# Chromatic Aberration



## Prof.

Mark,

I recently needed to make new borders for my screen, and decided to change the aspect ratio from 2.37:1 to 2.40:1.
This of course required the the prisms to be re-aligned, so at the same time I inverted the housing, to access the cover from the top..

Well the end result showed an improvement in general image quality, from one side of the screen to the other, but I noticed that the CA had changed as well..

The left side of the image showed virtually no CA or pin cushion at all, whereas previously it showed both quite clearly on both edges..but now only the edge on right side clearly showed CA and pin cushion effect..

Any ideas or suggestions as to what might be causing this? 
If I could get the right side the same as the left is now, I would have very little, if any CA..

EDIT..The other thing I forgot to mention is that now the housing has been inverted, it's moved the lens further away from the projector lens..
Could this have helped to reduce the CA on the left hand side?

Also the focus on the LH side is now razor sharp, since making the changes..


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## Blaser

lenses... :daydream:


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## Prof.

Not such a dream Ahmed...
If you have any handyman skills (and I'm sure you would have) you can put together, quite a reasonable quality lens for about $200.00!..:T:yes:
Quite insignificant compared to the price of your new projector..


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## Mark Techer

Hi Prof,

If the prisms are aligned 100% cirrectly, then CA will be symmetrical. Like you, right now, I seem to have more on the right hand side than the left, but I know that I need to remove that lid yet again. I have the image prefectly framed right now, so not game to alter the prisms just in case I mess up what I have. 

Slightly off topic, but thought I would share (actually tease). I have have prototype plastic case for my lens (MKIII) and have been experimenting with a cylindrical correction element, that, for these initial tests anyway, seem to almost completely remove CA. 

Clearly there is way more testing to do, but so far this is looking promising.

You can see a photo of the prototype HERE...

Mark


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## Blaser

Mark Techer said:


> Slightly off topic, but thought I would share (actually tease). I have have prototype plastic case for my lens (MKIII) and have been experimenting with a cylindrical correction element, that, for these initial tests anyway, seem to almost completely remove CA.
> 
> Clearly there is way more testing to do, but so far this is looking promising.
> 
> You can see a photo of the prototype HERE...
> 
> Mark


WOW !! A lens w/o CA??? That sounds really great! What price order are we talking about?


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## Prof.

Mark Techer said:


> If the prisms are aligned 100% cirrectly, then CA will be symmetrical. Like you, right now, I seem to have more on the right hand side than the left, but I know that I need to remove that lid yet again. I have the image prefectly framed right now, so not game to alter the prisms just in case I mess up what I have.


Yes, my feelings exactly..The overall image is so good that I don't want to mess it up..
But knowing me, I won't be happy until I try and make it even better.:bigsmile:



> Slightly off topic, but thought I would share (actually tease). I have have prototype plastic case for my lens (MKIII) and have been experimenting with a cylindrical correction element, that, for these initial tests anyway, seem to almost completely remove CA.


Mark..that would be a major achievement if you can almost eliminate CA..
Your lens would then be right up there with the best of them..:yes: 

By cylindrical element I presume you mean, as in a circular lens..like a filter shape..
Would this be near the projector lens..almost like an attachment?..Would it be something that could be added to your current models?




> You can see a photo of the prototype HERE...


That's quite an improvement over the MDF case..very nice:T
One of the first things I did with my lens was to line the sides with black felt...It does make a difference..


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## Mark Techer

Prof. said:


> Yes, my feelings exactly..The overall image is so good that I don't want to mess it up..
> But knowing me, I won't be happy until I try and make it even better.:bigsmile:


Yep, I am now tempted to try to re-align the prisms too...




> Mark..that would be a major achievement if you can almost eliminate CA..
> Your lens would then be right up there with the best of them..:yes:
> 
> By cylindrical element I presume you mean, as in a circular lens..like a filter shape..
> Would this be near the projector lens..almost like an attachment?..Would it be something that could be added to your current models?


Yes I am hoping this works out too :bigsmile:

The term cylindrical actually refers to the shape of the lens in plan. From the front, the lens may be a square, rectangle or round, but when viewed from the top, the lens appears to have had a cylinder cut out from it (concave lenses) or resemble a part of a cylinder (convex lenses)... 





> That's quite an improvement over the MDF case..very nice:T
> One of the first things I did with my lens was to line the sides with black felt...It does make a difference..


Thanks 

Yes, the more light control the better. I have also tried something new. Rather than block the ends of the prisms with flocking, I am just absorbing the light from the ends inside the case. And it seems to be working a treat too...

Mark


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## Prof.

Mark Techer said:


> The term cylindrical actually refers to the shape of the lens in plan. From the front, the lens may be a square, rectangle or round, but when viewed from the top, the lens appears to have had a cylinder cut out from it (concave lenses) or resemble a part of a cylinder (convex lenses)...


Aah!..Now I'm with you..It's either a Plano-Convex or Plano-Concave lens..

I have a Fresnel lens, which is 10" x 12", and as you may know, it's a flattened version of a Plano-Convex lens..
I was going to place it against the back prism, just to see what it did, but I don't have a decent enough saw to cut it to size, without damaging the rings, so I never tried it..
I don't think the quality of those lenses would be very good either... 




> Yes, the more light control the better. I have also tried something new. Rather than block the ends of the prisms with flocking, I am just absorbing the light from the ends inside the case. And it seems to be working a treat too...


That sounds like a very good idea..

As you may recall, I had the problem of light reflection from the adhesive of flock material, which was producing the side wall reflection..
I might try that with my lens..What's the best method for removing the flock?


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## Mark Techer

Prof. said:


> Aah!..Now I'm with you..It's either a Plano-Convex or Plano-Concave lens..


That is two types, there are others...



> I have a Fresnel lens, which is 10" x 12", and as you may know, it's a flattened version of a Plano-Convex lens..


I have a very small Plano-Convex here and I could not get it to focus. The out of focus image on the screen was also reversed...



> I was going to place it against the back prism, just to see what it did, but I don't have a decent enough saw to cut it to size, without damaging the rings, so I never tried it..
> I don't think the quality of those lenses would be very good either...


Again, proviced that your in its focal length...



> As you may recall, I had the problem of light reflection from the adhesive of flock material, which was producing the side wall reflection..
> I might try that with my lens..What's the best method for removing the flock?


I rigged up the new plastic case and had to apply flocking to the ends - massive ghosting. The flocking worked really well, so then it was off to try this "corrector" which I am proud to say did work as well. It didn't really reduce CA, but I was able to make it even on each side, so the image looked better, but did notice a slightly better corner to corner focus which is very good :bigsmile:

The best way to remove the flocking is peel it off using your thumb nail. It should just then lift off, but I do think your better with it on...

Mark


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## Prof.

Mark Techer said:


> I rigged up the new plastic case and had to apply flocking to the ends - massive ghosting.


Was this the white prototype case you were sent?. I would imagine that if it wasn't black inside, you would get a lot of internal reflections and ghosting..




> The flocking worked really well, so then it was off to try this "corrector" which I am proud to say did work as well. It didn't really reduce CA, but I was able to make it even on each side, so the image looked better, but did notice a slightly better corner to corner focus which is very good :bigsmile:


Are you talking about the new case with this "corrector", or your own lens in an effort to improve the right side CA and sharpness?



> The best way to remove the flocking is peel it off using your thumb nail. It should just then lift off, but I do think your better with it on..


Hmm...I thought you said that you weren't using the end flocking on your lens anymore, since you had lined the sides with a black material?


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## Mark Techer

Prof. said:


> Was this the white prototype case you were sent?. I would imagine that if it wasn't black inside, you would get a lot of internal reflections and ghosting..


Yes, so I had to line it with black non reflective material. I used a neopreme rubber...



> Are you talking about the new case with this "corrector", or your own lens in an effort to improve the right side CA and sharpness?


Yes I am testing the new case with one possible "corrector" now. This new element seems to correct the astigmatism (widening of images as they pan across the screen) and seem to be working a treat.



> Hmm...I thought you said that you weren't using the end flocking on your lens anymore, since you had lined the sides with a black material?


Nope, too many on screen reflections without flocking the ends of the prisms, so there are done now...

Mark


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## Prof.

Mark Techer said:


> Yes I am testing the new case with one possible "corrector" now. This new element seems to correct the astigmatism (widening of images as they pan across the screen) and seem to be working a treat.


