or Connect
AVS › AVS Forum › Display Devices › 2.35:1 Constant Image Height Chat › Cylindrical A-Lens Owner's Thread
New Posts  All Forums:Forum Nav:

Cylindrical A-Lens Owner's Thread - Page 18

post #511 of 623
Quote:
Originally Posted by Aussie Bob View Post

The improvement is vast. A cylindrical can be designed to do almost whatever you want, given enough curved surfaces. The more the better.

Prisms respond in one way only: no matter what angles you use in the prisms themselves, there's a performance bottom line that can't be improved, only subtracted from.

Modest angles in each prism mean that they have to be counter-rotated more, leading to much more astigmatism and CA problems.

Large angles add to weight and space problems: the fatter the prism the heavier it is and the more room it takes up.

The optimum is around 30 degrees (total), plus or minus a little, for the smallest angle. All prism systems perform exactly the same as far a geometry is concerned, as do the simpler anamorphic cylindrical lenses, given astigmatism performance benchmarks are kept to the relatively "high quality" end of the scale. Whatever you add to one side of the equation, you take away from the other side of the equation. Prisms have only flat sides... no degrees of freedom in design. You can tweak prisms, add correctors etc., but there's a ceiling of maximum performance you can't penetrate.

You may think you've got a great picture (as do many HTB and other prism owners), but until you see a really GREAT picture coming off a well-designed cylindrical, you're fooling yourself.

Fine.

How do cylindrical lens respond to variances in actual among different installations (and I take figures from your curve calculator, thanks):

1. 12ft throw, 40"inch height = Sagitta of 79mm, 30ft Curve
2. 15ft throw, 40"inch height = Sagitta of 65mm, 36ft Curve

Will the focus still be as good in both cases?
post #512 of 623
Quote:
Originally Posted by CAVX View Post

Watch these films: I Robot, Shooter, Australia and American Physco representing four different genre all of which contain some standout demo material that is very much in focus edge to edge.

Let me check I Robot next time. Perhaps animation and CGI would also fall in edge to edge focus.
post #513 of 623
Quote:
Originally Posted by dukeav View Post


Will the focus still be as good in both cases?

Whilst pincushion would increase for the shorter throw (requiring a smaller radius to correct it), the cylindrical lens's "continuously adjustable astigmatism correction" will allow you to bring the image into focus from corner to corner for both installations.

And I am still using a BenWQ5000 BTW.
post #514 of 623
Quote:
Originally Posted by CAVX View Post

Whilst pincushion would increase for the shorter throw (requiring a smaller radius to correct it), the cylindrical lens's "continuously adjustable astigmatism correction" will allow you to bring the image into focus from corner to corner for both installations.

And I am still using a BenWQ5000 BTW.

Are you saying that even at the same throw, focus can achieved for different curves?
post #515 of 623
Quote:
Originally Posted by CAVX View Post

Whilst pincushion would increase for the shorter throw (requiring a smaller radius to correct it), the cylindrical lens's "continuously adjustable astigmatism correction" will allow you to bring the image into focus from corner to corner for both installations.

And I am still using a BenWQ5000 BTW.

Assume a given fixed curve and throw,

1. Projector mounted at top of the screen with lens shift
2. Projector mounted at bottom of the screen with lens shift
post #516 of 623
Quote:
Originally Posted by dukeav View Post

Are you saying that even at the same throw, focus can achieved for different curves?

Not quite. Each Curve is specific to the throw. Therefore the shorter the throw, the tighter the curve. However what I will agree on is that once the specific curve is established, that the cylindrical lens can be 'dialed' in for each throw to give corner to corner focus.

Quote:
Originally Posted by dukeav View Post

Assume a given fixed curve and throw,

1. Projector mounted at top of the screen with lens shift
2. Projector mounted at bottom of the screen with lens shift

This should not matter. I've inverted my projector simply because there was a visible reflection of the globe on screen when shelf mounted, and inverting places that 'hot spot' off axis to my eye line, so now I don't see it. I am typing this right now using the projector as a monitor and my lens is currently sitting in the light path, so the image is stretched (I'm also using REAL on the BenQ for 1:1 pixel mapping) and entire desk top is pixel level clear from the clock in the bottom right to the words AVS Forums in the top left. Same goes for the other direction. There is no prism lens that can do that outside its sweet spot. Again the beauty of a cylindrical lens where it can be dialed in at ANY throw in its range.
post #517 of 623
Quote:
Originally Posted by CAVX View Post

Not quite. Each Curve is specific to the throw. Therefore the shorter the throw, the tighter the curve. However what I will agree on is that once the specific curve is established, that the cylindrical lens can be 'dialed' in for each throw to give corner to corner focus.

