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Panamorph User Guide - Anamorphic Lens Technology

Anamorphic enhancement relies on the electronics of your projector (or external video processor) to show a vertically stretched image, thereby using a lot more pixels when showing content that would otherwise have bars of unused pixels above and below the image.  By using a whole lot more pixels, the image quality improves dramatically but it also becomes distorted in the process - people become tall and skinny.  The job of the anamorphic lens is to optically fix this so that the image looks normal without sacrificing the brightness and clarity advantages of those increased pixels. 

There are three fundamental design approaches to anamorphic lens design (i.e. a lens that changes the image size more in one dimension than the other): prism-pair optics, cylindrical-pair optics, and curved mirrors.  All three approaches have been fundamentally around for decades, but the mirror approach results in a product so large as to be undesirable by almost all of the market.  Consequently, the first two approaches are what you see in today's products and what we will concentrate on for the discussion. 

The basics.  An anamorphic lens is a combination of lens elements.  However, instead of bending light equally around an optical axis like a simple magnifier, anamorphic lens elements fundamentally bend light only in one direction.  Of course you can imagine that if you do something in only one direction but not the other, the two different directions of the image may have a number of things different about them besides just image size.  For example, you can certainly use a single prism to change the size of an image in one direction.  But you know from high school science that a prism splits colors into a rainbow, and that would be bad for an image.  You could also use a lens element that has a curve in only one direction, which we call a cylindrical curve.  This certainly changes the size of the image in one direction as well, but in this case it would severely blur the image in that direction.  The way we get around these issues is to use two anamorphic elements in combination.  By doing this we can substantially, although not completely, cancel out the bad effects while retaining the ability to change the image shape. 

Cylindrical-Pair Anamorphic Lens (top down view)	Prism-Pair Anamorphic Lens (side view)
The core elements of an anamorphic lens design involve a pair of lens elements which operate only in the direction of anamorphic magnification.  The blue rays are those modified by the lens elements.  The gray rays show the unmodified beam.  Both approaches can be used to either expand or compress an image in one direction.

The residual imperfections not fully cancelled by the anamorphic pair of elements are called aberrations and the most significant of these include astigmatism, lateral chromatic aberration and distortion.  But first it's very important to realize that we have two ways to create a 2.35:1 image from 16:9 - Horizontal Expansion (HE) or Vertical Compression (VC) - and there is a significant fundamental difference between these approaches when it comes to lens design.

Anamorphic lenses are typically set up so that the center of the image looks the best while residual aberrations typically increase smoothly as you move away from the image center.  What's interesting about anamorphic lenses is that for the most part image aberrations happen in the direction of anamorphic change.  If we create the 2.35:1 aspect ratio by horizontally expanding the image, image quality generally gets worse as we venture left and right but it stays relatively the same from top to bottom.  Likewise, if we create the 2.35:1 aspect ratio by vertically compressing the image, we will keep image quality about the same across the image while diminishing it as we move vertically away from the image center. 

Now here's the thing.  Wide screen images are by definition wider than they are tall.  The fact that you have to move much farther from the center of the image to reach the left or right side means that aberrations are generally more pronounced there with a horizontal expansion lens than they will be at the top or bottom of the image created with a vertical compression lens.  Consequently, a VC lens does not have to work as hard to produce high image quality as a horizontal stretch lens does.  This mean fewer elements are required for a VC lens compared to an HE lens for about the same image quality, dramatically decreasing the cost.  With this in mind, on to the aberrations themselves.

Astigmatism is the inability of the image to come to focus in both vertical and horizontal image directions at the same screen distance and is arguably the most critical aberration in anamorphic lens design because astigmatism is the only primary aberration that reduces image clarity.

Historically, both cylindrical and prism design approaches reduce astigmatism by adding an additional set of regular “corrector” lens elements, one each before and after the pair of elements.  You see, astigmatism happens mostly when the light beam is already traveling to a focus, such as the beam from a projector being focused onto a projection screen.  If the light going through an anamorphic pair is modified so that it only comes to an image at an infinite distance away then astigmatism becomes greatly diminished.  So imagine two extra lenses in the simple layout diagrams above.  A first regular lens element would neutralize the focusing properties of the projector beam entering the anamorphic pair (making it focus at infinity) while a last regular lens element would restore the focus to form an image on the screen.  Of course, since the focal properties of the beam coming from the projector depend on the distance from projector lens to the screen (which varies in every installation), the ideal lens must have corrector lenses “tunable” to the throw distance.  The more expensive lenses indeed have such features, while in simpler corrected designs the corrector lenses are based on an assumed range of throw distances around a "sweet spot".  In addition, since the prime lens of the projector itself can be adjusted for infinite focus, to a large degree the astigmatism can be practically reduced by having only a single corrector at the exit (on the screen side) of the lens, which is fine as long as you don't need to move the lens out of the way.

Interestingly, prism-pair anamorphic lenses are less sensitive to astigmatism (and other less-dominant "blurring" aberrations) than cylindrical-pair lenses because the flat surfaces of the prisms create a more pure anamorphic bending of the light than the curved surfaces of cylindrical elements.  This means that once corrected for a single focal distance, a prism-pair lens still performs well throughout a relatively broad range of focal distances around that design distance.  In addition, this relative insensitivity combined with other optical properties results in a corrected prism-pair anamorphic lens producing a sharper image than a cylindrical-pair lens corrected for the same distance.  Consequently, these two advantages combine so that a prism-pair anamorphic lens properly corrected for astigmatism at one specific throw distance will generally outperform a tunable cylindrical-pair lens throughout a particular throw distance range. 

