
It’s hard to make that waterproof.
Imalent is like $150 and a review I saw showed the light levels are hard to choose on the low quality touchscreen. It’s too small of a screen to have such a low responsiveness.
There are some “dive” lights with a linear magnetic slider to regulate the brightness and some others which have a simple reed contact which you need to slide back and forth to adjust some different modes.
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I wanted forever one of the small 18350 lights with a magnetic slider but it seems that there are no new revisions made since quite a while.
I guess that it could be done mechanicly fairly cheap from a good factory like the convoy with good tolerances but then you have to make a new driver and it seems that this is often the point in which the cheap companies invest to less money…
it was imalent dm21. i don’t remember if it was freeme or madmax that had a coupon for it a couple of months ago. but now it’s $55 http://www.banggood.com/IMALENT-DM21-CREE-XM-L2U4-USB-Charger-Waterproof-18650-LED-Flashlight-p-994338.html
The Imalent DM21 is very interesting. I don’t know what I think of the presence sensor in back, it seems it would get shut off or turned on by accident.
If you can’t find perfect light, build one 
The 2 AA Explorer E83 is on Cnqualitygoods for $30ish
has 4 illumination levels
Kaidomian has the single AA Explorer E84 with 5 Illumination levels…l
Here’s a link to a Review of the single 18650 Rocher MC10 by FreeMe,
and a linked site that carries it.
( also with detents, not infinite variability)
BTW- my Niteye Eye10 is a favorite EDC.
IT has 11 (?) detents for great illumination selection, and great output with a 18350
It’s not so much the number of levels, it’s the way you use it. With a ‘volume knob’ you can move up until you meet the point where your eyes are currently adjusted. With levels, you end up going past one too many I notice. Mine have 5 levels now.
There are a few "ramping" drivers about that would be close to what you want. But of course, instead of turning a dial, you hold down a button.
Or there was this https://www.indiegogo.com/projects/function-flashlight-triple-emitter-1600-lumen#/
That is the whole point of the question in the OP: using a knob is sooo much faster, accurate and convenient than holding a button (and if you go past the correct position, start a different sequence to go back 
)
Btw, I do not see great difficulty in waterproofing a knob that goes inside a flashlight to a pot.
+1 
Yes, the experience would be soo much better. Imagine if you had to start and stop your car engine at just the right time to set the radio’s volume. You would be OK with it if everyone had the same experience, and if it had always been that way. But if your new car suddenly had a knob, you would never go back.
On a similar note, didn’t car manufacturers put in radios with all buttons on the radio in the 90’s, only to bring back the knob for volume in later years?
Joshk, perhaps you can start chasing Cryos Illumination to revive this project. 
I supported the Indiegogo campaign but unfortunately they never went forward with it.
Detailed thread here in the "other place" http://www.candlepowerforums.com/vb/showthread.php?360797-Cryos-Illumination-s-Triple-Function-Light
YouTube clip here:
