Chromatic Aberration

**Prof.** · **04-25-08, 07:30 PM**

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..

**Blaser** · **04-25-08, 08:30 PM**

lenses...

**Prof.** · **04-25-08, 09:36 PM**

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!..

Quite insignificant compared to the price of your new projector..

**Mark Techer** · **04-26-08, 10:47 AM**

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

**Blaser** · **04-26-08, 01:20 PM**

Quote:

Mark Techer wrote:

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?

**Prof.** · **04-26-08, 07:18 PM**

Quote:

Mark Techer wrote:

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.

Quote:

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..

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?

Quote:

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

That's quite an improvement over the MDF case..very nice

One of the first things I did with my lens was to line the sides with black felt...It does make a difference..

**Mark Techer** · **04-27-08, 08:01 AM**

Quote:

Prof. wrote:

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.

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

Quote:

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..

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

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)...

Quote:

That's quite an improvement over the MDF case..very nice

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

**Prof.** · **04-27-08, 07:41 PM**

Quote:

Mark Techer wrote:

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...

Quote:

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?

**Mark Techer** · **04-28-08, 03:58 AM**

Quote:

Prof. wrote:

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

That is two types, there are others...

Quote:

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...

Quote:

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...

Quote:

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

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

**Prof.** · **04-29-08, 12:16 AM**

Quote:

Mark Techer wrote:

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..

Quote:

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

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?

Quote:

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?

**Mark Techer** · **04-29-08, 09:43 AM**

Quote:

Prof. wrote:

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...

Quote:

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.

Quote:

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

**Prof.** · **04-29-08, 07:26 PM**

Quote:

Mark Techer wrote:

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?

**Mark Techer** · **04-29-08, 07:40 PM**

Quote:

Prof. wrote:

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

**Prof.** · **04-29-08, 08:04 PM**

Keep one aside for me..

**Prof.** · **04-29-08, 10:27 PM**

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..

I tried adjusting the back prism again the other night, only I made it worse.

..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..

**Mark Techer** · **04-30-08, 10:22 AM**

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

**Prof.** · **04-30-08, 06:51 PM**

Quote:

Mark Techer wrote:

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?

Quote:

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..

**Mark Techer** · **04-30-08, 10:05 PM**

[quote=Prof.;96166]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...

Quote:

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

That is good...

Quote:

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...

Quote:

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...

Quote:

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..

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

Mark

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