What are the advantages to longer FL fresnels? The ability to increase the triplet's field of view, is that it? Is that the only reason? Or is there something else I am missing? Currently I can focus cleary an image plane of 15" x11" with my optics at an FL of roughly 450mm. But that's only one triplet, I plan on having several fixed triplets.
Maybe I don't need that 650 fresnel? Can someone help me here?
Full Version: Longer FL Fresnels
The longer FL fresnel is to collect the light into a larger projection lens at a further distance. The Pro lens has a bigger diameter and a longer FL. The longer FL is to give us a longer throw.
Does that make any sense?
Does that make any sense?
QUOTE (Syscrush @ Dec 14 2005, 07:03 PM) 
The longer FL fresnel is to collect the light into a larger projection lens at a further distance. The Pro lens has a bigger diameter and a longer FL. The longer FL is to give us a longer throw.
Does that make any sense?
Does that make any sense?
Yeah, but 650mm. Is that the FL of the Pro Lens? Sorry, I should just look into that myself.
The FL of the fresnel has to be longer than the FL of the projection lens, because we don't want the light funneled down to a single point on the surface of the lens, we want it to be a cone shape that basically covers the lens in light.
If you make the projection lens larger in diameter, then the cone should be bigger where it strikes the lens, which means a longer FL for the collector fresnel. Similarly, making the projection lens have a longer FL means it has to be further from the panel, which also means that the cone of light has to be "stretched out" to correctly fill the triplet.
Does that help at all?
If you make the projection lens larger in diameter, then the cone should be bigger where it strikes the lens, which means a longer FL for the collector fresnel. Similarly, making the projection lens have a longer FL means it has to be further from the panel, which also means that the cone of light has to be "stretched out" to correctly fill the triplet.
Does that help at all?
This explanation might also help...(anyone chime in to correct me if I'm wrong):
I believe that a shorter FL fresnel would need to be placed closer to the triplet lens and farther away from the LCD by an amount equal to the difference in focal lengths. This will magnify the image of the LCD that the triplet "sees". It might become much larger than the field of view of the triplet.
You can experience this yourself by holding a fresnel above an object, with the grooves facing the object like it would an LCD. As you move the fresnel farther away from the object, the image grows in size as it gets magnified. It also becomes fuzzy and you can no longer focus very well through the fresnel - because fresnels are really not the best magnifying lenses.
But they are great for collecting light this way...
I believe that a shorter FL fresnel would need to be placed closer to the triplet lens and farther away from the LCD by an amount equal to the difference in focal lengths. This will magnify the image of the LCD that the triplet "sees". It might become much larger than the field of view of the triplet.
You can experience this yourself by holding a fresnel above an object, with the grooves facing the object like it would an LCD. As you move the fresnel farther away from the object, the image grows in size as it gets magnified. It also becomes fuzzy and you can no longer focus very well through the fresnel - because fresnels are really not the best magnifying lenses.
But they are great for collecting light this way...
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