Hi,
A question regarding a factory projector (not a DIY one): is it possible to reduce the focal lenght of projection lens, without any modifications to the projector? I mean adding some lens (like uncut one for making the reading glasses?) in front of the original lens, would it work at all? The goal is to obtain a larger screen from a given distance, I would like to enlarge the projection by about 20%. If so, how to pick the right lens (how to calculate its focal), and what drawback of such a solution one should expect?
Regards
Pawel
A question regarding a factory projector (not a DIY one): is it possible to reduce the focal lenght of projection lens, without any modifications to the projector? I mean adding some lens (like uncut one for making the reading glasses?) in front of the original lens, would it work at all? The goal is to obtain a larger screen from a given distance, I would like to enlarge the projection by about 20%. If so, how to pick the right lens (how to calculate its focal), and what drawback of such a solution one should expect?
Regards
Pawel
I'm not an optics expert by any means, but short throw lenses are not cheap. Some cost as much (if not more) than the projector they are meant to work with - SONY for example.
If you want an affordable "larger" image at a given throw, you might consider the DIY ANAMORPHIC LENS approach. Granted we are expanding the image only in one direction, but it might be possible to use four of the prisms (2 at 90 degrees to the other set) to form a lens that expands both horizontally as well as vertically. Given that your looking for 20% (we can do up to 33%) you might be able to do this with out too much trouble or expense...
Mark
If you want an affordable "larger" image at a given throw, you might consider the DIY ANAMORPHIC LENS approach. Granted we are expanding the image only in one direction, but it might be possible to use four of the prisms (2 at 90 degrees to the other set) to form a lens that expands both horizontally as well as vertically. Given that your looking for 20% (we can do up to 33%) you might be able to do this with out too much trouble or expense...
Mark
It's certainly possible to reduce the FL of your lens by adding another positive element, but this is likely to upset all the optical corrections designed into the original lens. The resulting quality *may* be good enough for your purposes, but it's unlikely. You will also need to arrange for the lens to object distance to be reduced in order to focus the image.
Why not try it and see?
Why not try it and see?
Why not try - because I don't have this projector. I'm considering Benq W100, but its lens focal is too long (screen too small in my opinion, considering my room size).
Mark: what would be a difference between expanding image both horizontally and vertically with two anamorphic lenses, and using just a single positive lens, which theoretically expands image in all directions?
My concern is the availability of such positive lenses of exact focal and diameter (eyeglasses maybe?), and possible deterioration of image quality, some geometric distortions, or chromatic aberrations, or problems with focus across the whole screen, I don't know. So far I have a DIY projector, still using it, so maybe I'll check the principles of this idea of reducing focal length., but this projector has a large lens, and I doubt I can find a corresponding positive lenses to fit it.
Regards
Mark: what would be a difference between expanding image both horizontally and vertically with two anamorphic lenses, and using just a single positive lens, which theoretically expands image in all directions?
My concern is the availability of such positive lenses of exact focal and diameter (eyeglasses maybe?), and possible deterioration of image quality, some geometric distortions, or chromatic aberrations, or problems with focus across the whole screen, I don't know. So far I have a DIY projector, still using it, so maybe I'll check the principles of this idea of reducing focal length., but this projector has a large lens, and I doubt I can find a corresponding positive lenses to fit it.
Regards
I am only guessing, but size and cost where the diy soloution is about $200 but about 8" Vs possibly many thousdands and a lens that is less then 3" dia.
A "Positive" optic (spherical lens) could be manufactured by almost any optics manfacture, but I am not sure of the cost. They are also limited in size.
I took a look at a pro-sumer "fish eye" and was surprised by how much CA it had when I was looking through it. The DIY lens will exhibit some CA at max stretch of greater than 33%, but you should not see any at 20% expansion...
How large do you want this image compared to how large is it native, and what AR do you want?
Mark
A "Positive" optic (spherical lens) could be manufactured by almost any optics manfacture, but I am not sure of the cost. They are also limited in size.