So I understand that given a throw, there is fixed curve associated with it, in which the focus is precise.
If for the same throw, a different curve was used (e.g SI Screens make 25ft and 35ft as standard), what would you comment on the focus?

To be exact, taking the above figures (i.e. pairing the 30ft curve with 15ft throw), 79-65=14mm would be difference at the edges between the curve of precise focus vs curve of the screen, of course ending at 0 difference in the middle...

Quote:
Originally Posted by CAVX View Post

This should not matter. I've inverted my projector simply because there was a visible reflection of the globe on screen when shelf mounted, and inverting places that 'hot spot' off axis to my eye line, so now I don't see it. I am typing this right now using the projector as a monitor and my lens is currently sitting in the light path, so the image is stretched (I'm also using REAL on the BenQ for 1:1 pixel mapping) and entire desk top is pixel level clear from the clock in the bottom right to the words AVS Forums in the top left. Same goes for the other direction. There is no prism lens that can do that outside its sweet spot. Again the beauty of a cylindrical lens where it can be dialed in at ANY throw in its range.

What I meant to ask in these cases was that with both these positions, the actual distance from lens to top of the screen and to the bottom of the screen would be different. I calculate 11mm as the difference in lengths (15ft throw, 40" height).


Or in question, do these differences cause any/much/visible change of focus across the screen?
post #518 of 623
Quote:
Originally Posted by dukeav View Post

So I understand that given a throw, there is fixed curve associated with it, in which the focus is precise.

Where the PINCUSHION is corrected. The lens will still bring vertical lines into focus at the same time as the horizontal lines.

Quote:


If for the same throw, a different curve was used (e.g SI Screens make 25ft and 35ft as standard), what would you comment on the focus?

Allot of screen manufactures make "standard" curves which are like "one size fits all", which of course it doesn't. You would still be able to focus the lens, but pincushion (the REASON you would use a curved screen in the first place) does not match 100%. I modded a commercial screen to make it thread adjustable to be able to precisely match the curve to the pincushion based on throw.

Quote:


Or in question, do these differences cause any/much/visible change of focus across the screen?

As I said last night, my image has pixel level clarity right to the edges. If for some reason it could not be brought into focus 100%, the cylindrical lens would still do better than a prism lens (which has NO adjustments for astigmatism) in the same system.
post #519 of 623
Quote:


So I understand that given a throw, there is fixed curve associated with it, in which the focus is precise.
If for the same throw, a different curve was used (e.g SI Screens make 25ft and 35ft as standard), what would you comment on the focus?

If you wanted to purchase a commercial screen fixed at a "standard" curvature, you could use my spreadsheet and the "Goal Seek" function of Excel. Fix as a constant either "Throw" or "Constant Screen Height" and then use Goal Seek to calculate the other variable.

As to astigmatism correction (or in plain language: focus), the lens will either be designed for best focus on a flat screen or a curved screen. In either case, there will be some small trade-off in sharpness from edge to edge if you vary the curvature of the screen.

Say my spreadsheet calculates a sagitta of 79mm (as in your example). Now project onto a flat screen and focus as best you can for edge-to-edge performance. Now take a piece of white card, walk to the edge of the screen and hold it 79mm in front of the plane of the screen. The pixels will be slightly softer on the white card than they would be on the flat screen at the same extremity. This is because you have in effect two screens: one 79mm (3.1") closer to the projector than the other. You will probably have to adjust your focus, by minutely sacrificing some centre detail to improve the image at the edges as displayed on the white card. Not by much, but a little.

If you can actually do this experiment, do so. It'll put your mind at ease.

No lens can focus perfectly on two different throw distances, except if it's been designed to focus on a curved screen... in which case it'll have slight problems with a fla screen!

The question is "How much difference?".

The answer is "Not much at all". If you can see pixel tiling edge-to-edge on a flat screen focus, you should be able to see it on a curved screen focus. It's just that the curved screen focus point will be slightly different than the flat screen focus point if you really want to be picky about it. More than a foot or two away from the screen the two focus points will appear just as good as each other even if you don't adjust.

