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On Screen Troubleshooting
These are some examples of common on screen issues, and their potential causes and remedies.
Focus
Focus Drift
The unpleasant phenomenon of focus drifting over the course of a reel, or "pulsing" in and out of focus. This is most commonly caused by warped film. A film with a minor amount of warping may require only a few small adjustments throughout the reel to stay in focus, but a reel with severe warping will require constant attention, and often it will only be possible to keep the center of the image in focus. Warped prints tend to play better on projectors with curved gates, which help bend the film back into position.
Focus drift can also be caused by mechanical misalignment of the gate, for example if the studio guides on a gate are "pinching" the film and causing it to buckle. Other common offenders are loose lens collars or excessive lens vibration.
A severely misaligned lamphouse can cause focus drift by burning one side of the film.
Polyester film should maintain nearly perfect focus and not warp, so if Focus Drift is observed on a polyester film or test loop it is a good indication of a mechanical issue.
Focus Uniformity
Jitter
Vertical unsteadiness. Common causes include improper gate tension, mechanical wear of the intermittent movement, excessively oily or dirty film, or preexisting perforation damage.
Jitter is often printed in - this can be especially evident on subtitled films with laser subtitles, where the subtitles will appear perfectly steady on screen but the image will still move. Printed in jitter is often more of a gentle bounce than a shaking movement and is commonly caused by high speed release printing, which was especially popular in the mid 90s - 2000's to satisfy high print runs with tight turnaround times.
Weave
Lateral (side to side) movement. The most common mechanical cause is worn, dirty, or damaged lateral guide rollers. Less common is excessive play in the Starwheel Shaft Thrust Collar, which would cause the intermittent sprocket to move from side to side during projection. Weave can also be caused by film shrinkage, and can also be printed in.