Lens FAQ

Status
Not open for further replies.
Jump to Latest
L

Lifter

Member
Joined 2002
#1
It think this thread is needed because I can't find any answers to simple specific questions.

- I've read a few comments saying that fresnels are not good for high res panels. Why not? What is the alternative?

- If you don't use a fresnel, does that mean you need a projection lens that is the same size as your panel?

- Why are fresnels from OHP's better than page magnifiers?

- What's the deal with CRT lenses. I've read that they don't work well w/ LCDs- yet people still use them. What gives?

- How many and what type of lesnes are used for various setups other than OHP systems?
 
#2
The reason you do not want to use fresnels as a projection lens is becuse they simply were not made for that reason. I first fresnel lens I bought was at wholesale from www.3dlens.com, and the quality of the lens was pretty poor. First of all, the lens was warped in a bowl shape. So the picture was blurry on the endges. The lens was bent from shipping becuse it was very thin plastic. All in all, fresnels should only be used for magnifing light sources, or if you have trouble reading the news paper. AND DO NOT EVER BUY A FRESNEL OFF OF EBAY unless you want to be charged 10 bucks for something you can get from www.3dlens.com for $1.50 or less. The reason OHP fresnels are sought after, is becuse they are made to magnify the light source so it is even on the item being magnified. They are also attached to glass, so you can be asured that they are very storng, good quality, and not warped. They are also bigger than 7X10. Sometimes lcds are bigger that that, and as a rule of thumb, the fresnel used to magnify light must be bigger than the lcd.

The projector lens does not have to be as big as your panel. This that was the case, there would be gigantic lenses out there. You simply place the lcd further back from the lens. There is a calculator someone made in exel that will tell your exactally where to place everything, and what size picture you will get. If you have exel, sent me an email and i'll send you the file. In theory, you can use just about any lens you want... to a certain extent.

The reason people have has trouble with crt lenses, is becuse they are made for a crt, and most crts have round screens, so the lens compensates for this... When you use a crt lens with a flat lcd... guess what... the edges are rounded. Now not all crt lenses will do this. Uvdee solved this problem by placing a frame around the crt that blocked the part that was not in foucus. I belive that he used a delta 64 lens. I do not think you will have that problem with the delta IV. I have one coming in on the 2nd, so i'll let you know.

I know the 4-1/2" DIA. FUJINON TV PROJ. LENS at the following link was made for a flat crt, so you probably would not have much trouble with that. Its only like 15 bucks too!


http://www.surplusshed.com/pages/Lenses.html

Good luck!
 
#3
Thanks for the great info. That helps a lot. There's just one thing I'm unclear about. I completely understand that high quality fresnels (like the ones from OHPs) are superior to page magnifiers, but is there a disadvantage to even the best fresnels? I read a post and someone said that using a fresnel for a high res screen is not a good idea. Why not? Do fresnel lenses themselves degrade the sharpness of a picture? Do they create a screen door effect? What is the alternative, a condenser lens? That's what real LCD projectors use isn't it? That's pretty much the only thing I'm unclear about as far as my planned project is concerned.
 
#4
Well, when I say people use fresnels, i do not mean to project the image. You place the fresnel between the light source and the lcd for create an even distibution of light. I am no expert, but from my experience, I have had no luck using a fresnel to project a clear image. I have had good enough quality to maby watch a poorly done cartoon like southpark (which is still a little blurry). Other people may have had better luck, but I do not recomend it for a first timer.
 
#6
I did mean using a fresnel to create an even distribution of light. It's good to hear that there's nothing wrong with doing that. However, I didn't realize that the screen door effect was due to having the projection lens far away from the screen. That makes perfect sense. Is there a way around this (aside from using a small panel)? Could there be a way to use a 3rd lens to shrink the image, thus allowing you to put the projection lens much closer to a larger panel?

I'm sure this has been discussed in the 2 monster threads, but I can't even open the printable version because my browser times out before it can download it all.
 
#7
Well for one, if your screen has a very high resolution, you can buy a line doubler for about 80 bucks and you generally get the SD effect if your projected image it too big. So just make it smaller. There is probably a lot more about the SD effect in this forum from people who know alot more than I, so do a search for it If you have not already.

How about a little experiment. If you look into a tv from far away, you see a solid image... no SD effect. But, if you stare into the tv from up close, what do you see? Each pixel in rows... looks like a screen door.
 
#8
Of course. But a lot of that is because of interlacing and the fact that TV's are low res. You don't see that effect nearly as bad on a computer monitor or HDTV. I'm just concerned about the pixels being spread apart because the lens is far away from the screen (like you told me about). I guess I'm just asking if there's a way to get the projection lens close to a large panel via extra lenses.
 
Status
Not open for further replies.
Top Bottom