A NOTE BEFORE READING:

I'm really hoping that not only will people comment on the idea, but to post their ideas
as well if they have any. Although it might not be a super useful thing for someone who
has a theater room in their house that they own where the projector isn't really going to
move at all for a long time, it would be really nice to not have to redesign if I end up
moving into a new apartment and having to re-adjust or purchase different optics just to
make it work in a new place.

I'm sure that figuring this out would be valuable, not only to people like me, but to the
people who are building smaller and more portable projectors... and also to the future
when we have affordable 2 and 3 inch LCDs with 1024x768 and are building things that
are similar to the semi-portable commercial projectors that are available now.

- end note -

So I'm not exactly a guru of optics or anything, obviously... but I had an idea that might
at least open someone else's mind on how to get it to work, beyond making actual zoom
lenses (which seems like a bigger pain than this idea).

Ok. Our projectors have fresnel lenses that shoot an image of our LCD screens at the
triplet which then magnifies and clarifies to a larger image. Let's say that on a 3" diameter
triplet that the image is slightly less than a square inch when it hits the triplet, then the
triplet does it's magic.

SO, in order to zoom all we have to do is keep that image hitting the triplet at the same
size and be able to move the triplet backward and forward in order to "zoom"... not sure
how much distance would make a difference in size.

Now would it be possible to use a lens to bend the light (much like our first fresnel before
the LCD) to straighten out the image hitting the triplet and keep it the same size no matter
where the triplet is?

Here's the idea as a picture... not sure if the lenses are right, but this is what
I'm talking about. So? Will it work?

Click to view attachment

If it doesn't work, would any simple changes make it work?

*** edit ***
Does the triplet not have the magnifying capability for this to work? Would we have to
use a CRT projection lens or something similar?

With the "right" combination of lenses you "may" get somewhat satisfactory results, but I am a guy with 50 lenses all different Focal Lengths and all of diameter of 5" or larger. Anyways, from just playing around with different combinations I can tell you from experience that you need to have something professionally designed for you're use. Optics is a very complicated field.

So basically it's possible... it's just that the math and mechanics are a pain.

Now, as long as you gave someone who knows optics well all the measurements that
they needed, they should be able to tell you how to set this up and what you would
need in order to accomplish it.

So my next question: Anyone an optic professional with a projector and some lenses
that can test this out, or can give me the measurements necessary?

Lets say not a triplet, but a single or doublet for ease if possible. At this early point in
the projector game, I'm not as concerned with clarity as much as cost and ease of
testing.

the lens are called focal length extenders,, usaully a negative lens, in astronomy they are called 'barlow lens'

however for this application,,, its not as easy as that

zoom lens have no less than five elements, and when zoom function is used several lens do a waltz inside the barrel some go forward some go back, and some go one way then back again

reason is that when you change gaps inside a lens all the colour corrections change to so hence the fancy waltz..

So if you were to throw the color correction out the window, would a zoom lens get
simpler to create?

Now, I'm not trying to recreate a zoom lens assembly, I'm trying to find out if we use
a lens to straighten out and keep the image the same size that it would normally hit
the projection lens indefinitely, what lens would (on the screen side) be able to magnify
that image from any distance (1"-12") without a ton of actual optical (not chromatic)
abberations?

How about a system for moving the triplet and fresnel together (on a split design) for a zoom?

I'll go with door number 1 and affirm your insanity

I'm not sure how moving the fresnel any decent distance away from the LCD
would affect the system. I can test that, though... so at least it gives me
something easy to try. I'd like to not have to move the whole front of the
projector, though, which is why I'm trying to figure out if this would work out
the way I want it to.

Well I figured that the fresnel provides for keystoning which is zooming one side of the image so if you move the whole fresnel you zoom the entire image.

Hey... anything is worth a shot at the moment. Thanks for the idea, even if it doesn't
work out.

***

I'm really hoping that not only will people comment on the idea, but to post their ideas
as well if they have any. Although it might not be a super useful thing for someone who
has a theater room in their house that they own where the projector isn't really going to
move at all for a long time, it would be really nice to not have to redesign if I end up
moving into a new apartment and having to re-adjust or purchase different optics just to
make it work in a new place.

I'm sure that figuring this out would be valuable, not only to people like me, but to the
people who are building smaller and more portable projectors... and also to the future
when we have affordable 2 and 3 inch LCDs with 1024x768 and are building things that
are similar to the semi-portable commercial projectors that are available now.

[I just added this to the first post since I didn't make this clear in the beginning... ]

This is not an easy task. A zoom lens operates something like Elken described. Its advantage is that it stays in one position while the FFL changes, it creates an imaginary lens further forward. In doing this its F# is increased meaning less light will pass though the lens. A focal extender has the same problem. A vari-focus triplet is the simplest, it moves an element inside the triplet to change its FL. When the FL changes the triplet itself needs to be repositioned as well. So if you were to modify a triplet or buy a ready made vari-focal triplet you will need to consider adjusting the arc distance to the rear fresnel to keep the arc image at the correct position inside the triplet. You should also remember that a vari-focal triplet won’t have the same image quality as a fixed triplet. And its quality varies at different adjusted FLs.

