HDTV lenses, MTF and picture sharpness

Rightly or wrongly, manufacturers of broadcast studio cameras and lenses decided long ago to radically simplify published specifications. This has probably saved the sanity of many a chief engineer perusing competitive specification sheets. But it has also obscured intractable realities that lens and camera designers must still confront.

Manufacturers of modern television cameras traditionally offer a horizontal-resolution specification and, sometimes, a separate specification for vertical resolution (with little correlation between the two) to describe the lens' contribution to picture sharpness. This is a legacy of the “specmanship” long practiced by camera manufacturers. For example, HD camera manufacturers typically specify a depth of modulation at a reference spatial frequency of 800 TV lines per picture height (TVL/ph) or 27.5MHz (for the 1080-line system). Some manufacturers separately quote a horizontal limiting resolution — the highest horizontal spatial frequency at which the depth of modulation is at least 5 percent.

Modulation transfer function

These published specification numbers are important in establishing a simple way to determine whether a given HD camera meets its resolution performance specification. But the numbers tell little about a camera's picture-sharpness performance. Long ago, engineers established that visual picture sharpness must be correlated with what is called the modulation transfer function of the lens-camera system.

A typical multiburst resolution chart contains groups of vertical black-to-white “picket fence” bars and lines that challenge the spatial frequency response of lens-camera systems. Thick, widely-spaced bars represent low spatial frequencies, while thin, closely spaced lines represent high spatial frequencies. When aimed at such a resolution chart, a modern lens-camera system can easily reproduce the full contrast of the bars that represent low spatial frequencies of about 50TVL/ph. The resulting full-amplitude video signal level acts as the reference contrast level.

Figure 1. MTF represents the change in the contrast level produced by an imaging sytem in response to a range of spatial frequencies as they pass through the system. Click here to see an enlarged diagram.

Lens-camera MTF and picture sharpness

For the higher-frequency lines on the chart, the system reproduces lower contrast levels. When aimed at a multiburst chart that contains frequencies ranging from the 50TVL/ph reference all the way up to, say, 1000TVL/ph, the lens-camera system produces a corresponding video envelope on a waveform monitor. It is as if the lens-camera system is modulating the contrast level over this frequency range. A graph can represent this change in reproduced contrast vs. frequency with spatial frequency on the horizontal axis and contrast on the vertical axis. (When expressing spatial frequency, video engineers prefer TVL/ph; optical engineers prefer line-pairs per millimeter.) Such a graph represents a form of transfer function for the contrast modulation, and is, therefore, called the modulation transfer function (MTF) of the lens-camera system. (See Figure 1.)

The MTF concept is one of the seminal works in the science of imaging. The most important result of that work was the revelation that visual picture sharpness for any system involving distant viewing (such as television or cinema) is proportional to the square of the area under the MTF curve. The implication is that the shape of the MTF curve over the useful passband of the camera is of vital importance to perceived picture sharpness. Indeed, it is much more important than the limiting-resolution specification.

Figure 2. This MTF curve for a generic 1080-line HDTV lens-camera system emphasizes the importance of an HD lens design that optimizes the MTF over the critical 200- to 600TVL/ph range. Click here to see an enlarged diagram.

Because the lens-camera system comprises two distinct components, the picture sharpness of any HD lens-camera system is ultimately determined by the shape of the lens' MTF curve multiplied by the shape of the camera's MTF curve. The shape of that composite MTF curve below 800TVL/ph — in the all-important range of 200TVL/ph to 600TVL/ph — is, in fact, the best objective way to determine the visual picture sharpness of a lens-camera system. Studio-lens designers must always consider this and include optical innovations that enhance the lens' ability to reproduce contrast over this spatial frequency range. (See Figure 2.) Because MTF is all about spatial-frequency contrast levels, it is important to note that the lens' inherent optical contrast performance (the degree to which the lens can distinguish between different brightness levels and its ability to reproduce a true black with no light contamination) is inextricably bound up in the lens-camera sytem's overall picture-sharpness performance.

Figure 3. Here, the MTF characteristic of a typical 1080-line HD camera combined with a typical HD lens is measured at picture center. Generally, manufacturers only publish this specification at the reference 800TVL/ph spatial frequency. Click here to see an enlarged diagram.

