HDTV lens design: Management of MTF
Pairing an HDTV lens to an SDTV camera will enhance the performance of that imaging system. Click here to see an enlarged diagram.
In the previous paper, “HDTV Lenses, MTF and Picture Sharpness,” Broadcast Engineering, January 2005, we related traditional discussions of lens/camera resolution to the more pertinent issue of the picture sharpness perceived when viewing a display from some distance (as is typical in both television and cinema viewing). We briefly reviewed the all-important concept of Modulation Transfer Function (MTF), its influences on picture sharpness and the optical design challenge of obtaining as even a distribution of MTF as possible across an image plane. As a reminder, MTF is a curve that describes the behavior of the contrast of increasing spatial frequencies across the total frequency band of the HDTV system.
The intent of this month's paper is twofold:
The HDTV challenge to lens/camera design
To discuss the technological challenges entailed in achieving the requisite MTF characteristic required by an HDTV lens in comparison with its SDTV counterpart.
To emphasize that lens design strategies can be quite different among manufacturers and that careful evaluation of lens-resolution characteristics may be far more important than evaluating camera resolution.
There are four significant challenges in extending lens-resolution performance from SDTV to HDTV:
Elevating the in-band MTF and then extending this to the higher spatial frequencies required for HDTV.
Maintaining that high MTF over the focal range of the HDTV lens.
Controlling the MTF as the object distance from the lens is changed.
Managing the lens MTF as the aperture is altered for different scene lighting levels.
The digital HDTV video signal has the potential to sustain up to six times more spatial detail than the best 4:2:2 digital 4:3 SDTV video signal (as defined within contemporary HDTV production standards). To fully exploit that tremendous increase in picture information, the lens must be capable of imaging and delivering sufficient spatial detail to the HDTV camera in order to satisfy its formidable digital information capacity.
Figure 1. MTF curves for contemporary HDTV and SDTV studio lenses. This diagram highlights the challenge to lens designers to significantly raise the MTF of HDTV lenses. Click here to see an enlarged diagram.
In the SDTV domain, if a lens/camera system combines a lens having the ability to resolve 30 line-pairs per millimeter (LP/mm) and the 480-line camera (having a 4:3 aspect ratio and operating at 30 interlaced frames/sec) has a bandwidth of 5.0MHz, then that imaging system can resolve a maximum of 400 television lines per picture height (TVL/ph). A modern high-end SDTV studio lens would be expected to exhibit a 90 percent MTF at 30LP/mm. This would typically fall to about 70 percent at 56TVL/ph (the higher HDTV optical reference frequency) and continue to fall off more quickly at higher spatial frequencies.
Figure 2. Comparative MTF behavior of an HDTV and SDTV lens across the image plane. Click here to see an enlarged diagram.
In the HDTV domain, the 1080-line HDTV system is far more demanding. It is sampling 1080 lines at 30 frames/sec and has 1920 horizontal samples per television line, and is — as a consequence — a far faster sampling system that devours bandwidth. If such a lens/camera system has a lens that can resolve 84LP/mm and the HD camera has a bandwidth of 30MHz, then the maximum horizontal resolving power of that imaging system is 875TVL/ph.
In the following examination, we will compare the MTF behavior of a typical SDTV studio lens in the higher HDTV spatial frequencies with that of a contemporary HDTV studio lens. It is recognized that the former was not designed to meet the aspirations of HDTV imaging, but this comparison does serve to effectively dramatize the challenge to the optical designers aspiring to reach the best in HDTV lens performance.
The first comparison is of a typical high-end HDTV studio lens MTF with that of a current high-end SDTV studio lens. (See Figure 1).
As discussed in the previous paper, maintaining a high MTF from picture center to the image plane extremities is one of the primary challenges in HDTV lens design. New optical materials, new element design techniques, and strategic element groupings all have combined to facilitate significant lens MTF improvements.
Managing MTF across the image plane
Figure 2 shows the relative performance of a high-performance SDTV studio lens and a typical HDTV studio lens (measured at the HDTV optical reference spatial frequency of 56LP/mm, which is about 580TVL/ph for the 1080/60i system). Note: The picture center, middle and corner dimensional positions are referenced in Figure 5 in our previous paper.
Figure 3: MTF variations over focal length range of SDTV and HDTV studio lenses. Click here to see an enlarged diagram.
Lens resolution is always changing in response to any and all optical controls that are exercised in the course of a normal production. Lens MTF will alter when:
The lens focal length is changed (when exercising the zoom control).
The distance of an object from the lens is varied (and the lens is refocused on that object) for a specific angle of view (fixed setting of the zoom control).