My experience is that projector lenses themselves (i.e. no anamorphic in the system) fall off somewhat when not perfectly straight on to the centre of the screen, and even then they can be uneven in focus. This is especially true of 3-chip projectors, as there is too much to line up - prisms, polarizers and imaging chips - to get everything perfect (except if you want to pay $0000s more for a "top of the line" 3-chipper... and I've still heard some horror stories). Because Home Cinema projectors are generally dimmer than commercial cinema projectors, and because HC projectors are in competition with very bright flat screen TVs, the manufacturers tend to make them as bright as they can by the lazy method of using lower f/-numbers on the projector lenses, which means the image is starting out using too much glass to achieve perfect focus across the screen. The wider the beam angle, hence the lower the throw ratio, the more edge-to-edge performance suffers. Remember: this is without an anamorphic lens, just the raw projector lens.

As a general rule, the smaller in scale anything is, the tighter the tolerances have to be to get near-perfect results that match the expectations of the user. Home Cinema situations test the extremities of optical projection design. The beam angles are large, much larger than in commercial cinemas. The throws are much shorter. And here's the killer: the ambitions of the users are usually much bigger. Add all these together and perfection suffers. It's up to you to decide where your exhaustion point is, where you're prepared to say, "That's great!" and stop worrying.

Having said that, I don't want to scare you. I'm just saying that "perfection" usually boils down to what you're prepared to accept. A well-designed cylindrical lens will certainly and quite noticeably outperform a prism system, when you take focus adjustability, better image geometry and, say, ease of use into account. The two examples you cite above are both over 2.0 in throw ratio, so you should get a pretty good result whether your screen is flat or curved.
post #520 of 623
Quote:
Originally Posted by dukeav View Post

So is it that pairing them with lower end projectors (e.g. BenQ W6000 which I have, and which shows CA on its own on the left side), may not show much difference?

That is one variable that may contribute to being able to see a difference between anamorphic lenses when viewing real world material. Viewer experience, visual acuity, image brightness, screen size, viewing distance, and other variables also factor in.
post #521 of 623
Quote:


So is it that pairing them with lower end projectors (e.g. BenQ W6000 which I have, and which shows CA on its own on the left side), may not show much difference?

An anamorphic lens of any type won't improve CA, but they CAN improve focus across the screen.

A quirk in my own HT is a case in point.

My JVC HD-100 (= "RS2") seems to have an ashperic element in its zoom lens somewhere that doesn't have the same curvature in the horizontal direction that it has in the vertical direction. This results in unfocused vertical tiling lines at the far right of the screen, particularly when I use offset (my projector is about 1 foot above and 2 feet to the left of the center of the screen due to structural considerations in my house). The horizontal lines are good, just the verticals are blurred.

This isn't CA, like in your case, because it happens with just the individual R, G or B channels switched on and projecting a checkerboard test pattern. If it was CA then you'd expect at least one channel NOT to exhibit the problem.

When I insert my lens it "pulls" these unfocused vertical lines back into focus. Not perfectly, but enough to see them pretty clearly.

At first I thought it might be some error in the lens build, but I've had 4 lenses do the correction now, so unless they all have exactly the same problem, I have to count this as an "unintended feature" of my particular set-up. My lens counteracts the error in the pj's own lens and fixes it somewhat.

Not making any big claims here, just noting serendipity.
post #522 of 623
Thanks for the replies and patience. Its very interesting that focus has that much tolerance.
post #523 of 623
Quote:


Thanks for the replies and patience. Its very interesting that focus has that much tolerance.

Focus has no tolerance at all. It's your eye, your performance thresholds and your expectations that have it.
post #524 of 623
Quote:


Focus has no tolerance at all.

What about the depth of field of a lens as it relates to it's aperture setting (f-stop)? Anyone who has used an SLR knows that this statement isn't true in the real world.

Changing the aperture of a lens from f16 to f4.5 makes a dramatic and obvious difference in the distance range of the image that is clearly in focus.... Of course, if your subject is 500' away, it's not going to matter. If the subject is 10' away, you can clearly see the impact of the depth of field associated with the f-stop you choose.

Vern
post #525 of 623
Quote:
Originally Posted by Vern Dias View Post

What about the depth of field of a lens as it relates to it's aperture setting (f-stop)? Anyone who has used an SLR knows that this statement isn't true in the real world.