I took a look at a pro-sumer "fish eye" and was surprised by how much CA it had when I was looking through it. The DIY lens will exhibit some CA at max stretch of greater than 33%, but you should not see any at 20% expansion...
How large do you want this image compared to how large is it native, and what AR do you want?
Mark
Mark: the original image would be about 94" wide, and I would like something about 114" wide (this is the screen I have made, and I'm used to watch image this big). Projector is equipped with optical zoom of 1:1.15 range (so very limited), and that 94" is at max zooming (shortest FL).
What is this "AR" that you asked?
Regards
What is this "AR" that you asked?
Regards
pepe303 said:
Ok did a quick google and it looks like this is 16:9 projector with 854 x 480 pixels or a SD 16:9 display.
In most cases when someone wants a "bigger" image, it is not the screen heigth, but the width that they want to increase. Normally, increasing the width means increasing the height, but if you were to employ an anamorphic lens like the very affordable one we are building in the linked thread, you can increas the image width by 33%.
Being a HT projector, I have to assume that your going to watch DVD movies. How many DVDs would you watch that are wider than the native 16:9 AR of the projector? If your like me, about 70% of the 300 or so DVDs I own are "delux wide screen" or cinema scope 2.35:1.
So do want to see black bars top and bottom, and do you realize that these black bars will use 25% of the vertical pixels taking your 480 down to just 360 vertical pixels?
What if you could use 100% of the panel (the full 480), and still maintain correct geometry and get an image 33% wider than the native 16:9 image?
What we are doing in the thread is using a pair of (one guy is using 4) crystal prisms to make a prismatic anamorphic lens.
We then "scale" the DVD to fill the full panel. This makes the image look "tall and thin" and removes the black bars. we do this by setting the DVD player to 16:9 then choose a setting on the projector like "letterbox" or "4 x 3 zoom".
We then optically stretch the "tall skinny" image to 2.37:1 to restore the geometry. Really cool stuff and fairly cost effective...
Take a read of my BLOG by clicking on the web icon. The thread is long, about 113 pages now, but the crystal prism talk didn't start until about page 60...
Mark
I understand the concept od anamorphic expansion/compression, but this not exactly what I would like to do with this projector. I would consider this in case of a 4:3 projector, to display a wide screen using full resolution. But this Benq projector is already 16:9, and there is no setting to display any other "wide screen" formats in "full screen" mode. Movie in a 2.35:1 will be always played with black borders on top and bottom. I don't want to use a PC, just a standard DVD player, so there are not many options to scale/resize image.
What I would like to do (or rather first make sure, that it's possible) is to enlarge the whole projection, to get a bigger screen from the same distance. A fixed distance from the lens to the screen is 410cm, screen width is 285cm, and max projection I can get from this projector is about 240cm wide. So I would like to enlarge this 240cm wide projection to 285cm. Lens focal is 20.4-23.5mm. How to calculate the required new lens focal length?
Regards
What I would like to do (or rather first make sure, that it's possible) is to enlarge the whole projection, to get a bigger screen from the same distance. A fixed distance from the lens to the screen is 410cm, screen width is 285cm, and max projection I can get from this projector is about 240cm wide. So I would like to enlarge this 240cm wide projection to 285cm. Lens focal is 20.4-23.5mm. How to calculate the required new lens focal length?
Regards
pepe303 said:
I am using a 16:9 (LCD) + anamorphic lens to produce images at that are way wider then conventional 16:9 could hope to produce.
I don't use a PC or external scaler either. The DVD player is set to 16:9 and I use 4 x 3 ZOOM to do the scaling of the 235 film and use 4 x 3 NORMAL for the scaling to do 16:9 images. So basically I can switch between 2 ARs just using the projector's remote...
pepe303 said:
Another thing to consider when enlargening the 16:9 frame is that the pixels that make the image also get larger, so more visible.
