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A Few Questions about Arri SR3 Film Gate


David Fitch
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I was wondering if someone might be able to help with a few SR3 questions specifically related to the film gate. In looking at some SR3 images I've found online, I noticed that in many of the pictures, there is what appears to be a small screw mounted a few millimeters above the center of the gate. Some of the documentation I've found has described this as an adjustment screw for viewing screen height / frameline position. Was this adjustment screw present on new SR3s from the factory, or is it usually only found on modified or overhauled gates or cameras? The reason I ask is that this seems like the last place you would want to mount a screw, given its proximity to the film path. I'm guessing that the screw must be flush mounted or recessed, but it seems that even a microscopic burr on the screw head could potentially scratch the emulsion and be disastrous. Is this a valid concern, or are there safeguards in place to prevent film scratches?

 

Secondly, I've read about some SR3 gates "without top or bottom rails on the gate". Can someone explain what this means, and if it's a feature on all SR3s or only on upgraded or overhauled cameras?

 

Thanks to anyone who can help!

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Hi David,

the set screw above the gate aperture is an adjustable stop for the ground glass which sits just behind, to set the viewing frame height. It should really only be adjusted by a trained technician, who would never introduce burrs to the screw! But it is recessed and lacquered in, and the screw hole itself is at a level well under the film support rails, so there's really no chance it will damage film. I've seen this on Standard and Super gates, so it goes back to SR2 at least. Can't actually recall any SR gates that don't have it, but it's been a long time since I saw an SR1.

 

Your second question I'm guessing relates to the new gate design that came in with the SR3 Advanced in 1999, where the old design with fixed top and bottom side rail blocks was replaced with two continuous side rails with sprung saphire rollers on the non-claw side. It solved the problem of wear to the side rails which could eventually cause lateral unsteadiness.

 

Newer gate on left:

post-46614-0-67238000-1390182557_thumb.jpg

 

 

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Dom,

 

Thanks for the information about the screw above the gate aperture. As for my question about rails at the top and bottom of the gate, I think I've figured out that question. In both photos you posted, note that above and below the gate opening there are two (silver colored) horizontal rails - one right above the opening and one below. Compare that to the image I found online and have posted below. Note that there are no horizontal rails at the top and bottom of the gate opening.

 

From what I've read, this modified gate design with no top and bottom rails at the gate opening is supposed to decrease the chances of dust or a stray hair getting caught in the gate. Can anyone else verify the purpose of this particular type of gate design, and/or pros and cons of this design versus the original factory gate design with the horizontal rails?

 

post-29599-0-27777600-1390269428_thumb.jpg

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  • Sustaining Member

Never actually saw one of those gates in person when I used to service the SR line of cameras. In theory I can understand the concept, but I would be more concerned that with the gate pictured above you would not get a clean frame line on the top and bottom of each image, possibly causing bleed between the images on your strip of film.

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  • 7 years later...
  • Sustaining Member
3 hours ago, Owen Sullivan said:

How would the film stay flat on the gate without the top and bottom rails at the opening? Wouldn't that allow it to sag inward? Also, does the pressure plate also not have top and bottom rails? 

The film is held in place by the pressure plate. The section of the pressure plate where the film is being exposed, has a little separate section that on the non-HR mag's, has it's own spring that holds the film in place. So not only is the film squeezed laterally between the rails, but also pinched in placed by the pressure plate on the magazine. 

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4 hours ago, Owen Sullivan said:

How would the film stay flat on the gate without the top and bottom rails at the opening? Wouldn't that allow it to sag inward? Also, does the pressure plate also not have top and bottom rails? 

 

On 1/21/2014 at 1:12 PM, David Fitch said:

post-29599-0-27777600-1390269428_thumb.jpg

 

If you're referring to this modified gate, I agree. There is nothing stopping the film from bowing in between the two side rails. It's not something I've ever seen to be honest - it's certainly not an Arri modification, but some third party creation.

However, there are a few reasons it might not be as problematic as it seems.

Firstly, fresh film stock tends to be fairly flat across it's width, at least with small gauges like 16mm. Because it is stored rolled - in other words curved along its length - any bowing tends to be in that direction, since it cannot bow in both directions. Old, or heat-damaged film is another story.

Second, the SR gate itself allows a little room for the film to have slight lengthwise curl, which minimises the potential for curl across the width. The main length of the SR pressure plate doesn't actually press on the film at all, but locates in the gate with it's four corner legs and creates a fixed channel slightly deeper than the thickness of the film. Only at the gate aperture itself is the film pressed against, by a second smaller pressure plate. It presses on the edges and in the middle.

787522535_SR2pressureplate.jpg.2d83a6dcedcc4d3787f5eda079e208e1.jpg

 

There are 16mm camera gates without a polished frame around the gate aperture, such as Aaton XTRs or Bolexes, and they tend to work fine, so I think it's probably not such an issue. The main thing is that the gate width is not too small, or the side pressure spring too strong, compressing the film across it's width, because that could very well create bowing.

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  • 4 weeks later...

Hi David,

 Yes, that is the (vertical) parallax adjustment for the viewing screen and it has been right there through every version of the 16SR camera. The set screw is flanged from the rear and cannot extract for enough to cause film scratching. I've never seen one damage film in 37 years!

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