Motion Control For Exposure Control
Published : 7th May 2006
Last week I had a call to do some motion control. What the DP wanted to do was quite interesting … because the location was so huge and windows so massive he wanted to shoot one pass at 3fps to get an exposure on the background and then the foreground at 25FPS with the action. As it happens there wasn't a cloud in the sky and the sun came through the window in streams and it could be shot at 25fps. So I covered up the rig and pretended I wasn't there.
I've often thought that this was a good idea ... but this was the first time I'd ever been asked to do it. Has anyone else ever used motion control for exposure control … I was thinking especially about HD shoots outside or in locations but I've never heard of that either ..
Specialist Camera Assistant
Justin Pentecost wrote :
class="style11">>Has anyone else ever used motion control for exposure control
I did a motion control shoot for a 35mm black and white commercial in Amsterdam a few years ago. We did multiple passes at different exposures, but this was because we had different lighting setups to suggest a passage of time in a location.
So yes is the answer but I'm sure you meant in a different context.
It might sound like a good idea, but unfortunately the depth of field would be different for each pass. On some shots, this might not matter much (i.e., pass 2 and above are shot in another location, like a blue set), but in a multiple pass situation it would matter quite a bit. And, obviously, anything that is moving in the shot - be it human, mechanical, or natural - would also present problems in terms of mismatched motion blur. The only time I would think this is a sensible approach is when shooting miniatures.
Cineworks Digital Studios
>It might sound like a good idea, but unfortunately the depth of field >would be different for each pass.
What about using ND? Should solve the DOF issue, no? Of course, I've never done motion control, so maybe I'm missing something.
Camera, Lighting, and Technology
class="style11">> It might sound like a good idea, but unfortunately the depth of field >would be different for each pass.
Actually, if you shoot different passes at different frame rates at the same aperture setting on the lens, you won't have any mismatch in DOF at all.
Two big caveats - obviously things that move (fans, flickering lights, pigeons, people, waterfalls etc will create a problem... but an empty room with actors in the "live action fg pass" and additional passes to get underexposed architecture to soft split into other areas of the frame should work.
The second big caveat is that different motion control rigs have very different dynamic properties. Depending on the move and the rig, scaling the time base of a move can cause the rig to wiggle, vibrate and jump differently on two passes shot at different speeds. Some rigs are very reproducible on multiple passes at a given speed but lose a bit of registration when the same pass runs at different speeds.
Additionally, while servos have many different advantages, scaling time with servo rigs can sometimes lead to servos "rounding corners" on higher speed passes, leading to believe you have a match move when, in fact you have some line up errors. This is not "steppers good/servos bad," but is an issue of which many clients are not educated.
With those two mechanical limitations in mind, and taking into account the specifics of a particular shot and roto issues involved if the actors have to appear over portions of the frame for which the slow speed pass is needed (which means some roto work cutting actors off the underexposed background which might be easy or might be murder), I respectfully disagree with Mike regarding the DOF issue.
I've done this numerous times...
On a recent commercial in NY for Cingular. This one was shot at Magic Hour, and we made separate exposures for sky, Manhattan Bridge, Manhattan Bridge lights (after sky was dark), Talent on sidewalk under FDR drive, reflection of talent in a puddle, then finally talent against moving green screen to be matted in to reflection puddle, and finally, rig removal pass for underside of FDR drive that was filled with lighting instruments in other passes.
I did one on a Jeep commercial where we "painted" the Jeep with light. Jeep was lit with panels that were in shot, then background landscape was shot at Magic hours, finally sky exposure was made for peak saturation, when the light was so dim that nothing else offered a decent exposure.
Last week on a set for a cosmetic project where we shot different exposures via different frame rates. We used ND on the lens to maintain consistent aperture and depth of field.
Another time on a commercial with a 4 wall kitchen set where the glass panels reflected the camera, lights, track. Mostly, this was rig removal multi pass, but there were different exposures to bring out the dark surfaces and brighten the background out the window.
So, yes Motion Control can be very effective to for exposure control, and can allow things to be lit in ways not otherwise possible through matting techniques, rig removal passes, and painting with light.
Gear+Rose Motion Control
I've done that in situations where half the frame is one exposure (big windows) and half the frame is another (ambient light). It then comes down to the editor/compositor to blend the split.
Not sure how easily that could be done if you're trying to hold windows in a shot plus the area immediately around them. Photoshop CS2 has a layering tool that allows you to combine multiple exposures and just use the brightness range you want from each one to create one photo with higher dynamic range than you could capture otherwise.
Anyone know of any tools like that in the average compositing kit? After Effects?
Director of Photography
Film | HiDef | Video
> I did one on a Jeep commercial where we "painted" the Jeep with light.
I'm amazed by some of the camera mounts for car photography I've seen. Really ingenious.
SFD Vfx & creative post
Santa Monica, CA
class="style11">> It might sound like a good idea, but unfortunately the depth of field >would be different for each pass. On some shots, this might not matter >much (i.e., pass 2 and above are shot in another location, like a blue >set), but in a multiple pass situation it would matter quite a bit.
Extreme ND...? Two polarisers?
Director of Photography
Film | HiDef | Video
Shot low budget corporate film. Had to follow professor telling class about ingenious new device. Of course his 13 students are him. Shot him against blue screen, and the various layers of him. Then needed to shoot background lecture theatre. No money, so chanced our way into the English Folk Dance Society building in Camden. Oak panelled room, perfect for University lecture theatre look. Forbidden to light the room, so undercranked the playback pass to get an exposure for the oak panels and another pass at 25fps for glass door into the reception and daylight beyond. Judicious use of ND’s to maintain same stop and thus DoF.
Recently had long conversations with SFX/DP about using our rig for lighting purposes. Wanted very low light levels, soft bounced light on singles and two shots. Could not lock camera off as wouldn't work with the rest of the material. Wanted roaming camera feel, so was concerned with reflectors/ poly etc., edging into shot. Agreed we'd shoot with dirty edges and then do clean pass to get rid of anything that edged into frame.
Ref the rest of this ..
Has anyone ever used stills flash to create the background ? I was wondering if any one has ever done a stop motion pass after a 25fps pass (perhaps with dialog). When I used to live in Kings Cross people would spend thousands lighting the gas towers. I looked into using a load of battery operated stills flash for a lock off…but then the production said they couldn't afford it.
Anyone want to do a test with this ?
Specialist Camera Assistant
+44 7973 317 241
class="style11">> Has anyone ever used stills flash to create the background ?
Matte painting of Bradbury Bldg. looking up stairwell to skylight (where blimp was flying above announcing great opportunities off world)
large format stills camera locked off - strobe being carried up stairwell and shot off periodically...or so I am told by a participant.
Lock-off rather than motion control, but same idea.