I pulled a double fresnel out of a OHP. I cut it down to my 7inch lcd side all correctly from the sides in.
I measure the FL of the fresnel joined. The back FL is 125mm while the front is 130mm. How is that possible since OHP objective lenses do not have anywhere near this FL?
How am I suppose to use this double fresnel behing the lcd with a standart objective?
If I separate the fresnels I measure the back FL as 220mm while the top fresnel as about 15 inches. Again, most OHP are not 15 inches so why is it this FL?
If I place both fresnels behind the lcd separated by say 2 cm with the FL of each be 220mm and 15 inches?
Will this work both fresnels behind the lcd? I assume this will produce more even lighting on the lcd as opposed to together even if both fresnels are behind the lcd.
I measure the FL of the fresnel joined. The back FL is 125mm while the front is 130mm. How is that possible since OHP objective lenses do not have anywhere near this FL?
How am I suppose to use this double fresnel behing the lcd with a standart objective?
If I separate the fresnels I measure the back FL as 220mm while the top fresnel as about 15 inches. Again, most OHP are not 15 inches so why is it this FL?
If I place both fresnels behind the lcd separated by say 2 cm with the FL of each be 220mm and 15 inches?
Will this work both fresnels behind the lcd? I assume this will produce more even lighting on the lcd as opposed to together even if both fresnels are behind the lcd.
I just separated the fresnels and placed them both behind the lcd. The first fresnel is at its Focal point of 220mm. The second is about 2cm away and then another 2 cm to the lcd. putting my mug where the projection lens would be things look good. The lcd is lit up evenly. The lamp arc seems to focus the sharpest at 27cm from the lcd.
focal length
Focal length is a fixed property of a lens or a fresnel. It does not change when you put the fresnel next to another fresnel.
What changes is the Effective Focal Length, that is the combination of the two fresnel focal lengths and the spacing.
When you seperate the two fresnels and point them each at the sun to measure their focal length, you are sending parallel rays into the fresnel. The focal length is the distance where those rays converge into a small image of the sun.
When you use the two fresnels together, the condensor fresnel takes a diverging cone of rays, and refracts it to some output rays. If the lamp arc is right at the fresnel's focal length, then you get parallel rays out. (Closer or further away, and you get diverging or converging cones of rays.) Then the field fresnel takes those rays and converges them. If it got parallel rays, then it will focus them at its focal length. If they were not parallel, then the focal point will be closer or further away.
In a non-split design, it all works best if your field fresnel focal length is about 20 mm longer than your LCD-to-lens distance. (For a split design, optimal is 20 mm less than the LCD-to-lens distance.) Then you can put the lamp arc at the condensor fresnel's focal length. But if your field fresnel is not the perfect focal length, then you can adjust the lamp arc to condensor fresnel distance until you get the arc image right to the center of the projection lens.
Focal length is a fixed property of a lens or a fresnel. It does not change when you put the fresnel next to another fresnel.
What changes is the Effective Focal Length, that is the combination of the two fresnel focal lengths and the spacing.
When you seperate the two fresnels and point them each at the sun to measure their focal length, you are sending parallel rays into the fresnel. The focal length is the distance where those rays converge into a small image of the sun.
When you use the two fresnels together, the condensor fresnel takes a diverging cone of rays, and refracts it to some output rays. If the lamp arc is right at the fresnel's focal length, then you get parallel rays out. (Closer or further away, and you get diverging or converging cones of rays.) Then the field fresnel takes those rays and converges them. If it got parallel rays, then it will focus them at its focal length. If they were not parallel, then the focal point will be closer or further away.
In a non-split design, it all works best if your field fresnel focal length is about 20 mm longer than your LCD-to-lens distance. (For a split design, optimal is 20 mm less than the LCD-to-lens distance.) Then you can put the lamp arc at the condensor fresnel's focal length. But if your field fresnel is not the perfect focal length, then you can adjust the lamp arc to condensor fresnel distance until you get the arc image right to the center of the projection lens.
The fresnels are back together as a doublet both behind the lcd separated by about 20mm from lcd as you said. Eveything lines up nicely with my 260-320 varifocal.
Maybe I dont understand you clearly but when joined the back EFL becomes 120mm. Another words testing the EFL of the joined fresnel its 120mm that the arc is visible nicely on my screen
(lcd, objective removed ofcourse)
But I cannot place my doublet fresnel at 120mm from the arc. Its all messed up and I cant even get the arc visible on the other side its just a big blob of light. Why?
Only 220mm back FL works which is the true split FL of the back fresnel.
Maybe I dont understand you clearly but when joined the back EFL becomes 120mm. Another words testing the EFL of the joined fresnel its 120mm that the arc is visible nicely on my screen
(lcd, objective removed ofcourse)
But I cannot place my doublet fresnel at 120mm from the arc. Its all messed up and I cant even get the arc visible on the other side its just a big blob of light. Why?
Only 220mm back FL works which is the true split FL of the back fresnel.
mistake
When you put a point source of light at the focal length of a lens (or the EFL of lens or fresnel combination), then it will refract the diverging cone of light into a parallel beam.
That is what happens when you place the lamp arc 120 mm from the fresnels. You said the arc is visible on your screen, so your arc-to-fresnel distance is slightly more than the EFL.
1/120 = 1/3048 + 1/x
x = 125 mm
This tells us that a distance of 125 mm would focus the arc image at 10 feet from the fresnels.
You don't want to focus the arc image on the screen. It has to be focussed in the center of the projection lens:
..EFL......D1.....D2
1/120 = 1/220 + 1/x so fresnels-to-lens (x) = 264 mm
1/120 = 1/215 + 1/x so fresnels-to-lens (x) = 272 mm
1/120 = 1/210 + 1/x so fresnels-to-lens (x) = 280 mm
and so forth...
When you put a point source of light at the focal length of a lens (or the EFL of lens or fresnel combination), then it will refract the diverging cone of light into a parallel beam.
That is what happens when you place the lamp arc 120 mm from the fresnels. You said the arc is visible on your screen, so your arc-to-fresnel distance is slightly more than the EFL.
1/120 = 1/3048 + 1/x
x = 125 mm
This tells us that a distance of 125 mm would focus the arc image at 10 feet from the fresnels.
You don't want to focus the arc image on the screen. It has to be focussed in the center of the projection lens:
..EFL......D1.....D2
1/120 = 1/220 + 1/x so fresnels-to-lens (x) = 264 mm
1/120 = 1/215 + 1/x so fresnels-to-lens (x) = 272 mm
1/120 = 1/210 + 1/x so fresnels-to-lens (x) = 280 mm
and so forth...
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