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Finding the perfect match with 3-D lenses

While you might think that the recent explosion of 3-D production throughout the United States, particularly for sports and movies, would be a windfall for manufacturers of broadcast and professional production lenses, the results have been decidedly mixed. It depends upon which vendor you ask.

Some productions to date have used existing Canon HD lenses pulled from the shelf and paired up for stereoscopic image capture, but this requires a certain technical skill to get the optical alignment right. Others have employed a new generation of 3-D-compatible lenses (from such companies as Fujinon and Thales Angeneiux) to get the job done, but there’s still some technical tweaking to be done.

Perhaps the most important issue for professionals is finding the right combination. Many think you have to find an identical pair of lenses and be sure to use those same two lenses together for every production to ensure perfect alignment and, thus, good (i.e., easy on the eyes) stereoscopic images. However, representatives for both Canon and Fujifilm said there is no such thing as a “perfectly matched” pair of lenses. Using sophisticated manufacturing techniques and careful QC processes, all of their standard 2-D HD lenses are typically designed to be 3-D-ready; however, there are some differences to be aware of.

Adjusting tolerances

Larry Thorpe, renown imaging expert and national marketing executive for the broadcast and communications division at Canon U.S.A., said most portable HD lenses contain approximately two dozen separate lens elements (large box lenses include almost three dozen). The optical design of the lens includes accommodations for adjusting the minute tolerances associated with all of these lens elements and their mountings.

“Although the present state of the art in lens design makes these tolerance differences virtually imperceptible in lenses used in normal 2-D production, the differentials between tolerances of any two lenses can become visible in a 3-D application,” Thorpe said. “Over the past year, based upon many 3-D projects in which we have become involved, our R&D team is presently exploring possibilities of further tightening control of these tolerances.”

Thom Calabro, director of marketing and product development for the Fujinon broadcast and communications products division at Fujifilm, said that his company has already figured out how to tighten these tolerances. While Fujinon clearly designates if a lens is 3-D-capable (including four new B4 mount lenses), it does not sell them as matched pairs. Basically, users can be assured of good results if they use any two Fujinon HD lenses with “T5DD” at the end of the model number.

“After our normal manufacturing process, we perform a final test,” Calabro said. “During this testing, we designate a certain number of these for 3-D. Each lens is carefully rechecked to ensure that the optical axis is of a very tight tolerance. Our 3-D-designated lenses can be mixed and matched, of course, with lenses of the same focal length. This makes it ideal for mobile companies who frequently move lenses between trucks. There is no need to keep track of ‘paired’ lenses.”

While Fujinon T5DD lens customers don’t need to designate a matched pair among their stock, Canon stresses the need to identify camera-lens combinations that work well together, even for 2-D HD acquisition.

Finding the right camera-lens combination

“You’re not just matching lenses; you’re also matching cameras,” Thorpe said, adding that there are two key issues to be dealt with when matching a 2-D HD lens-camera pair for 3-D operation

First, when a lens is mounted to a camera, there is an inevitable mechanical tolerance associated with the lens’ optical center and another separate tolerance associated with the camera’s optical center. These separate tolerances can be additive or subtractive in terms of the final optical axis for a given lens-camera combination because both tolerances entail plus and minus limits. Recognizing this, tolerance limits for both have been carefully established between the optical and the camera manufacturers.

“These (tolerances) are on the order of some tens of microns,” Thorpe said. “The resultant small miscentering of the lens-camera optical axis typically translates into tens of pixels (for a 1920 x 1080 HDTV video format). For a 2-D HD camera, this is of little consequence. For 3-D pairs, however, the differential can be troublesome.”

He said Canon has been training 3-D technical crews on how to take any two lenses, loosen their respective mechanical mounting plates and, by an interactive trial-and-error process while mounted on the two chosen cameras, converge the lens-camera optical centers to the fullest degree possible.

Of course, this process can be time-consuming and tedious, and Fujinon’s Calabro said that it’s not necessary in the case of T5DD lenses.

The second major issue associated with this fixed optical axis mounting tolerance is an additional dynamic movement of the optical axis when a zoom lens traverses its focal range from full wide angle to full telephoto. Such movement is a consequence of physical tolerances that invariably accompany the movement of multiple lens elements entailed in the action of zooming. Thorpe said current “tolerancing science” limits this movement to some tens of micrometers, which in turn translates to tens of pixels in video image movement. Depending upon the polarity of this tolerance, which can also have a plus and minus range, this tolerance can, over the entire focal range, add or subtract to the fixed offset of the lens-camera combination.

3-D lens adjustment tools

For synchronous control and zoom/focus/iris tracking of two lenses within 3-D operation, Canon has released its 3-D Lens Adjustment Software and a complementary 3-D Bridge Cable. Both are options that must be purchased separately from a lens or pair of lenses.

The Canon 3-D Lens Adjustment Software can be downloaded (at a Canon service center) into the digital drive units of all current-generation Canon lenses. It facilitates control of any two lenses of the same model, with the two zooms being simultaneously controlled by a single standard digital zoom controller and the two focus controls being simultaneously controlled by a single standard focus controller.

“The difference between any two lenses (of the same model) is largely an unpredictable fixed offset in tracking between the two zoom movements, rather than a cumulative error across the range (because of the 16-bit accuracy of the zoom encoders),” Thorpe said. “The new 3-D Lens Adjustment Software allows that offset to be compensated for within the respective digital drive units of each lens. Measurements made at the two extremes of the focal range (going from wide angle to full telephoto) exposes the amount of offset, and a correction for this can be directly entered into the digital drive unit using the display and associated push-buttons on the lenses. A different correction can be entered for the error (again associated with a small system backlash) when going from telephoto to full wide angle. The same procedure is used to correct for fixed offsets in the focus and iris action of the two lenses.”

Canon’s 3-D Bridge Cable, meanwhile, connects to the two 20-pin connectors on the digital drive units of all Canon lenses. The zoom controller connects to one of the 20-pin connectors on either of the two lenses. The single focus controller connects to the remaining 20-pin connector on the other lens.

According to Fujinon’s Calabro, additional software is not required to mount and precisely synchronize a pair of T5DD lenses on a 3-D rig when using Fujinon’s Synchronize control system and a zoom rate demand control unit.

In addition to these optional products, the digital drive units of Canon and Fujinon HD lenses employ highly precise miniature optical encoders to sense position related to the separate zoom, focus and iris physical movements within the lens. Both companies employ technology that senses as little as a tenth of a micron of movement and can make adjustments accordingly.

Accessing the choices

The good news is that there are a multitude of lenses for production companies to choose from. If you own Canon lenses, you do not have to buy special lenses for 3-D, because they don’t currently offer 3-D-specific lenses. Fujinon lens customers can use any two lenses within its T5DD series.

With or without the special software and other tools, the key to good results is that production crews must possess the skill to properly optically match the lens-camera pairs so, ultimately, movie audiences and TV viewers don’t get a headache.