Light Path Details

Here are details about the light path and things to watch out for with 35mm and 70mm projection. Please add new features or correct previous entries if they could be phrased better.

The Light Path

1. Lamp
2. Douser
3. Flicker shutter
4. Changeover shutter
5. Backplate
6. Aperture plate
7. Film
8. Lens
   1. XWide/Scope/Academy/70mm 1.85/70mm 2.2
   2. Anamorphic
9. Projection window
10. Screen

Faults

• Picture blurry - poor overall focus - In most cases this is simply a case that the lens position in front of the film is marginally wrong and so the film is running out of focus. This can be corrected by rotating the threaded focus control at the front of the projector to move the lens position and bring the image back into focus. If when starting this procedure you notice the focusing is getting worse then stop and rotate the focus control in the opposite direction. A common technique used to fine tune the focus of the picture is to move the lens position a marginally noticable distance either side of the actual focus position and then choose the mid-point. Be careful to ensure that the picture is actually in focus and don't constantly adjust the focus as the audience will notice this!

• Picture blurry - focus off-centre very poor - If the centre of the picture focuses reasonably well, but moving off to the sides, the focus becomes really bad, then this could be a sympton of an incorrectly placed backplate.

• Jumping picture (vertical) - Dirty Runners - In most cases unless the gate tension is obviously loose then this is caused by dirty gate runners. The best way to cure this is the next time it is possible to stop the projector, remove the gate and the back plate and clean throughly using a cleaning solvent if necessary.

• Jumping picture (vertical) - Gate tension too loose - Occasionally in normal day to day handling of the gate, or after resetting, or training sessions, the gate tension may me too low and the picture may start jumping vertically in the gate. In extreme cases you will hear a large nasty noise coming from the gate area. The reason for this is that the gate springs are not allowing the runners on the gate to put enough pressure against the film as it travels through the gate. This means as each frame is being shown the film is not stationary, but is still actually moving. Correct this by gently turning the thumbscrew located midway down the gate clockwise until the film becomes more steady. At the next opportunity remove the gate, check it is properly clean and manually reset the gate tension.

• Jumping picture (vertical) - Gate tension too tight - If the gate is too tight, it prevents the film from moving smoothly through the gate and speeds up the build up of dirt on the runners. This is usually indicated by the film being excessivly loud (straining sound) in the gate / intermittent. Very slowly loosen the gate. If you notice the volume and pitch of the noise slowly decrease, you are improving the situation. If the noise suddenly drops to a very low pitch you have gone too far and you are about to have jumping picture due to gate tension being too loose. Excessive gate tension also increases the wear rate of both the film and projector parts, especially the intermittent sprocket, you also significantly increase the risk of film snaps. It is easier to adjust when the projector is stopped and you can remove the gate and reset the get tension as described above.

• Jumping picture (vertical) - Intermittent sprocket fault - Sometimes a fault with the intermittent unit and sprocket may cause problems with picture stability. It is possible that under certain exceptional circumstances that the intermittent spindle may get very slightly bent out of alignment, or the seating of the intermittent sprocket on the spindle itself is not central. This can occur when a lacing problem causes the film to get pulled very tight within the projector, or after a 70mm --> 35mm conversion. Diagnosis of this is by looking at the moving intermittent sprocket at eye level and looking for any "bounce". As the circumference of the intermittent sprocket is 4 frames, then a intermittent misalignment will result in a vertical picture bounce of 6 Hz. If this problem is noticed, then the technical team should be notified so repair work can take place.

• Left-right picture movement - Film Weave - Occasionally you may notice that the picture appears to move side-to-side on screen, this occurs more frequently with old prints and 70mm film screenings. This normally occurs as a result of edge damage to the film and produces an effect known as film weave, i.e. the film is not quite the exact 35mm wide and so it is able to move from side-to-side as it travels through the gate. In most cases this is a fault with the film itself and there is little which can be done. A couple of things to try is to marginally adjust the gate tension, lace with a smaller top loop, ensure the film is running continuously within the confines of the top roller of the backplate.