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Discussion Starter #1
Just an odd thought.


Assumption: The single bulb pj we use are able to zoom in and out relatively effectively without introducing dramatic errors.


Shouldn't we be building lens that vertically shrink the the scaled and stretched image from 16:9 back down to 2.35 and then zooming the image larger?


Wouldn't this get rid of much of the visible distortion and chromatic abberation that we get from the relatively large stretch horizontally, vs the minimal shrinkage vertically needed to accomplish this?


(perhaps there are people doing this, and I've just missed the boat on it, do I need a flame suit now?)


For example, if you consider a typical situation, 80x45 16:9 screen converting to CIH of 106x45 2.35:1 screen.


A top row pixel would have to move only 4.5 inches vertically by lens changes....vs what we typically do now which is move a right column pixel by almost 13 inches.


Wouldn't we rather lens shift the 4.5 inches, and zoom the 13 with our projectors?


Should be doing other things, but instead, my mind is wandering to this puzzle.


Alex.
 

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Quote:
Originally Posted by AlexBen /forum/post/14293169


Shouldn't we be building lens that vertically shrink the the scaled and stretched image from 16:9 back down to 2.35 and then zooming the image larger?

Panamorph's first lenses were Vertical Compression lenses. They require you to zoom the image out to the 235 width, then optically compress the image down as you have described.


There are certainly pros and cons for both. The main reason many turn to Horizontal Expansion is that HE lenses allow you to move the lens to create a CIH system. Moving a VC would allow a CIW system.


The amount of distortion you see depends on the optics used where a basic 2 prisms lens (HE or VC) will exhibit CA and anstigmatism...


Mark
 

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Discussion Starter #3
Ton's of people read this post before some one spoke up.


Appreciate your information, and addition of trivia to the thread.


I have to do some more thinking about it, but it sounds like what you're saying is that most people didn't want to "unzoom" AND turn off the scalar, and move a lens......vs... turn off the scalar, and move a lens when watching less than 2.35:1 movies.


(meanwhile, I'm just happy to hear that a local Menards has 54 x 144 inch sheet rock in stock so I can mock up a play screen and see how this all looks....)


I'm just hung up on the observation that in a general sort of way... going from a basic 80 x 45 screen of 16:9 ... which has a certain square inches of area... to the same area using 2.35:1 takes away about 6 inches vertically, and adds 12 horizontally....and that I strongly suspect that a lens shift of 3 inches on top and bottom would be far less distorting than what we see by a lens shift of 6 inches on each side........


Food for thought for the purist in us....



Alex. (given my profession, its hard to think outside of the box)
 

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Quote:
given my profession, its hard to think outside of the box

True of us all



The idea is simply to magnify (at the same rate) the amount of scaling - IE we VS the image 33% and then use a HE to simply add 33% to the width of the image.


Mark
 

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I have heard that it was easier/cheaper to make a vertical compression lens that introduced fewer artifacts than a horizontal expansion lens, all other factors being equal. If this is really true or not, I have no idea. There are advantages and disadvantages each lens type - vertical compression (VC) and horizontal expansion (HE), generally speaking you swap the advantages and disadvantages between the two lens types.


HE advantages: can be moved out of the light path (or use pass-through) and the image height stays the same, no re-zooming required. Higher image quality - not due to any inherent advantage to HE necessarily, but because VC has been all but abandoned and the effort on image quality has been put into HE lenses. Works with a "normal" curved screen (image has pincushion), and with a torus (if somebody would just build one, love to see that).


VC advantages: brighter scope image for those projectors that put out more light with increased zoom (many projectors exhibit this phenomenon). Ability to mount the projector farther away from the screen.


VC exhibits barrel distortion (opposite of pincushion) - so it can't use a curved screen unless you flipped it around so that the screen bulge was outward towards the projector rather than inwards towards the front wall (never seen anybody do that, I wonder what the implications of screen material gain and viewing cone are?).
 

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Actually, right now I am using a VC lens for my CIH, zoom, and keep the lens in place all the time and scale for 16:9 and 4:3 presentations with my Mitsu HC1500. in reality, I end up with a stretched 4:3 but that is ok, since I rarely watch any 4:3 content anymore.


I do it because of my room size, screen size, throw of the pj, and the fact that I had a VC lens laying around, not a HE (although I have made some very good DIY HE lenses...)


Works for me!


Oh, by the way - here's a shout out to the OP - Arlington Heights! Just a hop, skip and jump for me in Crystal Lake!
 