HD lenses do not distinguish among different HDTV production standards. They are all high-definition lenses, so similar optical MTF criteria apply to all of them. Figure 3 shows the MTF curve of a typical 1920×1080 HDTV studio camera operating with a typical high-performance HD studio lens. Contemporary 1080-line HD cameras generally claim to reproduce a depth of modulation in the 40 percent to 45 percent range at the accepted reference spatial frequency of 800TVL/ph when used with a “typical” HDTV lens. But these camera specifications make no reference whatever to the all-important depth of modulation at spatial frequencies of 200TVL/ph, 400TVL/ph or 600TVL/ph.

1080- and 720-line HDTV cameras

The attenuation in a 720/60p system is less severe than in 1080i systems because of the tradeoff of spatial resolution for enhanced temporal resolution. As Figure 4 shows, the lens' MTF is high across the horizontal passband of this HD system, which is why it typically achieves a 50 percent depth of modulation at the reference spatial frequency of 530TVL/ph (or 27.5MHz in the video domain). This is one reason why 720-line 60p systems exhibit good subjective picture sharpness.

Figure 4. This graph shows the MTF characteristic of a 720/60p HD camera in combination with a typical HD lens measured at picture center. Generally, man-ufacturers only publish this specification at the reference 530TVL/ph spatial frequency. Click here to see an enlarged diagram.

You can't accurately establish the actual picture performance of an HD camera without including its associated HD lens. And, here, the technical plot thickens considerably. An HD camera's resolution performance remains essentially constant all over the picture raster. It is irrevocably determined by the spatial sampling of the imager, the optical low-pass filter and the electronic filtering employed prior to the camera's A/D converter. But the very nature of optical physics within the modern zoom-lens system dictates that its resolution performance is dynamic in three respects:

  1. Optical design constraints, manufacturing tolerances and the complexities associated with the concatenation of multiple optical elements within the studio lens system produce an MTF behavior that cannot be constant over the picture raster. There is an inevitable falloff in MTF from the center of the frame out to the four corners of the frame.
  2. Operating the lens' iris to control its aperture for different scene lighting conditions produces a variation in MTF. This is the result of fundamental optical physics associated with diffraction.
  3. Most importantly, the alteration of the lens's focal length during zoom operation further alters its MTF.

Lens realities in the MTF domain

To manage these variables, lens designers must consider more than just the performance at the center of the lens. One approach is to consider several points on the image plane, including some on the periphery. For example, >a href="#Fig5">Figure 5 shows nine separate spatial reference points used by Canon's lens designers within a 16:9 image plane. These points include the “picture center,” the “middle” (four points) and the “corner” (four points).

These points are used in conjunction with computer optimization programs to achieve the highest total MTF possible at all nine points for the lens' numerous optical elements. But this task is complicated by the fact that the optimization must also seek the highest MTF when the lens iris is being operationally exercised to alter the lens system's optical aperture. The task is further complicated if optimization also takes place when many of the optical elements are physically moved relative to each other while altering the lens focal length during a zoom operation. The sheer number of variables involved in such design optimization can be staggeringly large. Thus, only an optimum overall compromise is possible. It is testament to the sophistication of powerful modern computer-aided design techniques that some lenses achieve excellent spatial MTF (with special attention to the 200TVL/ph to 600TVL/ph region).

Figure 5. To manage the optical variables that occur during zoom-lens operation, lens designers consider multiple points on the image plane in their optimization calculations. Click here to see an enlarged diagram.

This dynamic nature of MTF is an inescapable technical reality for all lenses, regardless of manufacturer. Numerous constraints imposed by optical physics are in play here, and different lens manufacturers make their own proprietary design optimizations. Accordingly, there certainly are systemic differences in the necessary compromises made by each lens designer. These differences are what broadcasters need to explore in any HDTV lens testing before making a purchase decision.

Picture sharpness looms large in any assessment of HDTV performance, and the lens plays a major role in sharpness reproduction. It also predetermines the lens-camera contrast performance and plays a significant role in the system's color reproduction. Designing an HD studio lens involves an extensive number of variables. But manufacturers have been successful in overcoming this and other formidable challenges to produce remarkably high-performance lenses.

Putting it in perspective

Visit the following links to read part II and part III of this series:

Part IIHDTV lens design: Management of MTF

Part IIIHDTV lens design: Management of light transmission

Larry Thorpe is national marketing executive and Gordon Tubbs is assistant director of the Canon Broadcast & Communications Division.