Ok I understand. So I'll get better result with the fresnel on top of my LCD, and it will still focus, and the picture will be good?!
Property, here is a bastardized version of your picture. I hope this explains what I did not quite understand until yesterday. The important thing to understand is this. The fresnel lens has nothing to do with the objetive lens focusing on the LCD. The fresnel lens just directs the lightpath. If the objective lens is too far away from the fresnel's focal point, the image will be to big and not fit through the objective lens. This is why a top fresnel with a shorter focal length will allow you to move the objective closer to the LCD (so it is in focus) and the lightpath will still fit through the objective lens because it is closer to the fresnel's focal point.
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Well, I'm about to do a dremel job to my fresnels. I re-did my plexi so it's not blocking the air under the lcd ( I used a frame that come with the 1545v ). I did some test and something come to my mind. with split fresnels, I think lcds will less heat, why? Well 'cause the 2nd fresnel beam the light to a focal point, huh.. Everybody know that, but the light go to the middle of the lcd with that, so where does my lcd heats? In the middle, the side stay gently warm. So I'll check this out and give you more on this, could be a solution ( with fans ) to heat problem.
gui_999 said:
hey gui_999 when you cut the fresnel dont try to cut between them you have to cut the clear area off thats the glue good luck
and be sure you want to do this because when you cut them they will no longer fit in your 305sd they will be to small to reach those plastic holders you will have to find a nother way to mount it
cruser said:
Ok, that whole concept really confuses me. The bottom lens' focal length obviously doesn't change when you split them. Why would it need to be further away from the lamp now?
I understand this is the case, but I don't understand why.
Lifter said:
i dont know for sure why but i think its the condensor lens that is the isue here if you rember before you cut the top fo your ohp some of the fresnel was coverd because i think the condensor didnt spread the light enough hence if you move the condensor down and the fresnel up the beam will cover more of the fresnel
i know these fresnels have a EFL but there is defanetly some adjustment allowed because your light and objective lens both move
cruser said:
Well hehe, I didn't see your post before I began cutting it, and I'm cutting between the 2 fresnels
But it's going ok, 3/4 of the fresnel is cut, I'm giving my dremel and the cutting disc a break lol.
Finally, I've cut the corners to be able to split my fresnel, but the sticker "this side to lamp" is gone, and I don't know witch one is the one that goes toward lamp. How can I test them!?
I believe either side will work about the same but one will give you more aberration than the other. I believe the grooved side towards the light will be the one with less aberration, from what i've read.
Oh sorry, I thought you meant which side of just one of the fresnels.....Well, you could clamp them back together then figure out which one has which focal length by putting the light on one side and a piece of paper on the other and measuring when the light is focused to a point on the paper.
(If your light bulb is closer to 12" from where Fresnel usually sits then the 12" one should face the light, otherwise the 9" one)
(If your light bulb is closer to 12" from where Fresnel usually sits then the 12" one should face the light, otherwise the 9" one)
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