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What you're proposing is extremely unpractical and not very beneficial. Its unpractical because you're going to have to go through the hassles of both a lens and zoom setup. Its not beneficial because zooming will shorten the throw and exacerbate the distortions already introduced by the lens (even if you're talking VC's).


For CIH, use an HE lens or zoom

For CIW, use a VC lens or nothing (all 16:9 setups are CIW by default)


For either CIH or CIW you can always use a HE or VC lens in place all the time and scale the image accordingly.
 

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Quote:
Originally Posted by usualsuspects /forum/post/14300701


I have heard that it was easier/cheaper to make a vertical compression lens that introduced fewer artifacts than a horizontal expansion lens, all other factors being equal. If this is really true or not, I have no idea.

Having built both, there is no cost difference, but the VC did produce the better image (smaller pixles) from just 2 prisms, but the image was smaller. Zooming it out to be the same size as the image from a HE meant that there was no real gain and in my case, HE was preferred simply because of the flexibility including ease of set up. The key difference I noticed was that whilst CA was seen at the edges (vertically) with the HE, the CA was seen horizontally with the VC. Becuase I tend to focus on the centre of the image more, I found I could see the CA more with the VC.


The VC is usefull however when you need to increase the TR or when you have no choice but to mount the projector at the baclk of the room...


Mark
 

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Quote:
Ton's of people read this post before some one spoke up.

Myself, I clicked and saw it was another HE vs. VC thread and moved on since I'm apparently doing it "right" with my VC.
 

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I am currently using VC and leave the lens in place all the time and think even if I had a HE lens I would do the same thing just simpler and do not watch alot of 16/9. and like the fact that the brightness does not change.


Lots of IL in here.


Nate
 

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Discussion Starter #11

Quote:
Originally Posted by CAVX /forum/post/14308365


Having built both, there is no cost difference, but the VC did produce the better image (smaller pixles) from just 2 prisms, but the image was smaller. Zooming it out to be the same size as the image from a HE meant that there was no real gain and in my case, HE was preferred simply because of the flexibility including ease of set up. The key difference I noticed was that whilst CA was seen at the edges (vertically) with the HE, the CA was seen horizontally with the VC. Becuase I tend to focus on the centre of the image more, I found I could see the CA more with the VC.


The VC is usefull however when you need to increase the TR or when you have no choice but to mount the projector at the baclk of the room...


Mark

Mark, thank you for continuing the conversation, I appreciate it. I'm not surprised that there was no cost difference at the two prism level of VC/HE.


You make a very valid point, that in HE the CA would be displaced almost exclusively farther out to the peripheral vision sides of a wide screen, vs throughout the top and bottom of a VC image. THAT is the first and most appropriate reason from a technical aspect that I can see would be an issue to choose the HE over the VC.


Out of curiosity, would you say that its a reasonable position to take that given a 2 prism inexpensive Lens solution, that the amount of CA is less in VC than HE, but more noticeable because it stretches through the middle of the image at the top and bottom?


I do understand by the way that the convience aspect of zooming and moving the lens and CIH of the screen etc has been beaten to death in the forum now, but I was specifically interested in the actual potential decrease in CA going one way vs the other at the 2 prism level of budget.....


This concern of having CA horizontally throughout the image really would be an issue, and I can see that completely, and that alone would be a very good reason to abandon a project with VC......


For myself, I'm bummed at the need to do ANYTHING (move lens, take the scalar out of the video line, or adjust the pj to not scale) when going from 2.35 to 16:0.....so that I was thinking or hoping that VC was the answer to minimizing the inherent distortions of an inexpensive lens system.... and I'd put up with just a bit more effort....but I clearly neglected the concept that even if it was less actual distortion, I'd be moving it to a more visible and more disturbing location.


Message recieved, I'll go back into a hole, try to resume more of a lurk presence, and not rehash this again...... but I really, really appreciate the more knowledgeable answers that exist in forums like this. THANKS!
 

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Using a high quality HE lens like the Isco 3, there is no visible CA at any throw.


With a proper choice of equipment, the is no requirement to remove anything. Presets on the scaler take care of AR changes with the press of a button.


Vern
 

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Alex,


Your posts are just fine! Don't hesitate to post more! I like your thought processes...


I use a Prismasonic lens that has a motorized "pass / stretch". With a single press of my remote control, the image magically widens (and the audience goes "ahhhh...."). In "pass" mode, there is no noticable effect on the image quality. (Key word, "noticable".)


