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On Screen Troubleshooting
These are some examples of common on screen issues, and their potential causes and remedies.
Signs of contrast degradation are a washed out image with pale colors and milky blacks.
Degraded contrast can be caused by dirty projection ports or lenses; reflected light from walls, floors, or ceilings making its way back to the screen; ambient light from exit signs or aisle lights; reflected light from within the projection mechanism (for example a damaged shutter with a flaking non reflective coating); poor shutter alignment; or poor lamphouse alignment.
An individual print may have low contrast if it is intended for TV broadcast or transfer to video, is from poor source material (e.g. a 16mm dupe print), or was misprinted by the lab. In this case there is nothing much to be done.
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 is the overall evenness of focus across the entire image. If examining focus uniformity using a test film, it is not uncommon for the far edges of the frame to be slightly softer than the center. More problematic is when one side or area of the image is less in focus than another.
Uneven focus uniformity can be caused by anything in the optical chain being slightly out of alignment: if light from a poorly focused bulb is reflected onto a slightly out of alignment gate and magnified by a poor quality lens at an odd angle, the resulting image will almost certainly not focus evenly.
If this is the case, it is usually best to start by checking lamp alignment. If lamp alignment is correct, check the gate for any debris, sticking rollers, or damaged parts. A small bit of dirt buildup is enough to cause one side of the image to be significantly out of focus, and a sticking lateral guide roller can pinch the film in the gate so that the top or bottom of the image is soft.
In many installations, especially those with steep projection angles or short throws, it is not uncommon for the gate to be shimmed to be brought into alignment with the screen. This adjustment requires great care and patience.
Focus uniformity can also be compromised by dirty lenses or port windows.
Vertical unsteadiness. Common causes include gate tension that is too low, 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.
Jitter can be especially noticeable if frame lines are visible on screen due to overcut aperture plates or lack of masking, which will direct the audience's gaze to a jittering straight frameline.
Jitter caused by splices, damage, or low gate tension will cause chattering in the gate, while printed-in jitter will be silent.
Lateral (side to side) movement. The most common mechanical cause is worn, dirty, or damaged lateral guide rollers, or acetate film that has been trimmed or beveled to remove edge tears. 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.
Visible negative splices
If Scope, Movietone, full aperture, or 70mm films are projected slightly out of frame, if the projectors' aperture plates are overcut, or if a film has sloppily made negative splices, white flashes will appear at the top and/or bottom of the frame at every shot change (sometimes several dozen times over the course of a reel!). Care must be taken to monitor framing closely, as the exact location of these splices can change from reel to reel depending on how the film was printed.
The standard aspect ratio for scope films was eventually changed from 2.35:1 to 2.39:1 to account for thicker splices, which is helpful but does not fully eliminate the problem.
Many 16mm film negatives are cut single strand, so that even the most carefully made splices are visible on screen. In this case the projectionist doesn't have much control over splices appearing on screen, though you may be able to adjust the framing to split the difference.