This is something that has been known for quite some time, but is possibly not discussed frequently enough. Many producers, directors, DPs etc still think that you "can't zoom" in 3D unless you have a computer controlled system or a truck full of dudes and loads of expensive stuff tethered to your rig. In the July issue of American Cinematographer, Dariusz Wolski, ASC commented on his use of Angenieux Optimo 15 - 40mm zooms on Prometheus calling the short zoom "the most revolutionary lens in the industry right now." Of course the DP Rouge is not the same exact lens as as the 15-40, but there are more similarities between the two than there are differences, and the resultant image quality is virtually identical.
One key feature that the DP line has which the film only lenses don't, is the ability to calibrate zoom tracking with an extremely high degree of accuracy. This is SUPER important for 3D, and not only sports, live events or situations where you want to zoom during a shot. But most people do not do it, and probably because it just isn't brought up.
Here is how it's done. Just ahead of the flange, the DP Rouge lenses have a collar with eight small screws on it. Four are allen head screws which allow you to align the lens and four are phillips head lock down screws. The procedure to adjust this mechanism for 3D use is not immediately obvious, but once you know it, it's pretty easy and it is something that should be checked before any 3D shoot. Even if you do not plan to zoom during a shot, having your zooms track accurately will eliminate the need to realign with new lens size choices, and therefore save a great deal of time.
The first thing to do is set the tracking for each individual lens. I am stating the obvious to say ideally you should have one camera that is always RIGHT and one that is always LEFT and keep the same lenses with each camera. The zoom tracking alignment can be done with cameras mounted in a beam splitter or simply on a tripod. You may need to remove the lens several times to get it just right so be prepared for that. You will need a focus chart or just a simple piece of paper with a high resolution fairly small target like a dot or a cross hair. Set it up about 10 - 12 feet away from the camera. Before you mount the lenses on your camera, loosen the PHILLIPS HEAD screws ever so slightly. I mean like half a turn. If you make them too loose, undesirable things could happen. The lens may be floppy in the mount and you will may not be able to maintain focus throughout the zoom range.
Once you have the phillips screws lightly loosened, mount the lens and turn on the cross hair in the camera viewfinder or LCD. Zoom the lens to full tele (either 42mm or 80mm depending on which lens you have) and using the pan and tilt on your tripod, place the EVF cross hair dead center on top of the target cross hair. Now zoom out to full wide and you will very likely see the target go well off center. Make sure your tripod is very sturdy and totally locked down. Any movement of the head will throw off your calibration. Utilizing a 1.5mm allen wrench, gently turn the small allen screws on either side and top and bottom of the lens. You should see the image move as you turn the allens. You can only turn one at a time, so it will take some patience and a little bit of back and forth. Also be aware that if any of the allens are fully locked down, they may affect the other screws ability to influence the lens. This is a bit of a finesse operation, but once you see it work it is quite easy to understand. After you have your target cross hair back on center, zoom back in to full tele and check that you haven't moved the head at all. If you have, realign the camera cross hair with the target cross hair using pan and tilt on the tripod only. Do not adjust the allen screws at full tele. If you did have to readjust pan and tilt, zoom back out to full wide and continue with more allen screw adjustments. When you have the zoom tracking set properly, you should be able to go from full tele to full wide and keep the cross hair dead nuts in the center throughout the zoom range.
After you have done the procedure to both of your lenses, remove them and carefully tighten the four phillips head screws (sadly, this cannot be done without removing the lens). It goes without saying that you should gradually tighten the screws in opposing order so as not to influence the delicate alignment you just applied. I would recommend putting the lenses back on the camera and verifying that zoom tracking alignment hasn't been altered after you tighten the phillips screws. Often there may be a small change, you can still turn the allens a tiny bit to compensate, but hopefully you will not need to do this very much at all.
Now check 3D alignment on you beam splitter rig. I would recommend aligning at your max tele focal length. If everything went as planned and you have a competently aligned 3D rig with FIZ control from a C-motion or Preston type device, you should be able to zoom all the way out without introducing any noticeable parallax into the image. You are now ready to start your shoot day and zoom to any focal length without realigning your rig.
There are still so many false assumptions that people have about shooting 3D, not being able to zoom is only one. Even if you normally prefer primes, I would encourage you to consider high quality zooms for 3D shooting. Modern zoom lenses from manufacturers like Angenieux are absolutely beautiful and films like Prometheus prove that they are every bit as cinematic (even in IMAX applications) as prime lenses. 21st Century 3D is not affiliated with Angeniuex at all, but we are big fans and have been shooting with their lenses for many years. We have offices in New York City and Los Angeles and our staff of stereoscopic experts are happy to demonstrate this zoom tracking adjustment procedure and its awesome results in either of our facilities. Happy shooting.