Changing the aperture of a lens from f16 to f4.5 makes a dramatic and obvious difference in the distance range of the image that is clearly in focus.... Of course, if your subject is 500' away, it's not going to matter. If the subject is 10' away, you can clearly see the impact of the depth of field associated with the f-stop you choose.

Vern

Vern, I was referring to absolute perfect focus. There is only one point at which a lens is perfectly focused (i.e. as sharp as it's ever going to get)

Depth of field is a consideration, of course, but even depth of field still depends on how you define "acceptable" focus. Hence my caveat, "It's your eye, your performance thresholds and your expectations that have (tolerance)."

My experience with hyperfocal distances (as represented by the claimed depth of field range inscribed on a photographic lens) is that they're too wide. Objects the camera maker claims will be "focused" are not sometimes, at the margins of hyperfocal distance, sufficiently focused for my liking. That's where the "eye" and "expectations" come into it.

Anamorphic lenses have no formal f/number as they have effective "infinite" focal length. Their performance relies on how you place them relative to the projector, and the projector lens's f/number, plus their own focusing mechanism and optical quality to render the image.

If you place the anamorphic lens closer to the projector you'll use less cylindrical glass to refract the beam through the lens. It's commonly thought that this means the screen image must always be clearer, the closer the lens is mounted to the projector.

But mounting the lens closer also means the ray pencils - the "sub-beams", one from each point on the imaging chip - are wider passing through the anamorphic lens than they would be if you mounted the lens further away. The lens, paradoxically, has to do more "work" with this narrower beam, keeping all the individual rays pointing at that spot on the screen where they'll focus, once again, to a point.

The truth is that ultra-close mounting of the anamorphic lens to the projector is not the recipe for perfect imaging that many think. There is a "sweet spot" for mounting distance of anamorphic lenses, and this varies with set-up, lens and projector. Either side of that and there will be a fall-off in quality. A long way either side of it and the fall-off becomes apparent to the naked eye, even from a casual glance from a few feet away.

Once again, it's up to the user to decide whether this fall-off is worth the trouble of trying to fix it: "expectations" again.

I only think about all this detailed stuff because when I model a lens design I have to nominate how far from the projector it will be mounted, as an input variable. In my case I opted for a sweet spot in the medium distance - around 35mm - from the lens in order to account for recessed lenses. If I have to mount further away, or closer than that, the astigmatism adjustment can be tweaked to eliminate most of the fall-off, as my lens was designed with this ability built-in.

Any lens maker that tells you their lens just works perfectly wherever they place it, and whatever the set-up - projector, zoom angle, throw distance etc. - is either pulling your leg, or hasn't looked into the problem too deeply.
post #526 of 623
Quote:
Originally Posted by Aussie Bob View Post

The question is "How much difference?".

The answer is "Not much at all". If you can see pixel tiling edge-to-edge on a flat screen focus, you should be able to see it on a curved screen focus. It's just that the curved screen focus point will be slightly different than the flat screen focus point if you really want to be picky about it. More than a foot or two away from the screen the two focus points will appear just as good as each other even if you don't adjust.

I tried this out by moving a A4 sheet in front of the screen, and there was not any change which I could notice within a foot or two. Though it told me something else about my screen but that's not related to focus.
post #527 of 623
Quote:
Originally Posted by dukeav View Post

I tried this out by moving a A4 sheet in front of the screen, and there was not any change which I could notice within a foot or two. Though it told me something else about my screen but that's not related to focus.

Color shift?

Gary
post #528 of 623
As dukeAV has seen, it's not a problem in the real world as there is usually suffiicent depth of focus in the typical projection lens to handle typical scrren curvature with no problems.

In the numerical world of simulations and lens design applications, "absolutely perfect focus" may be attainable, but it doesn't translate to the real world because of potential imperfections in screens, lenses, and other optical components within the projector's optical system and also possibly within the human bodies optical system.

Vern
post #529 of 623
Quote:
Originally Posted by Gary Lightfoot View Post

Color shift?

Gary

The paper appears to be warmer, warmer as in Warm color balance setting. One on one, the white on the screen is no comparison.
post #530 of 623
Quote:


In the numerical world of simulations and lens design applications, "absolutely perfect focus" may be attainable, but it doesn't translate to the real world because of potential imperfections in screens, lenses, and other optical components within the projector's optical system and also possibly within the human bodies optical system.