Will this "corrector" be something that can be added to your Mk.1 and Mk.11 lenses?


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## Mark Techer

Prof. said:


> Will this "corrector" be something that can be added to your Mk.1 and Mk.11 lenses?



Yes and no. The corrector will work with the prisms from the MK I and II, but you really need the new case to hold it in place...

Mark


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## Prof.

Keep one aside for me..:T


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## Prof.

Mark, I thought I would put in some screen shots, showing the CA I'm currently getting..

The first two shots are close ups of the left and right CA..
As you can see, there's quite a bit of difference between the two sides..
The next two are extreme close ups, which show the CA more clearly..

I'm hoping that the RH side pic. will be a "before shot" and that a bit later, I'll have an "after shot" that will look the same as the LH side..:daydream: :whistling:





































I tried adjusting the back prism again the other night, only I made it worse.:hissyfit:..BUT, it did show me that if I adjust the prism in the other direction, from what I had tried...theoretically, it should improve the CA..


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## Mark Techer

Because we are using two prisms of the same size and angle, the displacement should be the same for both prisms, but given the current design, this is not the easiest thing to achieve, so why I have designed the plastic case with recesses for the prisms.

You won't totally get rid of the CA because both prisms are made from the same glass type, but when aligned correctly, the CA should be symmetrical with none in the centre of the image, but getting progressively worse towards the edges...

Mark


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## Prof.

Mark Techer said:


> Because we are using two prisms of the same size and angle, the displacement should be the same for both prisms, but given the current design, this is not the easiest thing to achieve, so why I have designed the plastic case with recesses for the prisms.


That sounds like an excellent idea..So the prisms just slip into the correct position, without any further adjustment needed?



> You won't totally get rid of the CA because both prisms are made from the same glass type, but when aligned correctly, the CA should be symmetrical with none in the centre of the image, but getting progressively worse towards the edges...


I tried adjusting the back prism again last night, and did improve the RH side CA a little..

How I got the LH side CA so good was to slightly tilt the front prism inwards..
Tilting the back prism outwards slightly, helped with the RH side CA, but not to the same extent for some reason..

I might try moving the prisms a little bit further apart and see what effect that has..

Overall, the image is very good, including the small amount of CA..but being a bit of a perfectionist, I'm always looking for ways to improve things..:bigsmile:


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## Mark Techer

Prof. said:


> That sounds like an excellent idea..So the prisms just slip into the correct position, without any further adjustment needed?


Exactly, so it makes alignment easier... 



> I tried adjusting the back prism again last night, and did improve the RH side CA a little..


That is good...



> How I got the LH side CA so good was to slightly tilt the front prism inwards..
> Tilting back prism outwards slightly, helped with the RH side CA, but not to the same extent for some reason..


Sometimes a slight tilt can also help with pincushion as well... 



> I might try moving the prisms a little bit further apart and see what effect that has..


That may or may not make it difference...



> Overall, the image is very good, including the small amount of CA..but being a bit of a perfectionist, I'm always looking for ways to improve things..:bigsmile:


Aren't we all, hence the desire to add more elements...

Mark


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## Mark Techer

This is not intended as a commercial post, but I am pretty excited about the latest developments of my new MK3 lens and thought I would share. 

I think it is safe to say that I have that CA problem sorted 

Mark


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## Prof.

Excellent work Mark..:T That is good news..
Are they the same size prisms..5" x 7".?
From the photo, it looks like the front prism is the Achromatic element..
The grid pattern looks really sharp also..Very well done!
Any idea of pricing at this stage?


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## Mark Techer

Prof. said:


> Excellent work Mark..:T That is good news..
> Are they the same size prisms..5" x 7".?
> From the photo, it looks like the front prism is the Achromatic element..
> The grid pattern looks really sharp also..Very well done!
> Any idea of pricing at this stage?


Hi Prof,

Both prisms are achromatic doublets, so therefore this is a 4 element (2 bonded pairs) lens.

The prisms had to reduced in size due to the angles required for the different glass types to actually cancel the CA, or the prisms would have been HUGE and imposible to make. 

I am working on the new case this week end, and hopefully this lens will have a foot print about the same size as Blu-ray Disc case, so much smaller than the previous lens.

