8mm Forum: Blurred Image

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Posted by Graham Ritchie (Member # 559) on October 01, 2008, 10:44 PM:

Reading an interesting article by Francis Williams from Movie Maker September 1984

Here is a small part of that article you might find of interest.

Bowing of the film in the projector gate is a problem that affects all gauges of film, up to and including 35mm. The reason why it is so very troublesome in the Super 8 is because of the very wide-apature lenses used for projection f/1.1 and even f/1.0 being commonly used in the more expensive projectors.

If we were to be content with dim pictures we could use a lens of modest apature-say, something like the f/1.3used in the old 9.5mm projectors- and the depth of focus would then be big enough to cope with a modest amount of bowing, and the focus shift would not be apparent on the screen. A lens of f/1 apature has very little depth of focus indeed.

The effect of heat is to make the film swell slightly, and so it tends to bow outwards {generally, but not invariably towards the projection lens}. This is what happens in a 35mm slide projector when an unmounted transparency "pops" in the gate. After repeated passes through the projector, some of this bowing becomes permanent.

An older and better idea was the curved gate channel, first seen in some of the early Kodak 16mm projectors Going back to the analogy of the rolled-up steel tape measure, we saw that film could not curl in two different directions at the same time. Similary, if film is taken in a curved path through a gentle curving gate, it will not be able to bow out.

The heat of the projector lamp makes the film curl {as in A}In a curved gate the small radius of curvature r is replaced by the larger and tightly controlled radius R {because film cannot curl in two directions simultaneously}.

Graham.

PS. So there you have it some of the focus problems we have with Super8 might come down to the design of the projector gate and the film itself. Note, The 35mm projectors I use at work have curved gates.

any thoughts on this subject?

Posted by Osi Osgood (Member # 424) on October 01, 2008, 10:51 PM:

Hmmm, I'm not so sure Graham. I'll admit I'm no expert in film properties, but I have notes that not all film ages at the same rate. Some Eastman stocks tend to warp somewhat quickly, while AGFA and L.P.P. tend to not have any warp at all.

I would think that the film has such short time in the film gate (and running through the projector in total) that any warping of any kind would be incredibly small.

I think it comes down more to the lense than the film gate. We can all agree that not all lenses are created equal. Some lenses I've ran into were so poor that nothing but the very center of the image would be focused. Put that same print on a higher grade projector and the whole image looks great.

That's my two cents worth.

Posted by Knut Nordahl (Member # 518) on October 02, 2008, 02:40 AM:

Where does the preasure plate fit into this cenario?

I do not understand how the film can be bowing out from the gate channel if there is a preasure plate in place?
If so that seems to imply that there is too much room between the gate channel and the preasure plate and that the preasure plate is faulty.

Posted by Paul Adsett (Member # 25) on October 02, 2008, 10:45 AM:

Interesting post from Graham. What he says about the curved gate makes a lot of sense. I can appreciate that the film would want to bow a little from the heat pulse of the lamp, and a curved gate might reduce the effect.
Was it the Heurtier's that had a ruby in the middle of the gate channel to prevent bowing of the film?
I still think the rear sprung gate is the correct way to do this to prevent focus drift due to stripe thickness variation. Too bad that the big manufacturer's like Elmo and Eumig stuck with front sprung gates.

Posted by Graham Ritchie (Member # 559) on October 02, 2008, 01:42 PM:

Hi Paul

Francis Williams does mention the Heurtier in this article written as follows

Over the years, designers have tried other methods of flattening the film in the projector gate. In the Heurtier super 8 projectors two polished rubies were positioned in the gate to bear on the middle of the film, one just above the aperture and the other just below. The idea was that they would forcibly prevent the film bowing out towards the lamp. They pressed on the shiny back of the film {not on the emulsion}, and as they were very smooth they did not scratch. Unfortunately the film doesn't usually bow that way. To stop it bowing the other way out towards the lens-the rubies would have had to bear on the emulsion side of the film, and that undoubtedly would have produced abrasion marks.

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