Based on the above strengths of a corrected, prism-based anamorphic lens, we took a close look at the actual cause for this aberration to see if, instead of making an infinite-focus beam, we could just cancel out the astigmatism in the design altogether.  The result is a patented hybrid design that includes both prism elements AND cylindrical lens elements.  The next breakthrough was to integrate these features into single elements to dramatically reduce the cost and size of a corrected lens, producing the exceptional quality and value that have made Panamorph lenses so popular.

Distortion refers to any geometric imperfection from the ideal image and has nothing to do with image clarity.  In most cases high quality anamorphic lenses do a very good job of making content look proper.  The most noticeable distortion effect is a slight bowing of the edges of the image.  Compression lenses will make the corners bow inward ("barrel" distortion).  Expansion lenses will make the corners bow outward ("pincushion" distortion).  In most cases this bowing is typically less than one percent of the size of the image (about 1.5% at the shortest throw ratios.  Consequently, this slight bowing of the image can be eliminated by slightly spilling or "overscanning" the image onto the screen border.  There are also internal distortion issues which are called "nonlinearities".  For example, a grid pattern of ideally even rectangles can exhibit nonlinearity by showing the center rectangles narrower than the rectangles out at the right and left edges.  While both designs have their relative strengths and weaknesses regarding distortion, the fact is that with quality lenses these things are only noticeable if you have a throw ratio (throw distance divided by native image width) less than about 1.7 and they all but disappear at 2.0, especially with video content.  And if you must install a system with a short throw ratio and you are using an expansion lens, you can always use a curved projection screen because the natural barrel distortion of the screen can partially cancel any residual pincushion distortion from the lens.

Lateral Chromatic Aberration (LCA) refers to the slight separation of the color components making up the image.  A properly designed and installed anamorphic lens will show zero LCA in the center of the image and any residual LCA will increase toward the edges primarily in the axis of anamorphic modification, with very little LCA in the other axis.  Chromatic aberrations can be corrected by adding additional optical elements into the design.  Of course, this makes the lens assembly larger, heavier and more expensive.  Consequently, there a few other tricks we can play to provide products small and affordable while maintaining high performance.

Since LCA happens in the axis of anamorphic modification, and since it increases with distance away from the image center, anamorphic modification in the vertical direction produces much less visible LCA in the image than modification in the horizontal axis because the native image extends much less from the center in the vertical axis.  Consequently, LCA at the top and bottom of a vertically compressed wide-screen image is much less than LCA at the left and right edges of a horizontally stretched wide-screen image, so much so that adding additional elements to do any final correction to a VC lens would add little extra value to the product relative to the additional cost. 

Unfortunately, LCA in the horizontal stretch case is so visible that some type of color correction is mandatory and all of today's popular HE lenses require extra lens elements to compensate for this (which is why HE lenses are so much more expensive than VC lenses).  However, we have another trick we can play as well.  LCA can be imagined as slight geometric scaling of each color component making up the image relative to some base color such as, for example, green.  If the projector (or external scaler) can slightly vary the stretch of each color to compensate for the optical stretch on the screen, the remaining color aberration can be neutralized to within one half of a pixel.  This patented technology has already been demonstrated.  However, actual implementation into projectors and scalers is something only the market can decide.  While the minor residual LCA of current vertical compression lenses is not enough to warrant this capability, it is possible that growth in the demand of a low-cost, high performance HE lens may provide enough motivation to develop it.

"Pass-through" versus moveable lenses for multi-aspect systems.  For those insisting on maximum brightness and resolution for both primary wide screen formats the obvious approach is to move a high performance,  2.35:1-designed anamorphic lens out of the 16:9 projector beam for non-2.35:1 content (in fairness, we note that the other "purist" approach of leaving the lens in place and using electronic scaling for all aspect ratios is just as valid for other reasons). 

There remains an alternative approach dating back to the original Panavision "Panatar" prism-based anamorphic lens from the early 1950s where the internal elements can be adjusted to produce a different aspect ratio - all the way to a "pass-through" condition that maintains the original aspect ratio of the projector.  The benefit of this is a somewhat more compact multi-aspect imaging system since space and additional hardware are not required for the "lens out" position.  However, the trade off is a sacrifice in image quality for both aspect ratios because the design must necessarily be a compromise over a range of element orientations inside the lens.  For example, astigmatism varies with anamorphic magnification, requiring either a) a costly variable astigmatic correction based on fundamental Panamorph technologies or b) the historical and lower quality approach of creating an infinity-focused beam inside the prism pair system along with refocusing optics.  In addition, chromatic aberrations must be optimized for all aspect ratio settings as well.  Because of these trade offs, pass-through lens designs are not practiced by leading lens manufacturers since the entire purpose of a multi-aspect system is to provide the best performance for both wide screen formats.  While a moveable lens optimized for 2.35:1 is the best solution for multiaspect ratio imaging, the next-best alternative is actually to use a fixed, high performance lens with electronic scaling for all aspect ratios since the higher pixel count of a pass-through lens design is offset by the optical degradation of such a lens.