Fujinon HA13XPF lens with Precision Focus. Precision Focus Assist employs the contrast method for auto focusing, which derives its focusing points by means of optical path differences.
While HD video can produce pristine images, occasionally the resulting display leaves something to be desired. Often that something is a sharp focus.
A slight misfocus of an image is hardly discernable in SD, but in HD it is clearly obvious. To overcome this problem, Fujinon has developed the Precision Focus Assist system for HD lenses. It addresses the precise focus issues in HD production stemming from the format's shallow depth of focus and the requirement for small, low-resolution camera viewfinders. Although automatic in nature, the system places the priority for focusing on the operator, providing assistance when needed.
Camera operators focus by viewing the image on a 2in or 7in viewfinder. With a typical resolution of 450tvl, these devices are quite sufficient for SD focusing. However, today's HD displays provide a vertical resolution of 640tvl, making the camera viewfinder woefully inadequate for precise focusing and highlighting any focus mistakes.
Figure 1. HD lenses have a smaller optimum focus range than do SD lenses. Click here to see an enlarged diagram.
Focus problems in HD
Focusing a camera requires the operator to first zoom to the most telephoto position of the lens and then adjust focus while observing the edges of the subject. When done correctly, the high-frequency component of the subject's outline is maximized and therefore sharpest.
With HD, the operator must always be at a maximum zoom position whenever focusing is required, making it difficult to focus anywhere else in the focal range. A key distinction between focusing in SD or HD is the difference in depth of field between the two lens types. Figure 1 illustrates this phenomenon. The irises on both SD and HD lenses are fully open, the subject is at a 10m distance, and the lenses are in the telephoto position. Note that the focus range of the SD lens ranges from 6.8m to 18.5m. Yet, the focus range of the HD lens is only from 8.2m to 12.6m. Now let's see what effect misfocusing would have on actual picture quality.
Figure 2 displays the difference in depth of modulation for a pair of lenses, one SD and one HD. At 1000tvl, an HD lens has a modulation of approximately 82 percent modulation depth, and the SD lens' modulation is approximately 65 percent. If the SD lens is misfocused to the point where we see a 10 percent drop in modulation (i.e., 65 percent - 6.5 percent = 58.5 percent), the same amount of misfocus would cause a 50 percent drop in modulation for the HD lens (i.e., 82 percent - 41 percent = 41 percent).
Figure 2. A misfocused SD lens results in a 10 percent drop in MoD. That same misfocus in an HD lens results in a 50 percent decrease in MoD. Click here to see an enlarged diagram.
There are several methods to accomplish auto focusing, typically categorized into two groups. The first group is the focus detection method, which searches for the focus position. The second is the range finding method, which achieves focus by measuring the distance from the lens to the subject.
The focus detection method has two variations: the contrast method and the phase matching method. The contrast method detects the amount of misfocus based on the contrast of the image. Home video cameras typically use the contrast method. The phase matching method, often used in still cameras, detects phase differences in the image.
The contrast method is preferred for video cameras. When a lens is precisely focused, both the contrast and the amount of high-frequency energy in the image is maximized. Typically, the lens' focusing group is moved until it finds the point of contrast where the high frequencies peak. The focusing group of lenses undergoes a “wobbling” process to search for that maximum point. There are several advantages of the contrast method. No range-finding sensors are required, critical focus will be obvious, and the point requiring focus can be viewed in the viewfinder because there is no parallax with the image.
Unfortunately, there are some drawbacks to using the wobbling process with the contrast method. Simply put, today's large and heavy lens elements are difficult to move at high speed. Also, the slight changes in focus caused by the wobbling can be easily seen. Furthermore, moving objects such as a spinning football might be compared at different times, which could result in errors. Finally, the process may not be quick enough for high-speed objects, such as those common in sports.
Figure 3. All of the automatic focus circuitry is contained within the lens, making the feature easily portable to other cameras. Click here to see an enlarged diagram.
Fujinon developed the Precision Focus Assist system using the contrast method, but without using the wobble process. Instead, the system uses an optical path difference signal.
The focus assist replaces the lens extender by a half-mirror in ENG-style lenses. (See Figure 3.) This half-mirror works as a prism. The Precision Focus CCDs are arranged so that the focal plane is at the same position in the optical path as the camera focal plane and the two CCDs are equidistant from the focal plane point. The camera gets a simple software patch to allow the operator to specify the focus point. Because the entire auto focus system is incorporated in the lens, the lens can be freely used on various cameras.
The camera operator first specifies the focusing point in the viewfinder, and then selects either auto or manual focus. Finally, an LED shows the operator when the proper focusing point has been achieved.
The system must first determine which direction to adjust the lens for focus. The circuit compares the values of CCD A and CCD B. The relationship between A and B is described in Figure 4. When focused, A is equal to B. When not focused, A is larger or smaller than B (i.e., A B). The required adjusting direction can be determined from these values. The focusing group is then moved to reach optimal focus position, while simultaneously keeping the movement of the focus lenses minimized.
Figure 4. Image position and direction is calculated from the outputs of the two different CCDs. When the electrical outputs are identical, the lens is focused. Click here to see an enlarged diagram.
All HD cameras use CCDs with an aspect ratio of 16:9. However, to minimize weight, costs and power consumption, 2/3in SD CCDs with 4:3 aspect ratios are employed. Although the SD CCDs have many fewer pixels than their HD cousins, experiments show that this does not degrade the performance of the focus circuits.
Tests also show that the focus assist area needn't examine the entire image area. In fact, this solution limits the examined area to approximately 90 percent of the image seen by the HD CCDs.
There are several advantages of the Precision Focus Assist system:
- Precise focusing is available even with wide shots.
- Images slightly out of focus, which are often difficult to recognize in the viewfinder, can be quickly corrected.
- The focus indicator tells the focus position, which helps the camera operator to more precisely manually focus the lens.
The technology is available on the company's XA101×8.9BESM HD zoom lens, XA87×9.3ESM HD zoom lens, HA13×4.5BRD-S28K wide-angle lens, HA22X7.3BRD HD EFP lens and HA26X6.7BESM studio lens.
Thom Calabro is national sales manager for Fujinon.