Reader Feedback
Dec 1, 2005 12:00 PM
Camera resolution
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Dear editor:
For years, all camera manufacturers quoted horizontal lines of resolution in their specs. It was through this measurement that you had a ballpark idea of the resolution of a broadcast camera.
By reading the horizontal lines of resolution, you could tell if a camera had great resolution or not. The finer the lines and the closer the lines were together, the greater the resolution. When the lines became fuzzy or blurred together, that was the horizontal resolving limit of the camera.
Now none of the manufacturers list horizontal lines of resolution in their spec sheets. Therefore, an old guy like me can't tell which camera has the best resolution.
How do you discern the resolving power of new video cameras?
Jim mitchell
Response from Larry Thorpe, national marketing executive, Canon Broadcast & Communications Division:
The perceived picture sharpness performance of an HD lens-camera imaging system entails a complex assimilation of the total spatial resolution portrayed on the HD display. Television camera specifications do not do justice to the realities of their picture sharpness capabilities.
Historically, camera manufacturers quoted horizontal lines of resolution as the prime measurement of the performance of their cameras. These numbers referred to the limiting resolution or the horizontal resolving power of the lens-camera system. It was an attempt to simplify a complex performance assessment to a singular number. Regrettably, this became an industry yardstick — a competitive horsepower number (in that bigger was allegedly better).
This has always been a flawed approach to the assessment of a lens-camera's true performance with respect to picture sharpness. The horizontal limiting resolution specification has no bearing on what actually stimulates the television viewer.
Fortunately, today's camera spec sheets do quote an additional horizontal number — usually called modulation depth — in their spec sheets. This is a more important number than the limiting resolution specification. The significance of this percentage number is that it provides a spot reading of the modulation transfer function (MTF) of the HD lens-camera system at one specific spatial frequency.
MTF is a plot of the contrast behavior of image detail of increasing spatial frequency, and it directly relates to perceived picture sharpness. The shape of the plotted MTF curve across the frequency band of the system is what realistically describes the performance of a given lens-camera system (perceived picture sharpness being a function of the square of the area under that curve). The overall MTF information is the only true assessment of the practical resolution performance.
For a contemporary 1080-line HD camera, a typical spec might read: “Depth of modulation: 45 percent or more horizontally (at 800 TV lines at picture center, or 27.5MHz, with typical lens).” Given that the 30MHz bandwidth (defined by a prescribed digital filter) of the 1080-line HD standard is equivalent to 872 TVL/ph, the quoted specification is telling us that the MTF curve near the extremity of the system bandwidth (at a spatial frequency of 800 TVL/ph — equivalent to 27.5MHz in the electronic frequency domain) is reproducing 45 percent of the contrast at low frequencies. Thus, it is a useful indicator of how well the horizontal MTF is holding up across the useful HD band. But, it by no means tells the full story. In comparing lens-camera systems, there is no substitute for actually measuring the depth of modulation at 200, 400, 600 and 800 TVL/ph and evaluating the MTF profile.
In addition, the above specification is confined to the picture center because the camera manufacturer understandably does not wish to engage in specifying the resolution behavior of the associated HD lens. Unlike the camera, all HD lenses have a variable resolution from picture center to extremities, which is compounded by further alterations over the focal range, with object distance from the lens and with aperture setting. No lens manufacturer publishes such specifications. Given the expense of HD lenses, it is recommended that they be carefully evaluated at different settings of the zoom and the iris.
Curiously, no manufacturers presently specify the vertical resolution for their HDTV cameras, though they continue to quote a limiting vertical resolution number for their respective SDTV cameras.
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