GETTING IT ON FILM #4 - STROBING, OR HOW FAST IS TOO FAST?

Nov 97 - Gord Harris


A phenomenon as old as the movies is the broken and jerky motion commonly known as "strobing", which leads to the familiar sight of spoked wheels turning backwards on film or TV. Strobing refers to the visual effect produced on the screen when the illusion of smooth motion breaks down, and on screen action appears instead as a series of stuttering jumps. Strobing is also sometimes called "image judder", and gets its name from its similarity to the effect produced by a stroboscope, flashing at a slow enough rate to produce a series of flashing static poses.

Strobing can be caused in a large-format film by moving either the camera or the subject too fast and causing a broken and jerky motion. Any fast panning over high contrast, static objects can be troublesome, especially if viewed at high screen brightness levels. Producer/editor Toni Myers of the Imax Space Team relates: "...it's always there ready to happen, like a headache. Every film that I've ever worked on with Imax from North of Superior to Mission To Mir, has at least one or two strobing shots... and sometimes, if there's no other alternative, you just have to use them." An example is the Russian nose cone scene in the Soyuz roll out in Mir.

Strobing can also occur in scenes as simple as a CG title with moving letters, or in the lively signature films commissioned for theatres, typically filled with continuous camera and subject motion.

The single biggest cause of strobing problems is simply moving the IMAX camera too fast, at the same pace the filmmaker or animator is used to using for video playback or 35mm feature films and commercials. What initially looks good on an AVID video screen in the studio or a small video monitor from the camera video tap, can strobe horribly on an IMAX or IMAX Dome theatre screen.

Pans are particularly troublesome. Many years ago we did a technical test by panning an IMAX camera at controlled rates while shooting technicians waving flags in front of a brick wall. At the normal 24 fps camera rate, the image began to noticeably degrade, soften and strobe at an on-screen rate as low as 3 degrees/second. Director/cinematographer Greg MacGillivray advises that it takes 30 seconds to pan smoothly from one side of the IMAX screen to the other on a 40mm beauty shot, if you cannot follow action.

To minimize strobing, producer Lorne Orleans tells camera operators to slow down their panning rates significantly, and then halve the rate of what they think actually works. Cinematographer James Neihouse even uses the term, "funeral pace." Imax optics specialist Hugh Murray advises new 15/70 directors and screenwriters to "forget about ending a shot by getting the character to walk out of frame - it may waste 15 seconds!"

Fortunately, there are many useful tricks to reduce strobing and achieve faster on-screen action in normal 15/70 films projected at 24 fps. One well-known trick is to follow a moving subject - the audience won't care if the background strobes, or is soft, if they have a clear centre of interest to focus on that is sharp and strobe-free. Second, use wider-angle lenses or animations - they are more forgiving than telephoto lenses (or narrow angle CGI virtual lens "taking angles"). A physical pan rate of only 1-degree per second is necessary for a 110mm telephoto in order to get the same sharpness as a 3-degree per second 40mm shot.

Third, keep in mind that higher contrast, higher brightness subjects will strobe worse than lower contrast, more monochromatic subjects. Toni had to reject some flyover train shots from Heart Land due to strobing of the black trains and tracks, against the white snow and overcast sky, an especially problematic situation.

Similarly, avoid shooting with narrow shutter angles and consider deliberately rendering scenes with motion blur in CG or with motion control camera movement during single frame animation. Softer edges help to hide strobing on the giant screen.

Both James and Toni advise it is better to think in terms of objects tracking diagonally or into and out of the screen rather than left/right action - for example, moving from top left to bottom right or dollying into a clear centre of interest. Lateral dolly moves can be dodgy, such as the too-fast sideways dolly move past the hydroponic plants in the professor's lab in L5 - First City in Space. For shooting from side door mounts in helicopters, James usually angles the camera forward to reduce strobing, and often overcranks the camera beyond the normal 24 frames per second to a higher 36-48 fps to slow motion and reduce image strobing. This also gives you a longer usable shot between aircraft bumps or lurches.

Finally, if you must use a fast camera or subject move in a shot or animation that you know will likely strobe, best to get it over with quickly, with no attempt to avoid strobing, and then slow down smoothly so the audience can see the detail a second or two later.

Ultimately, there is simply no substitute for watching your own rushes up close in an IMAX theatre, or to be even more critical - in an IMAX Dome theatre. Don't sit in the last row of the theatre or judge everything from video (which tends to blur and soften edges that can strobe in 15/70). If you must use a video computer monitor, get the largest screen you can find, sit up very, very close (so it fills 90 degrees of your horizontal field of view) and view them through a large dioptre (magnifying glass) to get your head to the proper IMAX theatre viewing angle relationship. If you follow these tips, we hope you will quickly learn "how fast is too fast."

(Reprinted from The IMAX Experience November 1997 Volume 1 Issue 6 with permission from IMAX Corporation)

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Copyright © 2003, Gord Harris, go-R&D Consulting. IMAX®, IMAX® Dome, IMAX® 3D, IMAX® 3DDome, IMAX®
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