Which is another way of saying "Good focus is what you're prepared to put up with and pay for."
post #531 of 623
Thread Starter 
IMO, why mess with curving the screen when a flat screen can give you better focus. While it does remove pincushion a properly designed system with 2.0 and larger throws pretty much nullifies it anyway. Unless you have an extremely short throw my guess is it's for the aesthetics of it, a certain cool factor.
post #532 of 623
Quote:
Originally Posted by coolrda View Post

IMO, why mess with curving the screen when a flat screen can give you better focus. While it does remove pincushion a properly designed system with 2.0 and larger throws pretty much nullifies it anyway. Unless you have an extremely short throw my guess is it's for the aesthetics of it, a certain cool factor.

My throw is longer than 2.0:1 and on my curved screen, I now have flat horizontal lines, not ones that slightly (even ever so slightly as the case may be) curved ones.

Cylindrical Lens and flat screen = perfect focus with some pincushion.
Cylindrical Lens and curved screen = perfect focus with no pincushion.

There is a little extra set up to achieve this though.
post #533 of 623
Now I am utterly confused. I have a jvc rs-1 with projector to screen distance 11feet and looking at a 2.40 :1 screen 125 inch diagnal projected from under a table upward. So, with one of the cylindrical lenses would a curved screen such as the elite lunnette 125 be best or flat screen?

Hope someone can help.

Thanks
sarkleshark
post #534 of 623
As best that I can tell, the projector will not illuminate a 125 diagonal at a throw distance of 11 feet. Perhaps I mis-read the projector central calculator or maybe you have a zoom different than that in the calculator.

Best is vague at best (no pun intended). Price? Prestige? Pincushion? Resale? Ease of install? It is your equation to insert the valuation parameters into.
post #535 of 623
Thread Starter 
Quote:
Originally Posted by sarkleshark View Post

Now I am utterly confused. I have a jvc rs-1 with projector to screen distance 11feet and looking at a 2.40 :1 screen 125 inch diagnal projected from under a table upward. So, with one of the cylindrical lenses would a curved screen such as the elite lunnette 125 be best or flat screen?

Hope someone can help.

Thanks
sarkleshark

It can be confusing. Just remember if your using an A-Lens you use the 16:9 ratio (ie: 125" 2.35 is a 100" 16:9). Its this 16:9 screen measurement thats needed. When zooming you use the width of the 2.35 image. This is yet another benefit of and A-Lens. Largest available screen width per depth of room.
post #536 of 623
Quote:
Originally Posted by coolrda
IMO, why mess with curving the screen when a flat screen can give you better focus. While it does remove pincushion a properly designed system with 2.0 and larger throws pretty much nullifies it anyway. Unless you have an extremely short throw my guess is it's for the aesthetics of it, a certain cool factor.
Regarding curve screens when watching 16:9 content removing the lens does the pic sit ok on screen? Or do you need lens on?
post #537 of 623
You can end up with barrel distortion for 16:9 if you remove the lens with a curved screen.

Gary
post #538 of 623
Quote:
Originally Posted by Gary Lightfoot View Post

You can end up with barrel distortion for 16:9 if you remove the lens with a curved screen.

Gary

Thanks Gary
post #539 of 623
Quote:
Originally Posted by Franin View Post

Regarding curve screens when watching 16:9 content removing the lens does the pic sit ok on screen? Or do you need lens on?

The anamorphic lens introduces pincushion. The curved screen is used to correct this distortion. If you remove the lens for 4 x 3 or 16:9 program on a curved screen, the correction the screen provided (for the lens in the light path) will now have the opposite effect and create a new distortion called barreling. IMO, if you are going to curve the screen, then you are best to leave the lens in place all the time as scale the image. Others may disagree.
post #540 of 623
Quote:
Originally Posted by Franin View Post

Regarding curve screens when watching 16:9 content removing the lens does the pic sit ok on screen? Or do you need lens on?

The anamorphic lens introduces a distortion we know as pincushion. The curved screen is used to correct this distortion. If you remove the lens for 4 x 3 or 16:9 program on a curved screen, the correction the screen provided (for when the lens is in the light path) will now have the opposite effect and create a new distortion called barreling. IMO, if you are going to curve the screen, then you are best to leave the lens in place all the time as scale the image for all ARs. Others may disagree.
New Posts  All Forums:Forum Nav:
  Return Home
AVS › AVS Forum › Display Devices › 2.35:1 Constant Image Height Chat › Cylindrical A-Lens Owner's Thread