DJ

Good info, definitely. I understand how a zoom lens works, mainly due to Wikipedia... since
they have a great description of how it works with several pics. But I'm trying to figure out
if there is a different way to zoom.

If the image that hits the triplet is basically resized to be the correct size (focus) no matter
where the triplet distance is set to, will it work as a zoom? If what I'm trying to show is
correct, you wouldn't be changing focus when moving the projection lens, just using it as a
magnifier.

*** edit ***

Also, when looking at the picture, it doesn't need to be a triplet. I'm starting to get the feeling
that a triplet NEEDs to have things set up a certain way, whereas if we used some kind of
single magnifying lens, we may have some chromatic abberation, but it would work for the
purpose I'm trying to do.

From looking at your original idea and the posts that followed, I respectfully suggest that you really don't know how a zoom lens works.

It seems like you're missing something fundamental here - your diagrams show the lenses herding the light, but they don't show the actual focusing/imaging. It seems like the basic concept of what a lens does and how it works is just totally missing.

I don't mean this as an insult, so please don't take offense. And I'm no expert either, so it could just be that I'm missing the point. But I really suggest that you do some basic research on how a projection lens works, and what focusing and zooming actually are, how they relate to focal length and how brightness relates to F-number.


Take it easy,

I understand that in a normal zoom lens that there is a lens or set of lenses inside
the whole of the lens, where the entry lens and exit lens stay in the same position
while the other lens or lenses change the FL and correct for chromatic and optical
abberation.


That's what I'm saying... I really don't know how lenses act and proposed what I did
because I wanted other people who knew to hopefully be able to tell me how it could
work in a simpler manner because a normal zoom lens works to correct the abberation
where I really don't care about it unless it's going to be in horribly huge amounts
making the image unviewable, which this idea may very well do.

My hopes were that these people that know about optics might be able to tell me or
give me measurements and focal lengths of lenses that I could try it out with... which
I just now realize I never posted. Sorry.

*** This was my thought process regarding the idea:

In regards to how I show them herding the light... I was taking the idea off how we
use the fresnel lenses in a split setup. The first fresnel straightens out the light so
that it will pass through the LCD mostly in one direction. Then the second fresnel
takes the light passing through the LCD and shoots it at the projection lens. If we've
already got the focus set up to be at a certain spot, can we not then use another lens
in a similar manner to the first fresnel and make the smaller image of the LCD shoot
straight at another magnifying "projection" lens? Then when that lens gets moved, it
will change the size of the projected image while not changing any focus because only
it's distance away from the screen will change.

Believe me, I'm not getting huffy, I'm just trying to clarify myself as much as I can
so that what's in my head will hopefully become more clear to everyone who can help.
This is one of the main reasons that I dislike forums and texting, because you don't get
to hear my tone of voice and inflections...

I'm just mega intense about figuring the things out that pop into my head. It's also
really hard to find good information about lenses that someone who's not quite versed
in it will understand easily just by searching on the internet. I also just have no idea who
I could ask locally that could help me understand it better, you know?

Ahh well... I'm still tracking this topic if anyone wants to add more to it or to
help me figure this out. I'm going to keep researching and hopefully figure out
some way of testing it, and if I do I'll post more info.

Thanks so far for talking about it at least...

The problem with your theory is that the fresnels have very little to do with the actual focusing. The fresnels don’t create an image of the LCD at the triplet, they create an image of the arc at the triplet. The fresnels are only refracting rays so that they enter the triplet. The actual image of the LCD is created at the screen.

DJ

If you had an unsplit design, you could make the distances for the bulb , fresnels and LCD adjustable, and by moving them it would create a zooming affect; while still allowing good light flow. And with a triplet focusing mechanism of course. Is this correct?

So that makes me a little confused... as if that weren't already apparent.

Of course the real image of the LCD is created at the screen, but I was speaking of the
virtual image which I've heard talked about quite a bit. Light does carry color, and we're
focusing the light with the fresnel down to the triplet, correct?

Actually, for zoom you could just move everything from the LCD forward if you
were to do it REALLY simply... but I don't know how much of the light we would
lose...

Looky what I have....


this is a variafocal lens from a 3M 9500 overhead projector.
I started to use it in my PJ, but it is too small for a 15" panel.
It has good focus and no pincushining, but has dark corners.
The way it works, I think???? Is it moves one of the glass lenses,
and the other two stay put.

I can take pics of the triplet if you are interested. I still think, though
that moving the fresnel and lens together will do the same thing.
If you are interested in buiding a PJ with a smaller panel like 10" or
less, a lens like this might work for you. I put it in my PJ and really
liked the ability to focus, which also changed the screen size by about 4-6".