So, I keep my lens in place all the time... but I also have it on a swing mount in case I ever change my mind. There are motorized sleds that accomplish the same thing, but do your homework because some lenses will require refocusing or zoom shifts when you put them in place. Sliding a lens in front of the projector is not a hassle, but having to readjust throw, focus and/or lens shift is more time consuming.


Good luck... and continue asking those questions!
 

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Discussion Starter #14
On a purely hypothetical note.....


If one had a pj, and did some experimenting with 16:9 screens and figured out what the largest screen size for projecting 16:9 original ratio movies was....In other words, decided that any bigger and the pixels were visible, or the brightness was insufficient.... and so determined what that was the max size screen in 16:9 that they could enjoy.....


Wouldn't it make sense then to use the area of that screen to determine the dimensions of the 2.35 screen they they wanted to implement?


In other words, if you found that 3600 sq inches of screen was the biggest you could go before brightness or pixel size was an issueon a 16:9... then doing the math to calculate (X)(2.35X) = 3600 would yield the vertical dimension of the optimal CIH screen.....


or, is the fallacy in this that if pixel size had determined the max area enjoyable in the premise, that the lens would be stretching the pixels when implemented, and they may become objectionably large?


As one might guess, I'm torturing myself over selecting a new screen size.


I've been accused of thinking too much...yes....
 

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Quote:
Originally Posted by AlexBen /forum/post/14313091



You make a very valid point, that in HE the CA would be displaced almost exclusively farther out to the peripheral vision sides of a wide screen, vs throughout the top and bottom of a VC image. THAT is the first and most appropriate reason from a technical aspect that I can see would be an issue to choose the HE over the VC.

So whilst the 2 prisms HE lens causes CA to expand towards the edges, the VC tends to compress it horizontally, so it may not be as obvious anyway.


The other sore point often raised is pincushion for a HE and barrel for a VC. I found that the barrel of the VC was no where as bad as the pincushion of the HE. I built a curved screen for pincushion correction for my HE, but I would not have bothered with the amount of barrel distortion with a VC.


An interesting point to note too. The cinema I went to last night is CIH, yet there is a turret that turns in front of the projector that holds the lens and it does not just have one anamorphic, but rather a secondary zoom for the primary - I am thinking a reduction lens. The reason I noticed this was during the film, there is a key scene that reveals geometric distortions, but rather than the usual bowing outwards towards the edges of pincushion, the image bowed in - this was barrel, not pincusion.


If they are using a VC lens, they would use a reduction lens on all other program to keep the cinema CIH as VCs extend the throw. During the end credits I was able to see into the projection booth and could see the two lenses - one round, the other, the classic oval shape of the anamorphic...


Mark
 

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All theaters projecting 35mm films use two lenses. One is a cylindrical lens used for flat, and the second fis an anamorphic for 'Scope.


The 'Scope lens can either be a normal cylindrical prime lens with an anamorphic attachment screwed on the front or an integrated combination containing both the cylindrical elements and the anamorphic elements combined in a single lens.


You will never find a VC lens used for 35mm projection in an indoor theater. Only a Drive In with a very long throw would use a VC lens and that is done by simply reversing the HE lens end for end and rotating it 90 degrees. This was usually done with the Bausch & Lomb 4" diameter anamorphic attachment.


Vern
 

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Quote:
Originally Posted by Vern Dias /forum/post/14321963



You will never find a VC lens used for 35mm projection in an indoor theater.

No disrespect Vern, but how would one explain the barrel distortion I saw last night? This was clearly opposite to the pincushion I am used to seeing on other cinemas as well as CIH HTs with a flat screen. The screen in this cinema was flat too by the way...


Mark
 

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Quote:
Spherical lenses can exhibit aberrations including barrel distortion and pincushion, not just anamorphics.

Absolutely true. ANY lens can exibit all kinds of nasty problems, not just limited to pinchushion, barrel distortion, chromatic abberation. If a movie was shot, for some reason, with a very short focal length lens, the distortion could be in the source as well. Take a look at "Around The World In 80 Days" for some very obvious barrel distortion in shot's made with certain lenses.


Just like in the HT world, there are many grades of lenses available to the cinema world, from ultra cheap with all kinds of optical problems to ultra expensive which can achieve optical nirvana.


Just noticed you comment about the screen being flat, so deleted my alternate explanation.


Vern
 

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I am Legend has a nice shot with barrel distortion too, in the first 1-2 minutes they show the deserted city and you see a scyscraper from top to bottom, in that scene the barrel distortion is very evident

i like that scene, looks nicely surreal
 
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