CIH gives 33% wider images at the same height, so the vertical pixels remain the same size. With a CIH system, your always using the max vertical rez of the projector and as human sight is more sensitive to vertical image height than width it make sense not to go too tall just to increase the width...
Mark
required focal length
You want a magnification 285/240 times bigger, keeping the same distance between object and image. This means a focal length smaller by the same ratio. This works because the distance to image is a lot larger than the distance to object.
You would need a very strong glasses lens, over +9 diopters, which would almost surely add unbearable distortions. You may have better luck trying a photographic 0.7x wide angle add on lens. It should work better given a projector is a camera in reverse. Besides, the digital cameras they're designed to complement have focal lengths close to those you mentioned for the Benq.
You could try instead to project into a mirror first, to lenghten the throw, or to project diagonally and dekeystone.
You want a magnification 285/240 times bigger, keeping the same distance between object and image. This means a focal length smaller by the same ratio. This works because the distance to image is a lot larger than the distance to object.
You would need a very strong glasses lens, over +9 diopters, which would almost surely add unbearable distortions. You may have better luck trying a photographic 0.7x wide angle add on lens. It should work better given a projector is a camera in reverse. Besides, the digital cameras they're designed to complement have focal lengths close to those you mentioned for the Benq.
You could try instead to project into a mirror first, to lenghten the throw, or to project diagonally and dekeystone.
zznobi: thanks for this explanation.
I just checked for protographic wide angle converters, this thing is pretty expensive. And I wonder about the focusing range, as far as I understand, shorter focal means that lens should be placed closer to the object (which is a DLP chip in case of this projector). Is there a risk, that with this add-on wide lens there would be no available focus range, as projector lens should be moved too close to the chip, beyond the available range? Or this add-on lens does not interfere with focus, as it would be hardly usable with cameras that have no replaceable lenses?
Regards
I just checked for protographic wide angle converters, this thing is pretty expensive. And I wonder about the focusing range, as far as I understand, shorter focal means that lens should be placed closer to the object (which is a DLP chip in case of this projector). Is there a risk, that with this add-on wide lens there would be no available focus range, as projector lens should be moved too close to the chip, beyond the available range? Or this add-on lens does not interfere with focus, as it would be hardly usable with cameras that have no replaceable lenses?
Regards
Hello
Actually there's no way to calculate exactly such a magic glasses lens, because we have inssuficient data. And I don't think it's worth the trouble, unless you already have such lenses around. Let me explain: these projector lenses are optical systems very carefully designed. They are optimised by matching variables like surface curvatures, various distances and different glasses, with selected refraction indexes and dispersion numbers. You can't just add a miracle glass lens, of which you don't even know much data, and expect to work as well as before. Any slight variation destroys that fine balance.
I once opened a simpler Taylor triplet just for kicks, and upon reasembly I commited but one small error: I flipped the middle divergent lens. It looked perfect on the outside, and the middle lens was almost simmetrical to the eye, but the projected diascope image was total garbage in this new 'configuration'.
However, a simpleton approach, if we could approximate the objective by an ideal thin lens, would be to add next to it a glasses lens such that the sum of the objective diopters with the add on lens diopters equals what you need. The diopter power is the inverse of the focal length.
So your lens has 0.0204 FL, that means +49 dioptries. You want to end with 0.0204*240/285=0.0172 FL, meaning +58.2 dioptries. Therefore you would need a glass lens of +9.2 dioptries, ie a convergent (positive) lens, for hypermetrops.
In reality you will need more dioptries because you'd probably place the lens at some distance from a thick system. But as you said, this will also require replacing the entire new 'optical system' closer to the DLP. If you'd be willing to get this far you'd better switch the whole objective with a photographic one, although 17mm FL is unusually short for classic 35mm cameras. Maybe a film projector lens comes closer.
