# Throw Ratios and Corrective Elements



## Mark Techer (Jan 17, 2008)

Generally we know that in oder to find our TR (throw ratio) we divide the native 16:9 image width into the distance from the screen to the projector's lens. However, those using prisms based anamorphic lenses with correction elements need to be aware that just because you have the right TR, your corrective element may not allow the lens to resolve pixel level clarity.

I'll provide examples to demonstrate. 

Lets say we choose a TR of 2.0 when using a massive 14' screen. The 16:9 portion is simply 14.0 x 0.75 = 10.5'. 10.5 x 2.0 gives us a projection distance of 21'.

Now lets say we only had 8' screen (same projector, same lens) and we still wanted out TR of 2.0.

8' x 0.75 = 6' and 6 x 2 = 12'.

The TR is still 2.0, but 12' is a far cry from 21'.

Now here is a problem. Fixed focal corrective elements are designed for a given focal length. This means that they are designed to be used at set distances and not just TRs.

If a corrective element was designed for 17', it is not going to give optimal results at either example above even though the TRs for both are the same at 2.0:1.

Comments and feedback welcome...

Mark


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## Prof. (Oct 20, 2006)

That's very interesting Mark..
So any lens that uses corrective elements, is limited to a certain projection distance!..
One would have to be very sure that they have the correct element, otherwise the image is not going to very sharp..


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## Mark Techer (Jan 17, 2008)

Or as sharp as it could be. They still work, but not quite as intended. There has been a few Panamorph owners that has brought up focus issues with their set ups on another site, and when looking into it, it is not the lens as fault, but lack of understanding from the owner or installer. 

Therefore the point here is to try and make a few aware that they really need to ask the right questions before they make a purchase, or they are going to come away with a sour taste in their mouth... 

Mark


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## Mark Techer (Jan 17, 2008)

It has been a wile since I posted here and whilst going through the threads, though I might update those that are active readers.

If the mods object to the nature of this posts, please feel free to remove them. 

Given that this thread was about the Correction element, I'd like to share the success I have had with the CAVX Corrector. The CAVX Corrector is a single cylindrcial lens that sits in the light path just behind the rear prism of the MK3. When in place, I rename the MK3 the MK3+C which now makes the unit a 5 element anamorphic lens. 

So whilst it does correct astigmatism, the curvature is not enough to correct grid distortion, however I have a plan for that in the pipe line, though I can't post about that just yet... 

So for those that are not sure why the CAVX Corrector was created, basically, prism can only be focused in one plain at one time (generally a prism pair pass the horizontal lines and blur the vertical), so when you add a prismatic lens without additional focal correction, you essentially have to rock the focus of the projector to get the image to what is considered "back in focus". However this does not work as well as it should because of the limitations of the prisms themselves, so rather than getting both H and V lines perfectly in focus, you actually defocus the H lines to get the V line more in focus. 

So the corrector helps solve that issue by assisting with the focus of the V lines whilst the prisms pass the H lines. This then allows the image to be projected without having to rock the focus. Because it is a single lens, it is limited to a select range. In testing and and in the few practical installations I've been able to see, the 'sweet' spot for this corrector seems to be about 5.5m. 

Mark


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## Mark Techer (Jan 17, 2008)

I just got back from installing an MK3+C and thought I needed to share this screen cap. 









Not too bad for a single lens solution.


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## Prof. (Oct 20, 2006)

Mark,

Could you explain what a Mk.3+C is for those who are not familiar with your lenses..


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## Mark Techer (Jan 17, 2008)

Prof. said:


> Mark,
> 
> Could you explain what a Mk.3+C is for those who are not familiar with your lenses..


Sure. In its most simple terms, the CAVX Corrector is a cylindrical (it has to be such to prevent the height from changing) lens that assists with the focus of the vertical lines. 

All HE anamorphic lenses naturally pass the horizontal lines. This is because their vertical magnification power is 0.00x. Horizontally, the HE lens is designed to magnify the image, so as a result, the vertical lines tend to be affected. In video, horizontal lines are actually the vertical rez and vertical lines are actually the horizontal rez. Therefore each pixel is affected to a degree. Top/bottom is unaffected, left and right sides become blurred. 

The MK4 has lenses that can focus light in both the H and V direction at the same time. The key is to find the exact air gap between these lenses to make an infinite focal point. Once found, the lens passes both H and V lines and why the images created by this lens are so sharp.

Prisms on the other hand can only focus in one direction at one time. As mentioned above, typically H lines are unaffected. However because V lines are, we tend to rock the focus to bring both into a more uniform form of focus. In other words, we de-focus the H to bring the V more into focus.

So by adding a focal element, we can have a better overall image. The additional lens is designed to do the work the prisms can't. 

The limit is that the corrective lens is a single lens and has no other lens to work with, hence it has a sweet spot and if your in the sweet spot, all good and if your not, then the lens may cause the image to worse than without. 

As the image shows, my correction lens sits behind the prism pair.


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