The lens aperture is altered (when exercising the lens iris control to restore video level with scene lighting changes or to alter depth of field in a given scene).
The dynamic behavior of HDTV lens MTF
Each of these will be briefly examined in turn.
The digital HDTV video signal has the Exercising the zoom control in a broadcast lens involves precise relative movement of two groups of lens elements. This has an inevitable impact on a variety of lens imaging characteristics — and on lens aberrations. One impact is on the lens MTF. Because of the far greater clarity of the HDTV picture, it is necessary to institute a much tighter control over this variation. Figure 3 outlines the relative MTF variation of a typical SDTV and HDTV studio lens — measured at a reasonably high spatial frequency of 56LP/mm (a popular optical reference frequency) — being approximately 600TVL/ph for the 1080/60i system and roughly 500TVL/ph for the 720/60P HDTV system.
Figure 4. This illustrates the significant improvement that had to be made to the behavior of the HDTV MTF when refocusing as a subject moves from one position to another. Click here to see an enlarged diagram.
Studio production regularly entails specific choices in framing for each shot (requiring appropriate adjustment of the zoom control to achieve a specific angle of view) and subsequent talent activity within that framing. For example, a principal actor may be 6ft from the lens (and sharply focused) — but then may move away to 25ft from the lens (and is refocused — with no alteration of zoom or iris). Or, the camera operator might “rack focus” between two actors separated by such a distance or greater. Lenses will change their MTF between those two focus settings — again, a consequence of lens elements physically moving during a focusing action.
Figure 5. The variation in MTF across the image plane at a singular setting of the iris (for different focal lengths) — shown here at the aperture of f-4.0 common to broadcast television studio operation. Click here to see an enlarged diagram.
Figure 4 illustrates, by way of a reference, the behavior of a high-performance SDTV studio lens at the higher spatial frequencies (where SDTV/NTSC design really does not need to concern itself). But, because of the high sharpness expectations of HDTV, the HDTV lens does need to pay close attention to MTF behavior in this critical spatial frequency region of 56LP/mm (or roughly 600TVL/ph for the 1080-line HDTV system).
Maintaining high MTF over the focal range
Optical science is confronted by some fundamental physics, and part of that is diffraction. Diffraction is a consequence of the wave nature of light and has the effect of modifying an infinitely small point source of light by “spreading” it as it passes through an optical element.
Maintaining MTF with changes in object distance
In a perfect lens, which would be diffraction-limited, MTF would be at a maximum when wide open. This would progressively lower as the lens aperture is stopped down. In the real world, however, lens designers use a variety of optical techniques to manage the linkage between MTF and aperture setting. Depending on the application of the lens (such as studio, field, ENG or cine) designers will optimize its overall MTF profiles (while also paying close attention to MTF changes with focal length) to be at their very best at the anticipated lens aperture settings for those applications. Broadcast studio lenses, for example, are generally operated in the vicinity of f-4.0, and consequently the design may optimize MTF at that aperture and at the medium focal length that is more common — allowing it to lower (to a modest degree) when the lens aperture is wide open. Long telephoto field lenses will have different optimization criteria. Cine lenses will have yet different criteria.
Figure 6. The variation in MTF across the image plane as the lens aperture is fully opened. Click here to see an enlarged diagram.
Figures 5 and 6 show a typical HDTV studio lens and how its MTF profiles vary between two aperture settings. The MTF is measured at 56LP/mm (again in the vicinity of 600TVL/ph for the 1080-line system), and its variations across the image plane at three different focal lengths are shown. Figure 5 shows the characteristics when the aperture is modestly stopped down to f-4.0. Figure 6 shows the alterations to those same profiles when the lens iris is opened wide. Note the favoring of the wider angle at this setting (because this is where the scene detail is at its highest).
The effect of aperture settings on lens MTF
It should be apparent that management of MTF in an HDTV zoom lens is a Herculean technical task. Yet, marvelous technical progress has taken place over the past decade. The HDTV lens today exhibits extraordinarily high quality despite the multifaceted optical challenges within this enormously complex system.
It should also be clear from the imaging characteristics shown in Figures 1 through 4 that coupling an HDTV lens to an SDTV camera will significantly enhance the performance of that imaging system. Overall, SDTV picture sharpness will be visibly enhanced. This means that an investment in HDTV lenses today is worthwhile — even if you don't plan to transition to an HDTV camera for some years to come.
Visit the following links to read Part I and Part III of this series:
Part I: HDTV lenses, MTF and picture sharpness
Part III: HDTV lens design: Management of light transmission
Larry Thorpe is the national marketing executive and Gordon Tubbs is the assistant director of the Canon Broadcast & Communications Division.