Such lenses are getting obsolete and can be had for cheap these days. Try e-bay or places like www.surplushed.com
Actually there's no way to calculate exactly such a magic glasses lens, because we have inssuficient data. And I don't think it's worth the trouble, unless you already have such lenses around. Let me explain: these projector lenses are optical systems very carefully designed. They are optimised by matching variables like surface curvatures, various distances and different glasses, with selected refraction indexes and dispersion numbers. You can't just add a miracle glass lens, of which you don't even know much data, and expect to work as well as before. Any slight variation destroys that fine balance.
I once opened a simpler Taylor triplet just for kicks, and upon reasembly I commited but one small error: I flipped the middle divergent lens. It looked perfect on the outside, and the middle lens was almost simmetrical to the eye, but the projected diascope image was total garbage in this new 'configuration'.
However, a simpleton approach, if we could approximate the objective by an ideal thin lens, would be to add next to it a glasses lens such that the sum of the objective diopters with the add on lens diopters equals what you need. The diopter power is the inverse of the focal length.
So your lens has 0.0204 FL, that means +49 dioptries. You want to end with 0.0204*240/285=0.0172 FL, meaning +58.2 dioptries. Therefore you would need a glass lens of +9.2 dioptries, ie a convergent (positive) lens, for hypermetrops.
In reality you will need more dioptries because you'd probably place the lens at some distance from a thick system. But as you said, this will also require replacing the entire new 'optical system' closer to the DLP. If you'd be willing to get this far you'd better switch the whole objective with a photographic one, although 17mm FL is unusually short for classic 35mm cameras. Maybe a film projector lens comes closer.
Such lenses are getting obsolete and can be had for cheap these days. Try e-bay or places like www.surplushed.com
OK, thanks for this explanation, that's about the same as I managed to calculate, +10 dioptries. But today I looked for information on those photographic wide angle converters, and it seems that these are negative, not positive lenses. Check this out, it's a DIY wide angle add-on, made of a strong negative lens (description is in Polish, but there are pictures):
http://www.cyberfoto.pl/fuji/52801-nasadka-szerokokatna-jak-zrobic.htm
And now I doubt whether I should consider positive or negative lens.
Regards
http://www.cyberfoto.pl/fuji/52801-nasadka-szerokokatna-jak-zrobic.htm
And now I doubt whether I should consider positive or negative lens.
Regards
DIY wide angle converter
Looks indeed like some negative lenses http://www.helixcamera.com/Video/century/dsfewasb.html
And to add to the confusion I read some converters act both as wide angle and telephoto by just flipping them in front of the lens. Just like those anamorphic prisms.
I guess they're placed at enough distance in front of the objective so that our little calculus does not apply. I'd say go with a negative spectacle lens then, if you can live with the field distortion. The edge bluriness is not that bad.
Nice doggie! Did the guy make those photos with just one divergent lens or two? And what diopters?
Looks indeed like some negative lenses http://www.helixcamera.com/Video/century/dsfewasb.html
And to add to the confusion I read some converters act both as wide angle and telephoto by just flipping them in front of the lens. Just like those anamorphic prisms.
I guess they're placed at enough distance in front of the objective so that our little calculus does not apply. I'd say go with a negative spectacle lens then, if you can live with the field distortion. The edge bluriness is not that bad.
Nice doggie! Did the guy make those photos with just one divergent lens or two? And what diopters?
Can I say just one thing? Galilean telescope.
With the camera lens being the front objective, so naturally used in reverse it should render a wide angle, but distorted, fish-eye picture, like all binoculars do. It just struck me wondering how it works.
Let us know the outcome for your projector.
Cheers
With the camera lens being the front objective, so naturally used in reverse it should render a wide angle, but distorted, fish-eye picture, like all binoculars do. It just struck me wondering how it works.
Let us know the outcome for your projector.
Cheers
But I didn't purchase this projector yes. This is Benq W100, which is within acceptable price range for me, and has all the features I would expect, but its projection ratio is too small for my room. I'm used to my DIY projector, that projects huge image, so I'm investigating how it would be possible to adapt this W100 for my room and for my present screen. Anyway, if I have any success or failure, I'll let know here.
Regards
Regards
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