Figure 1. Shown here is the same degree of pincushion (a positive geometric distortion) for a 4:3 aspect ratio image and a 16:9 aspect ratio image. The percentage of distortion is visibly greater in the widescreen image. Click here to see an enlarged diagram.
Despite their excellent performance, however, these latest HD lenses are all still challenged by multiple optical impairments that can be collectively minimized but never totally overcome. Optical impairments generally fall into two categories:
- distortions to the optical image (in shape and size); and
- aberrations, which are image defects associated with the fundamental behavior of light rays passing through a lens element.
Figure 2. Light input to a lens element, consisting of an array of infinitely small point light sources that will stimulate the lens focusing aberrations. Click here to see an enlarged diagram.
Optical image distortions
Optical image distortions Distortions are an inherent part of all lenses; they relate to the unwanted alterations to the overall size and shape of the image created by the lens.
Geometric distortion Geometric distortion alters the accuracy of the geometric representation of an object scene in the optical image plane. A distortion of the image shape — described as a pincushion effect (positive geometric distortion) or a barrel effect (negative geometric distortion) — and the amount of that distortion is generally expressed as a percentage of the picture height as outlined in Figure 1.
Typically, a zoom lens will exhibit barrel distortion at the short focal length (that is, at the wide angle) and pincushion distortion toward the telephoto extreme. In addition, the wider 16:9 aspect ratio of HDTV imposes a more severe challenge to managing geometric distortion over the entire zoom range. Within the 2/3in image format size, a 1.5 percent level of pincushion or barrel distortion in a 4:3 image translates into almost 2.2 percent in the 16:9 format. This exacerbation of geometric image distortion in 16:9 widescreen is illustrated in Figure 1.
Focus Breathing Focus breathing is not strictly classified as a distortion. In the eyes of practitioners, however, it behaves as an image distortion. The term refers to the phenomenon of the change in image size when operating the focus control. It is an unwanted alteration in picture angle of view that is a consequence of moving optical elements during focusing (an undesired result of zooming). While traditionally accepted in ENG shooting, it can be totally unacceptable in high-end drama and movie shooting.
Optical image aberrations
Figure 3. Spherical aberration (exaggerated for visibility) at the lens output. Click here to see an enlarged diagram.
Aberrations that are independent of wavelength Long ago it was mathematically predicted that there are four aberrations associated with the passage of monochromatic (single wavelength) light passing through a single lens element. The geometric distortion described earlier is also an additional element within this prediction. If a fine point of monochromatic light is sent to a single lens element, it will emerge with some degree of all of the four defocusing impairments described below.
- Spherical aberration: Spherical aberration relates to an aberration common to all lenses made up of spherical elements. It manifests itself as a differential focusing, with those rays (emanating from a physical point on the axis of the lens) that pass through the outer edges of the lens, converging at a focal point that is closer to the lens than those rays that pass through the central optical axis of the lens. Forming doublets of lens elements having equal and opposite spherical aberrations can considerably reduce spherical aberration. Recent spherical lens element designs (a significant manufacturing challenge) have also contributed to reducing this problem.
- Coma: When a lens is completely corrected for spherical distortion it may still create other forms of aberration on points in the scene that are off the central axis. If the rays from a point object are incident at an angle to this central optical axis, they exhibit a comet-like tail instead of forming a focused point. Such distortion is called coma or comatic aberration. The blur sometimes seen near the edges of the tail is often termed a comatic flare.
Figure 4. Comatic flare (exaggerated for visibility). Click here to see an enlarged diagram.
- Curvature of field: Curvature of field relates to a defocusing phenomenon where the lens fails to focus a plane scene object as a plane optical image. When the center of that object plane is sharply focused, the edges are out of focus. Conversely if the edges are symmetrically focused, the center of the image is out of focus. The lens, in effect, is forming the optical image in a bowl-like shape.
- Astigmatism: A lens that has been corrected for spherical distortion sometimes will still not properly focus an off-axis point in the scene. Instead, the point adopts an elliptical shape or becomes a line in the lens-created image. Adjusting the lens focus, the image can change between two ellipses at right angles to each other — failing to achieve a sharp point focus.
Aberrations of light transmission
Figure 5. Note the progressive center-to-edge defocusing associated with curvature of field. Click here to see an enlarged diagram.
Different wavelengths of light encounter a different index of refraction within a given optical material. The phenomenon is referred to as dispersion. A single lens element will accordingly form a number of images — one for each color present in the light beam. These are technically described by two separately defined aberrations: a longitudinal chromatic aberration (different focus plane for constituent colors) and a lateral chromatic aberration (the fact that the focal length of colored light rays vary causes an associated variation in the lateral magnification, which produces an effective misregistration). Because this is an important and challenging topic within HDTV imaging, it will be separately dealt with in some depth in our next article in this series.