No pricing has been set yet, but this lens will be listed about $1900AUD. The really cool part is how nice the optically pollished faces are as opposed to the standard "trophy" finish of the MKI and II.

The image is sharper simply because the colours are converged now and not spread out like they would be with standard prisms. So white lines on a black back ground are now white lines, not white lines with blue and yellow boarders...

Mark


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## Prof.

Mark Techer said:


> Both prisms are achromatic doublets, so therefore this is a 4 element (2 bonded pairs) lens.


Wow!!..two doublets..That's getting pretty serious and right up there with the best of them..:T
Did you find a local manufacturer, or did you have to go overseas to get them made.?



> The prisms had to reduced in size due to the angles required for the different glass types to actually cancel the CA, or the prisms would have been HUGE and imposible to make.
> 
> I am working on the new case this week end, and hopefully this lens will have a foot print about the same size as Blu-ray Disc case, so much smaller than the previous lens.


That sounds like it should be a nice compact unit..
How does the weight compare to the previous models?



> No pricing has been set yet, but this lens will be listed about $1900AUD.


That's quite a jump up from the MK.11, but I'm sure it's worth every penny..
It should be an excellent alternative for the US guys..considering our de-valued dollar..


> The image is sharper simply because the colours are converged now and not spread out like they would be with standard prisms. So white lines on a black back ground are now white lines, not white lines with blue and yellow boarders...


That's probably because the lens is now closer to an optical lens, rather than a prismatic lens..

Looking forward to seeing some screenshot's from the new lens..and some closeups of the edges..not showing any CA...

Just one other thing...Has the new lens improved pincushion effect at all.?


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## Mark Techer

Prof. said:


> Wow!!..two doublets..That's getting pretty serious and right up there with the best of them..:T


Well that has been my goal from the beginning...



> That sounds like it should be a nice compact unit..
> How does the weight compare to the previous models?


The weight is less but the second glass type is densers, so the prisms still weight a fair amount...



> That's probably because the lens is now closer to an optical lens, rather than a prismatic lens..


Well it is still prismatic, just the prisms are more precise. The surface quality is like chalk and cheese compared to the MKII glass...



> Looking forward to seeing some screenshot's from the new lens..and some closeups of the edges..not showing any CA...


There is a link in the link to some from the James Bond film. I don't think CA can be totally removed as even the projectors lens adds some... 




> Just one other thing...Has the new lens improved pincushion effect at all.?


The key to reducing pincusion is to increase the TR, so some is still there, the but the amount is too small to worry about. Right now for testing, I am projecting onto a solid flat scope screen. My Curved AT Scope screen is in storage...

Mark


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## Prof.

I should have read your Blog more carefully...I missed the link to the screenshots..:R
Very nice shots..I can just see a little a bit of CA on the back of his chair in the second shot..Certainly nothing to worry about..

I must take those same shots with my lens, to see how bad my CA is. :bigsmile:..


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## Mark Techer

Prof. said:


> I should have read your Blog more carefully...I missed the link to the screenshots..:R
> Very nice shots..I can just see a little a bit of CA on the back of his chair in the second shot..Certainly nothing to worry about..


Thanks Prof. What I have found is that the BenQ's lens actually exhibits some CA itself. Since those shots were taken, I have re-aligned the prisms and clamped them in a make shift case and there is no CA to be seen. I found what appeared to be CA, but when I removed the lens, the blue and yellow line was still on screen. Because the lens optically expands the image, the lines appeared much worse and looked like CA... 



> I must take those same shots with my lens, to see how bad my CA is. :bigsmile:..


I have tried to take a screen shots using the MKII and then the MK3. The shots didn't quite work out (PC issue) and I will try again, but what a difference the new lens makes...

Mark


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## Prof.

Mark Techer said:


> What I have found is that the BenQ's lens actually exhibits some CA itself.


Mark..I'm very surprised to hear that..
A good quality multi-element, optical glass lens shouldn't have any chromatic or spherical aberrations..
By that account, it would seem that Benq (and probably others) are using very basic, low number of elements in their projector lenses..
I guess that's one of the reasons you pay big money for the top line projectors..A lot of that extra cost is for a better quality lens system..

Any idea of how many elements Benq use in their lens on the W5000.?


