Internal filters and back-focus question

[Tom Doolittle]

While I was mucking around inside my Beaulieu last night, I noticed the clear gel was missing from the filter holder. I later found it laying in the bottom of the outer shell, but I decided not to put it back in place. I also went ahead and popped out the 85 filter, because I never trust those internal filters and always use screw-on filters on the lens.

In another forum, there was a lengthy discussion about how removing internal filters may throw out the back-focus. I can see how this might be theoretically true, but I was wondering if anyone here had any practical experience with this?

For the record, I measured the gels with a micrometer. They are 0.0035" thick. I'll shoot a test this weekend, but something tells me I won't notice any difference. Any thoughts?

[sciolist]

Although the effect of these gels is very small, their removal does make a difference when the zoom lens usually fitted is used at its shortest focal length at full aperture (e.g., 6mm at f/1.8 or 8mm at f/1.9). The softness that results isn't apparent in the viewfinder, only on the projected film. When the built-in filters are removed from a Beaulieu 4008 series camera, the flange focus distance should be adjusted (by machining the camera body) from 17.57-58mm to 17.52mm for optimum performance.

[aj]

In another forum, there was a lengthy discussion about how removing internal filters may throw out the back-focus. I can see how this might be theoretically true, but I was wondering if anyone here had any practical experience with this?

For the record, I measured the gels with a micrometer. They are 0.0035" thick. I'll shoot a test this weekend, but something tells me I won't notice any difference. Any thoughts?

The lenghty discussion was recently staged here too. Search for it.

For years it was explained that the rear of the lens held an adjustable part to collimate it to the camera. Only experts could do this. New technology is now to take some material of the camera to let the lens(es) go in deeper.

On average S8 filming (average aperture, average distance, hand held) nobody is going to notice if your back-focus is off by some micrometers.

[Tom Doolittle]

...the flange focus distance should be adjusted (by machining the camera body) from 17.57-58mm to 17.52mm for optimum performance.

Really? Where did you find this number? If I can be reasonably certain this is a proven adjustment I can have the mount turned down to these specs. I have a machinist standing by... Well, almost.

[sciolist]

I recently had my 4008 ZM II overhauled by Bjorn Andersson. As a part of the service, Bjorn removed the gel filters at my request, machining the camera body to reduce the flange focus distance to compensate. I do not know if these measurements apply to the S 2008 camera.

[sciolist]

These built-in gelatin filters were a convenience when the cameras were new, but, as you've found, they don't always age well. I use daylight color reversal film exclusively, so the #85 is of no use to me anyway.

[metaT8]

Although the effect of these gels is very small, their removal does make a difference when the zoom lens usually fitted is used at its shortest focal length at full aperture...
...When the built-in filters are removed from a Beaulieu 4008 series camera, the flange focus distance should be adjusted...

I don't get it.

Are you saying that when you engage and disengage the internal filter there is another back focus change taking place as well? How is that possible?

If internal filters altered the back focus wouldn't B&W & daylight film have a different focus characteristic than tungsten?

To the best of my understanding the filter is simply placed in between the lens and the film -- and does not alter focus any more than an external filter would.

I've had more problems with jittery carts and fluctuating back pressure from carts recently than I ever had in the past. Personally I think that's are far more serious issue.

All that said. My tests have proved that some of my internal filters are crap and they DO alter the image quality. So I use external filers.

best,

m

[sciolist]

Are you saying that when you engage and disengage the internal filter there is another back focus change taking place as well? How is that possible?

If internal filters altered the back focus wouldn't B&W & daylight film have a different focus characteristic than tungsten?

To the best of my understanding the filter is simply placed in between the lens and the film -- and does not alter focus any more than an external filter would.

An internal gelatin filter is always in the optical path in the Beaulieu 2008/4008 series cameras. Either the UV filter or the #85 is in place. There is no setting that allows filming without one of the two filters in place as you seem to suggest is possible.

Back focus is affected when these behind-the-lens gelatin filters are removed. Front mounted filters do not affect the back focus in any perceptible way. Professional camera technicians who post to this forum from time-to-time (e.g., Bjorn Andersson and Bernie O'Doherty) can testify to this. However, as others have indicated, unless one is using short focus lenses at full aperture, the effect of the focus shift resulting from the removal of the gelatin filters won't be detectable.

[aj]

Professional camera technicians who post to this forum from time-to-time (e.g., Bjorn Andersson and Bernie O'Doherty)

You are mixing up forums. Neither of these two posts here.
Unless under nome de plume (ie a different name).

[Andreas Wideroe]

Not correct. Björn Andersson posts here.

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