Holy Hot Spot Batman, 824kcd! (Formerly, 100mm Aspheric Lens at Fasttech)

Hmmm.... I don't know optics so I may be totally off base with this but:

That low profile lens looks like it may have a long focal length. Long distance from the emitter would mean more light lost off to the side and less light focused out through the lens?

DrJones? Anyone?

LOL. Love it Garry. I like that better than the most common last heard words from a redneck, "Hey y'all, watch this!".

I'm hoping, Stockton-Rob. I need to start playing with upside down reflectors to see if I can approximate a Waven Collar. There will be some room to play around with this setup. Something tells me the lens may be a let down given it's cost though.

That makes sense Keltex78. If I understand what Djoss told me once, longer focal length means tighter hot spot. Hopefully, the bigger diameter makes up some for the longer focal length. Let's just hope it's not too long.

Yes, throw with an aspheric is trade-off: if the diameter is the same the thicker lens will catch more light and puts it in a bigger hotspot, the thinner lens will not catch a lot of light (the rest is lost), but puts that little bit into a smaller hotspot. So throw will not be too far off in the two cases, but I prefer the efficiency of a thicker lens with large hotspot.

Thank you Djozz (Sorry for misspelling your screen name above). This lens appears to have an convex curve (Not sure this is the right word, but the glass curves away from emitter going towards the center) on the inside face too. I know it would just be an educated guess without knowing the specifics, but do you have any thoughts on how that may impact the beam?

I do not see that, in the FT pictures the underside looks flat to me.

One could experiment with two of these directly on top of each other. You will loose some light from the extra reflections off the extra surfaces, but it will more or less act like one thicker lens :-)

I just ordered this a few hours ago. Going to resurrect an old old build. It's actually a perfect fit.

I don't think sandwiching two is the best idea, the amount you lose at the interface between the two lenses can be exemplified by cracked glass.

Yeah, this is how I understand it, assuming everything else is the same,
100mm lens with low convex profile - long focal length, project smaller spot (lesser light captured)
100mm lens with higher convex profile - shorter focal length, project larger spot (more light captured)

But they both have the same light intensity in the spot.

You may use dual-lens system to ‘shorten’ the the focal length of the 100mm lens - resulting a larger projected spot with still the same intensity.

(I might not use the correct technical terms here.)

Maybe focal length is 140mm same as I got 2 from DX.

http://dx.com/p/99mm-replacement-glass-lens-for-flashlights-43043

I ordered one of their smaller lenses but don’t have it yet.

It adds a couple extra reflective surfaces but back to back, the focal point should be closer and capture more light. Bridging that thin gap with optically clear silicone could eliminate or at least drastically reduce those reflections.

Very clean. No I haven’t tried it yet. I thank you for showing us this lens. It might come in handy if I need some lens-changing in the future.

I have no doubts that that lens would make one hell of a thrower. That’s obvious because its close to frisbee sized.

LOL VFM.

Way to go Ramblings. Now lets see what we can do with these bad boys (Well, I hope they kick ass).

POK wrote:

Maybe focal length is 140mm same as I got 2 from DX.

http://dx.com/p/99mm-replacement-glass-lens-for-flashlights-43043

Interesting. Says 80mm in the link, but I'm not surprised it is wrong. Are they actually 24mm thick as advertized? If so, they it would seem the lens in the OP would have an even longer FL since it is only 19mm thick. How are the lenses working for you?

“How are the lenses working for you?”

badddddd

Sorry to hear that. Thanks for reporting. I take it it doesn't focus clearly?

I'm starting to like DX, but it seems their line of aspheric lenses is quite sub-par. Lets hope this FT lens is a good one.

Deal Alert. OP updated

Yowza! 790k from XML2, especially 400k from MTG2 is quite… insane! Especially one you can build at lower budget.

What’s the approximate focal length of this lens?