A few years ago… I was trying out various lenses on lights to see what the results were.
I live close to Surplus Shed; and if you want to grab lenses off the shelves and try them on lights while you’re there, the owner doesn’t care.
Of course if you buy a few after your experiments, they appreciate that.
If you buy some, give good feedback on forums that might entice others to purchase a few lenses… they like that even more.
I mentioned this on another flashlight forum; and posted the results of some of the lens/light combos. I included the stock numbers of the lenses if there was anything anyone liked and wanted to try themselves.
I got lectured that you can’t just grab lenses and stick them in lights, you have to do all the math first.
One member was (politely) being outright insulting.
Anyway… here’s part of the “lecture thread” I had saved as a Word file (as it has some useful information):
Re: Aspheric lenses - 52mm/37mm fl - 52mm/33mm fl - 50mm/35mm fl - 50mm/38.5mm fl ~ B
Melles Griot Asperic Lenses.
52mm with 37mm focal length is here.
This is why I wanted to know the distance from the emitter to the back of the aspheric lens:
See, Fb is 21.9mm - so the pest possible placement of the LED from the back of the aspheric lens would be 21.9mm. MrMimzu found the distance to be 27mm, so moving the emitter slightly closer to the back of the lens should provide more throw.
The 52mm w/ 33mm focal length, the Fb is only 16mm - which would be bad.
Conversely, the 50mm w/38.5mm focal length, the Fb is 24.3mm - so if the emitter placement is indeed 27mm, then the 50mm w/38.5mm focal length should throw better.
The moral here is for maximum throw, the LED should be Fb distance from the back of the asperic lens for best throw. Having the emitter at the exact focal point of the lens should focus the most rays straight ahead - of course the focal point is measured from the middle of the lens, so we have to look at the Fb number.
Optimal distances from back of asperic lens to emitter:
52mm w/33mm fl = ~16mm
52mm w/37mm fl = ~22mm
50mm w/35mm fl = ~23mm
50mm w/38.5mm fl = ~24mm
So, considering the Fb for the 52/37 and the 50/35 are only 1mm apart - I would be surprised if they threw that much different.
You can get a magnesium fluoride AR coating on the 50mm lenses for $20 and on the 52mm lenses for $40.