Hiya, this is my second attempt at building a PJ. This time I hae followed the plans to the letter (almost) but still I found that when I did a piliminary test I found that my lens needed to be even further from the LCD to focus.

I have used the LL lens kit and the front of my PJ was 228 cm away from my wall.



I thought the lens had a focal length of 320mm ish. So I had hoped that my 360mm should be bags. Any idea how I should get round it without making another box?

I have both fresnel lenses close to each other in the first panel you see. (in case you wondered :0))

You really need a focussing mechanism with roughly 2" of travel. There are a variety of methods in the plogs and reference section.
This is about the only way to focus short of moving the PJ forward and backward until focussed.

Bigger the image the closer the lens needs to be to the LCD.

Yeah, thats the thing though. My lens is 360MM away from the LCD panel at the moment. I havent screwed the front panel its just loose at the mo.

To focus I need to hold the lens another 3 - 5cm further forward so my box isnt long enough. :0(. Im just wondering if its due to anything as its much further away than the focal length of the lens.

the smaller the projection diagonal, the further the lens has to be from the lcd, the standard plans are justt hat..standard, you have to vary it according to the size of the projerction as I found out when I tried to project a 60" diagonal and had to make my box longer, I thin it has to be 390, almost 400 mm away from the lcd for mine.... look at the focal calculator in the build and calibration tools forum....
ps: looks like it needs to be 372mm at least for a 228cm throw

Hi, I am a little confused with focusing and picture size. I have used the FocalCalc II to get my distances correct. My picture size cannot exceed 83 inches, and my distance from wall to projector lens is 13 feet. What is the Triplet FL size of 640mm?

Does this mean I will have to increase the distance between the LCD and the Triplet lens to 640mm. See attached pic for details.

Thanks.

Click to view attachment

FocalCalc is telling you that to have an 83 inch image with the projector 13 feet from the screen you will need a triplet with a focal length of 640. If you have to have an 83 inch image then what you want to do in FocalCalc is click the radio button next to screen distance, then the box for triplet focal length will turn white.

Enter your triplet focal length in the box and FocalCalc will tell you how close your screen has to be to keep an 83 inch image.

David.

what would happen if he used a 650mm front fresnel and moved the standard triplet to 660mm? could the triplet still focus on the lcd? as long as the triplet and the object its focusing on are at least a little further than the triplet focal point wouldn't it still be able to focus on it? would the projection be smaller because the triplet FOV would be so large at that distance?(meaning that at that distance the triplet FOV would be maybe 3-4 times the size of the lcd)

To tell you the truth, I do not know if that would work but I think it will not. Here is what I gather from what I have read.

He would still need a 640mm FL triplet to throw an 83 inch image from 13 feet with a 15 inch panel. From what I understand the fresnel has some effect but most of the throw is determined by the focal length of the triplet.

Actually if he did find a 640mm triplet he would also need a 650mm or more focal length for the field fresnel.

What I have read in other posts is that the triplet focuses on the panel, not the fresnel. The fresnel is there to bend light toward the center of the triplet to gather as much light as possible into the triplet.

David.

If you truly want a 83" image at 13 feet.
Using Standard LL optics 320mm triplett and fresnels.

Using a 7" LCD (Lilliput) you would get 85" diagnal.

So what we are all saying here is that the triplet and focal length are 2 totally different things? My intention was, in order to keep my 83" picture with a distance of 13 feet, I could build a folded projector tall enough to have a longer focal length.

From what I am reading here it is not the overall focal length that is the problem, but the triplet length. Right?

Or, I could reduce my LCD screen size to 7" as suggested by SPK.

This is not as easy as I though it would be.

yeah, thats why I thought using a 650 mm fresnel to gather the light, and then put the standard triplet at the brightest point..... I know the triplet will focuse on the lcd panel when it is 400mm away(about a 4' throw.... I think it might be worth a try, if someone had a 650mm fresnel to test out, I don't know how light would be in the projection though...seems to me, as long as the triplet is further from the lcd than its FL it should be able to focus on the lcd.... and the lcd will be a smaller portion of its FOV so will projection be smaller also...normally this would be a very dim picture since the light from the bulb is "gathered" by the fresnel at 320mm .... but its just a guess.... I reckon I can see if the triplet can focus at 600 mm ...hmm...plus I could measure it...just a dim picture since the light from the fresnel will be diverging pretty bad by then since I only have 320mm fresnel


on second(and third and fourth) thought, I'm pretty sure I'm wrong... but I'm not sure why....

or you can use powerstrip to reduce the size of your active screen area on your lcd... I use a 1280x1024 17" and since my fresnels aren't that big I have it set at a 1024x768 window within the 1280x1024...... but you lose resolution..... it would end up being a little less than 800x600 I think

The triplett is the driving factor on image size and projection distance. The fresnels are what gather and distribute the light. That is why it is important to match the field fresnel with the triplett focal length.


13' is a long distance to project a small image with the standard optics. A pro set will help but the image is still going to be larger unless you reduce the LCD panel size.