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## Mark Techer

Sorry Prof, I have no idea, but I do know that they changed the lenses from the W9000/W10000 range (which used the same quality optics as the infamous PE8720) when they were electroniclly controlled to a new lens for the W5000 and W20000 which is now manual zoom and focus. There were many faults reported with the first batches including CA and even visible scratches that can be seen on screen. Mine has one on the left side of the screen about an inch from the left side masking. The new 4 element anamorphic lens shows this clearly, where the previous 2 element lens did not. 

I am not sure if that should have been covered under warranty (it probably should have been) but it does not bother me unless I look for it and it is really only visible during static image like a grey field during calibration, not during actual video.

I don't intend to keep this projector that long, so I am not that worried about it at this point in time.

Mark


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## Prof.

Mark Techer said:


> There were many faults reported with the first batches including CA and even visible scratches that can be seen on screen. Mine has one on the left side of the screen about an inch from the left side masking.


Wow!! scratches!!.. That's pretty bad..It makes you wonder just what sort of quality control they have!




> The new 4 element anamorphic lens shows this clearly, where the previous 2 element lens did not.


That speaks highly for the new design..:T 




> I don't intend to keep this projector that long, so I am not that worried about it at this point in time.


Oh!..Got something else in mind?


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## Mark Techer

Prof. said:


> Wow!! scratches!!.. That's pretty bad..It makes you wonder just what sort of quality control they have!


I think QA has lifted since I bought mine as I have not heard too many complaining about the faults us early adopters found...




> That speaks highly for the new design..:T


Yes it resolves very well. The finish amazes me. I thought the previous glass was clear, but this is something else...





> Oh!..Got something else in mind?


I want to stay with DLP but we'll see what I get next. The RRP price has gone up since the Oz dollar fell, so my options may be limited. Maybe I just need a new BenQ W5000 :thud:

Mark


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## Mark Techer

Allot has happened since I last posted in this thread.

Here is a very quick update. 

I bought the BenQW5000 in early 2008. 
I went on to develop the Mk3 late 2008.
I went on to develop the Mk4 late 2009.

The images are sourced from 1080P BD using 1080 projectors. One is a 2 prism (trophy lens) and the other is from using the MK4. For those wondering why a good lens costs what it does, I think the answer is clearly shown here.


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## Prof.

Wow!..those shots are like chalk and cheese!! A massive improvement with the MK.4 lens! :T:T
You can certainly see from those shots why you have to pay a price to get that sort of quality..:sad:


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## Mark Techer

Prof. said:


> Wow!..those shots are like chalk and cheese!!


And it is not until you see them side by side (or vertically stacked as is the case here) that you understand the real nature of good optics. I was the guy that pioneered the "trophy" lens, so I've come from humble beginnings to what I have today. It is not until you have a quality lens that you actually understand the limits of everything else. 

A while ago now, a guy posted his screen caps from DIE HARD 4.0 whilst using an Mk2 lens. The shots looked great to me and I (for some reason) sent the link to a well known ISCO user. I remember taking his reply as negative at the time. The fact is, his reply was pure truth as he pointed out the limitations of a non corrected lens and the interesting thing for me was, I was oblivious to these limits because I had nothing to compare them too at the time.


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## KelvinS1965

That certainly shows the difference betweent the two lenses. I don't know if 'real' film would be even more revealing as I've usually found that animation can be more forgiving of the display (or in this case the lens). However, I'd have thought that putting up text or even just the projector menu would have shown the softness of the trophy lens, but I guess that back then expectations might have been lower.


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## Mark Techer

KelvinS1965 said:


> That certainly shows the difference betweent the two lenses. I don't know if 'real' film would be even more revealing as I've usually found that animation can be more forgiving of the display (or in this case the lens). However, I'd have thought that putting up text or even just the projector menu would have shown the softness of the trophy lens, but I guess that back then expectations might have been lower.


Text is the most demanding. Real life video seldom contains detail that is that fine.

See if you can spot the limitation on this image. Image created using MK3 with CA correction and NO astigmatism corrector.


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## KelvinS1965

Mark Techer said:


> Text is the most demanding. Real life video seldom contains detail that is that fine.
> 
> See if you can spot the limitation on this image. Image created using MK3 with CA correction and NO astigmatism corrector.


