@islisis interesting, I didn’t thought before how to prove theoretically/mathematically but it sound like you are in the right track at first look. unfortunately i forgot to make partial differentials, because are more that 15 years that don’t exercise on math. If i find some time will open my books and will try to do it myself just for fun if i can. pretty sure some math software will make it very easy.
In practice there is like 0 difference or like ~2% from what I measure, but that can also be part off human mistakes or tolerances in light meter or other variables.
pretty much my sec mod on UF-T20 is done.
from around ~100K cd with dome on de-domed now is ~180K cd
I would love to manage to put some kind of small light collector inside but with tools I have is almost mission impossible. still I am looking around for some random object that can be suitable for the task just some small gain like ~30% will be perfect
about dedoming I almost never manage to double lux, but max ~80% is this normal?
waiting night for some beam shots.
meantime i make yesterday some photos:
ok I give up for today trying to learn mouse over thing (no more free time at my disposal for today)
i tried to rely on intuition arguments and didn’t research the problem at all but i came across a random thread with a link to this explanation: “as the focal length is reduced, the beam divergence increases at a greater rate than the total beam power”. that might be easier to understand…
I got my T20, in spite of the pictures, partly because it was well liked on another thread here and partly because I thought I might be able to use the lens in my Yezl T-9. The build quality really is good, perhaps the only better build I have is a C8. I like the brass pill, trapezoidal anodized threads, etc. But it is pug ugly and feels just as bad in the hand, so I don’t want to use it as a regular white light.
Instead, I took its lens and put it into the Yezl, put the DX lens that didn’t quite fit in the Yezl in the T20 and put my deep red led into the T20. The pill cutouts for driver and star were nearly right and it is functionally excellent. It works well in that role.
just some small gain like ~30% will be perfect 