Figure 6. Exaggerated case of lens astigmatism. Click here to see an enlarged diagram.
As that light passes through the lens, it is subject to a variety of impairments that can contaminate the darker portions of the optical image. Such contamination of the black portions of an image can severely detract from the contrast performance of the lens, which, in turn, impairs the lens MTF and consequently, the overall picture sharpness. The overall optical image can also be impaired with unique optical interferences created by the passage of strong light sources from the object scene.
These transmission impairments are, in a sense, an optical “signal-to-noise” issue. Just as the design of a digital HD camera system struggles to enhance all of those attributes that collectively ensure the highest quality video output while simultaneously attempting to submerge noise and other electronic artifacts (such as aliasing, shading, flare, etc.), so too, a similar technological struggle is inherent in the design of an HD lens system. Here, the collective of all of these light-transmission aberrations is analogous to the optical “noise” of the system.
As discussed in our previous article, additional degrees of freedom in managing the multiple variables inherent in lens design are facilitated if more lens elements are used. This does, however, increase the number of air-to-glass surfaces and this increases the total amount of reflected light from these boundaries. This reflected light will impact the camera image sensors in a variety of forms — as discussed below.
- Flare: Flare is an impairment that can arise from strong highlights in the scene and is most obviously manifested in dark regions of the image. The highlights can interact with the multiple optical surfaces within the zoom lens, causing light to spread around sharply defined highlights. These highlights may even originate from regions of the object scene that lie outside of the specific picture being imaged. The highlights may also reflect off the interior lens barrel, or the iris blades, or even off the camera sensor itself. The advent of exotic multi-coatings on each lens element has provided a powerful means of alleviating these impairments. These coatings are thin transparent film materials deposited on the optical surfaces to reduce reflection and to increase transmittance. Multi-layer coatings in high-performance have become more popular because of their increased protection. High-quality optical materials, extremely tight control of manufacturing tolerances, and carefully blackened internal mechanical surfaces must all be mobilized to lower this aberration.
- Veiling glare: Veiling glare is an unwanted diffuse stray light that can abruptly “fog” across the image plane under certain imaging conditions, consequently detracting from the image contrast. Scatter from optical element surfaces (usually the front elements), or reflections from the surfaces of iris shutter blades and the internal surface of the lens barrel causes veiling glare. Again, precision optical design, special materials, exotic coatings, and tight manufacturing tolerances are all harnessed to control such interference. The appropriate use of a lens hood when shooting in strong sunlight is also an important protection against the possibility of out-of-image strong light interference.
- Ghosting: Ghosting refers to the creation of a sharply defined reflection — caused by the sun or other unusually strong light source in the scene — that can stimulate a complex series of reflections among the multiple lens surfaces. It often manifests itself as an image of the mechanical diaphragm that constitutes the iris control and it usually appears in a position symmetrically opposite that light source (thus, manifesting a ghostlike appearance). In severe cases it can show up as multiple such images. Depending upon the lens, these images may be monochrome or separate colored reproductions. The ghost image can be introduced by the highlight source even when it is outside the picture area. To reduce such impairments special coatings are applied to lens surfaces.
Summary By now it is recognized that the nature of the multi-element HDTV lens system is a far more complicated and unyielding system than a modern digital HD camera. The digital camera is quite technically disciplined (by virtue of precision fixed sensor structures and precision digital processing). In comparison, the lens is —by the very nature of optical physics — highly dynamic in its management of many parameters when dealing with a wide range of light levels and a wide range of focal lengths. As a consequence, design optimization strategies must vary considerably between categories of HD lenses (such as the larger long-zoom field lens, studio lens, and the portable EFP, ENG, and cine lenses). The specific application of these lenses determines the priorities assigned by the designers as they seek the best performance for each category.
Our next article will remain on the subject of optical lens aberrations and will examine perhaps the most vexing aberration of them all — the chromatic aberrations. These stem from the most fundamental of optical physics associated with any transparent material. Managing them to an acceptable level within the physical dynamics of a zoom lens has tasked optical designers for decades. In the context of the highly challenging imaging goals of HDTV — within the small 2/3in image format — the chromatic aberrations remain the most daunting image impairment of them all.
Larry Thorpe is the national marketing executive and Gordon Tubbs is the assistant director of the Canon Broadcast & Communications Division.