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Focus Verification


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Sorry if this topic has been discussed before. I couldn't find anything on it with my limited search skills.

I know there is a method, for film cameras, to verify accurate focus. In the sense that, if the image is in focus in the viewfinder, it would be in focus on the film plane. Is there such a test, and how to conduct it?

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If you mean focusing by the rack-over process with a turret camera, it’s as simple as peeping through the camera’s focuser, setting a lens its iris fully open, and taking it to the position in front of the film. Lastly, you relocate the camera on a rackover support by the stretch that lies between the focuser and the exposure aperture.

The Keystone K-56 for example has a two ports turret and a ground glass focuser. It works for sharpness, not in view of framing because the turret plate totters. A mechanic can improve that, though.

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For modern cameras with a groundglass and reflex mirror, the only accurate method is to use an auto-collimator and a calibrated test lens. Most rental houses or cine lens technicians with film camera experience would have this equipment. You would use the auto-collimator and test lens to check infinity focus on the ground glass, an optical measurement since you can’t physically measure the distance from lens mount to groundglass as reflected off the mirror surface. For the matching focal flange depth, which is the distance from lens mount to film plane, a flange depth gauge and backing plate in the gate is used to physically measure the distance. Any discrepancies can be adjusted back to spec.

Without access to these tools, you would need to shoot a focus test by eye and check that the recorded image is in focus, but this is not as accurate and only tells you if something is off. A technician would still need to measure and recalibrate the settings if there was a discrepancy. 

The distances of flange depth and ground glass depth are more critical with shorter focal lengths and open apertures, so ideally you would shoot a focus test with a wide lens at maximum aperture. Maybe an 18mm or 25mm for S35. But this is also harder to judge perfect focus by eye, and lens aberrations can introduce softness that might be mistaken for a focus error. A longer lens is easier to critically focus in the viewfinder, but has more room for error at the film plane, so it will not necessarily show up a small discrepancy in ground glass depth.

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17 hours ago, Dom Jaeger said:

Without access to these tools, you would need to shoot a focus test by eye

Wow. I'm stuck with that for now I guess. I was thinking about shooting a slanted tape measure and focus by eye on 39" for example and then check the developed film to see what number actually came in focus. But that is not easy with wide lens. Even wide open a whole bunch of number will be in focus. This is for a Beaulieu R16. It's not a rotating shutter. 

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6 hours ago, Raymond Zananiri said:

I was thinking about shooting a slanted tape measure and focus by eye on 39" for example and then check the developed film to see what number actually came in focus.

Now we can follow your thoughts. Not wrong, yet a very difficult task. Be assured that the depth of focus seen on a fine ground glass is what will be exposed or, as an old Bell & Howell slogan went: What you see, you get.

The measuring-tape method yields a base to determine hyperfocal distance. For an exact distance choice one would need the mathematics of a given lens. The pull-out (from the infinity stop) could be made by measuring the angle of rotation of the focusing thread whose pitch must be known.

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18 hours ago, Simon Wyss said:

Be assured that the depth of focus seen on a fine ground glass is what will be exposed

Thanks for the help Simon. Isn't there a chance that the ground glass might not be seated correctly? Or I shouldn't worry about that? Is that a problem that is very rare with film cameras?

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It is with the exception of Pathé WEBO M. Generally, the ground glass is glued in with amateur products but fitted with professional equipment. To verify correct seating is a technician’s task. Do you have a doubt about a camera of yours?

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2 hours ago, Simon Wyss said:

It is with the exception of Pathé WEBO M. Generally, the ground glass is glued in with amateur products but fitted with professional equipment. To verify correct seating is a technician’s task. Do you have a doubt about a camera of yours?

Maybe you’re right about glued ground glasses with some cameras Simon but none of the major camera manufacturers like Panavision or Arri does that. Panavision’s ground glass simply slides into a frame, while Arri’s is loaded into a spring loaded frame. We would always need to pull them out regularly for cleaning or changing formats. 
 

As for checking the accuracy of ground glass focus, there is a very simple test for that.
 

1. Take a sheet of newspaper with all print (no pictures) on it - the classified section is good for this. Any large sheet of paper with rows of fine print will work. 
 

2. In the middle of the page, draw a line through a row of print. Make your line with a colored sharpie such as red so it stands out against the black print. 
 

3. Take the sheet of paper, tape it on a flat wall with the print oriented vertically, NOT horizontally. 
 

4. Take your camera with a long lens, wide open at minimum focus, in order to get shallow depth of field. Place the camera on an extreme raking angle towards the news print, placing the vertical red line right in the middle of the frame. Fill the frame with the rows of print. Eye focus carefully on the red line so it is sharp and you can see the other rows falling off out of focus fore and aft in your frame. Roll camera. 
 

5. When you view the footage and the red line is sharp, your ground glass is properly collimated. If it’s not sharp, it’s not. Now you know if you can trust eye focus or not. 
 

G

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In case one has a replacing part for a mirrored shutter, a mechanical and therefore positive adjustment can be made. If I take the Arriflex 16 for instance, I’d install a steel disc instead of the shutter, it needs to be of equal thickness specifically. Then I’d wring a steel prism to it, the dimensions of which I know (because they can be measured very precisely). Last, I can bring the prism still in contact with the disc to the position where the ground glass will be and scrutinise the seat. Basically, this procedure is essential to check shutter position relative to lens seat and GG seat.

Some insight into the work I do.

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12 hours ago, Raymond Zananiri said:

Thanks for the help Simon. Isn't there a chance that the ground glass might not be seated correctly? Or I shouldn't worry about that? Is that a problem that is very rare with film cameras?


There is a chance, but in my experience having checked hundreds of cameras over the years it generally tends to remain at the factory setting.

Even if a mount is changed or shimmed, it will affect both flange depth and ground glass depth the same, so they remain locked together. The only time it changes is if the gate gets exchanged for instance, and is now sitting at a different depth from the ground glass. Or occasionally vibration or an impact can shift a setting like ground glass depth or the mirror position but in my experience it’s pretty rare. 

So usually, eye-focus is more reliable than going off the lens distance scale, which may be off if either the lens or the camera flange depth are a bit out of tolerance.

 But because it’s such an important setting in film cameras, I would always check the ground glass focus as well as flange depth of any film camera before it went out on a job when I worked at a rental house during the film days. Every now and then I found one that needed a slight tweak, for whatever reason.

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