Harder to tell with this shot, but the cabling over the shoulders does look a little bit blurred/ringing, but that could be the compression used by the forum, or even the way that shot is taken/filmed. The image at the sides is out of focus anyway, so this will hide any softening by the lens, whereas a shot with the background in focus, perhaps with tree branches, hair or brickwork or similar might expose the lens' shortcomings more obviously, so that shot is a bit easier on the lens. 

My point was that it's pretty easy to check the effect of a lens just by putting up the projector's menu. If it wasn't important to have a lens that can display text sharply, then we'd all still be using trophy lenses surely...IMHO it doesn't wash that real life video doesn't need to be sharp. 

When setting up/adjusting my lens I move the projector's menu around the screen as I find it helps optimise the image sharpness. Once I can see that text is sharp over the whole screen, then I'm happy that all the detail in the film will be shown, even if it's rare to have detail as fine as text, if it's there I want to see it, not have it blured.


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## Mark Techer

KelvinS1965 said:


> Harder to tell with this shot, but the cabling over the shoulders does look a little bit blurred/ringing


Nailed it in one. I will be receiving a batch of CAVX Correctors (hopefully) tomorrow and then I will be able to re-shoot this shot with and without an astigmatism corrector. The MK4 of course resolves every bit of endo detail. But again, without being able to compare the image to a higher quality one, you may not release what is wrong. 

When I bring up the Menu on the projector using the MK4, I can clearly see the interpixel gaps on the text from my back row (3.2x the image height).


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## KelvinS1965

Mark Techer said:


> When I bring up the Menu on the projector using the MK4, I can clearly see the interpixel gaps on the text from my back row (3.2x the image height).


Wow that's sharp...though you might want to try a JVC as I can't see the gaps with or without the lens unless I'm only a few feet from the screen.


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## Mark Techer

KelvinS1965 said:


> Wow that's sharp...though you might want to try a JVC as I can't see the gaps with or without the lens unless I'm only a few feet from the screen.


I think a JVC X3 is going to be my next projector. You don't see pixel gaps in a film because there is nothing that fine.


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## KelvinS1965

Yeah, me too. Though I'll probably wait until they appear used in the classifieds on a UK forum I'm on. I may even go to an X7 if the prices drop enough (possible if JVC introduce an LED powered replacement, but unlikely I reckon).


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## Mark Techer

KelvinS1965 said:


> if JVC introduce an LED powered replacement, but unlikely I reckon


From what I have seen, only DLP chips work with LED because it is the LED engine itself that provides the both the colour as well the light. I got to see a THIS projector in action at CEDIA last year.








Very impressive stuff. Colours were over saturated of course, but it was bright and the colour definition was like nothing I've from a lamp driven device short of the 4K monster from JVC which uses a Zenon Arc Lamp, not the traditional UHP lamps. 

But at $15KUSD :rolleyesno: it would be some time before it is ever added to my cinema.

It seemed to have a good lens and it scaled for CIH too


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## KelvinS1965

These LED powered models are looking more tempting by the day: I've just read a post about the RS50 and how long throw setups may produce less brightness than previous models (ie mine). I just about get enough brightness from my HD350 so wouldn't want any less (and there seem to be too many reports about the lamp diming faster on these models.

The other option to move the projector closer is a complete non starter as I tried moving it closer today as a temporary setup to help a UK forum member to see if it would zoom big enough for his room (it did...just). I tried my Isco II in place at this (max) zoom and got severe vignetting. Even with the zoom pulled back to cure the vignetting, there was much more pincushion at this throw than I would be happy with (I have a flat electric screen) so any future projector will also have to go at the back of the room.

Even though on paper these LED models may look like they aren't as bright, I'm finding after 700 hours, calibration and long throw that I'm getting in the mid 300 lumens from my HD350. Depending on the effect of long throw on the LED models, this might still be possible and more importantly, it will remain constant for much longer.

Now if only they were about £3K...


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## Mark Techer

KelvinS1965 said:


> Even though on paper these LED models may look like they aren't as bright, I'm finding after 700 hours, calibration and long throw that I'm getting in the mid 300 lumens from my HD350. Depending on the effect of long throw on the LED models, this might still be possible and more importantly, it will remain constant for much longer.


Yeah that is what I am hearing too. That LEDs just don't drop off like UHPs do.


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