Well….i havent been so upset about a flashlight lately, i waited for it for like 3 months, and finaly it arrived !
Great body, good finnish, very solid and easy to mod construction, a blass pill, at first i was thinking finaly, the new t20! A great body for my IRs and XPG2s!
And volia, with a good old XPG2 at 4.5 amps it gave only 90k candelas! I checked, doublechecked everything but it seems that those lesnes can give only 90k…a shame realy!
And the beam profile is the best i zoomie profile i have ever seen!
Will post detailed body photoes when i have the time
Functionally these flat fresnel lenses function very similar to an aspheric lens. The main differences:
Weight - a flat fresnel lens is a thin piece of plastic. It can weigh a lot less than a comparable diameter aspheric especially if a short focal length is used. In an aspheric lens as the focal length gets shorter, the lens gets thicker. Not so with an aspheric. unfortunately, my understanding is the HT08 uses a fairly long focal length fresnel. Shorter focal lengths increase the width of the hotspot in spot mode, but do not increase throw.
Length - because a fresnel lens is flat, you can make your light much more compact than if you instead used a protruding aspheric lens.
Better Flood mode - maximum width of the flood mode is usually governed by the edges of the bezel needed to protect the lens. Shorter focal length aspherics tend to have narrower floods because the lens is thicker. With a flat fresnel lens very little bezel needs to protrude to protect the lens. As a result you can get a wider flood mode than with any aspheric lens.
Worse Throw - the biggest downside of a fresnel is throw. Some light is lost at the edge of each ridge. The result is the throw from a flat fresnel will never be as good as a from the same width aspheric lens. Throw is further reduced by the need to have a second glass lens on top of the aspheric lens to protect it. This reduction in throw can be substantial. A cheap fresnel lens might only have 50% of the throw of the same diameter aspheric. Even an expensive frensel lens might only have 70% of the throw. Even a sub $1 aspheric lens will out-throw a $30 fresnel lens.
I think he just meant a flat glass lens to keep the fresnel from getting dirty or scratched. Dirt and stuff can get stuck in the ridges of the lens and be hard to clean, if left unprotected.
Edit: I just saw your comment in the other thread regarding negative focal length fresnel lens. I did not know about those.
My findings are similar to Firelight2’s regarding throw using fresnel lenses. Their effectiveness can vary a lot depending on the lens’s quality and “ridge density” also.
Also, it might be worth mentioning that the fresnel lens can be put in backwards. If the side with the ridges is towards the LED it will produce an image but the beam will be very dim. The ridges should be on the outside, away from the LED.
I assume a negative focal length aspheric is one where you have the ridges on the inside facing the LED instead of on the outside where they can get damaged.
Probably less available.
When I did my modding a few years ago I ordered fresnels from Edmund Optics. I don’t recall seeing any negative focal length aspherics for sale.