Beaulieu Super 8 camera internal gelatin filters

[joelpierre]

I have read on a french forum that if the internal gelatin filters are removed of the cameras, it is necessary to re-calibrate the internal optics (especially for very short focal lengths).

http://www.repaire.net/forums/film-arge ... ciale.html

Google Traduction: http://translate.google.fr/translate?sl ... ciale.html

But it seems that cameras with interchangeable lenses like Beaulieu (2008-4008) and Leicina Special have no internal lens that could be re-calibrated.

Can we remove the internal filters of Beaulieu (2008-4008) without focus problem (with no other changes)?

[aj]

There is plenty to read on that topic on this forum too. If you can locate the threads. Use google on this site.

The built-in filters are made from optically neutral material. Taking them out causes no loss of quality.

Most important is to not drop the pressed out filter into the camera mechanism

Still, others persist (contraire theory) that measures must be taken to alter the back-focus distance of lenses used on such cameras. I.e. you would need to collimate the lens to the camera or a hair's thickness would need to be shaved off the c-mount flange. Of course at price.

Considering usual super-8 camera-handling ((extra)sloppy) worries about a few microns appear like measuring sea-level using a millimeter scale.

[bolextech]

If you are the owner of a Beaulieu or a Leicina, you are after a certain level of quality. Chances are you will also like to experiment with different lenses.

The presence of a gelatine filter in the path of image forming rays has a significant impact on focus, especially in the smaller formats. Of all motion picture formats, the 8mm and super8mm formats have the least amount of depth of focus at the film plane. Something on the order of 0.011mm.

The smaller the format, the less depth of focus.
Shorter focal length lenses have less depth of focus.
Using wide apertures decreases depth of focus.

That's why if your filmmaking is limited to shooting on bright sunny days at f/5.6 - f/8 with moderately wide focal lengths, you'll never notice a difference if the filters are removed.

In short: if you have the filters removed, have the flange-focal distance and the groundglass reset at the same time.

Interesting reading: http://en.wikipedia.org/wiki/Depth_of_focus

Cheers,
Jean-Louis

[joelpierre]

The smaller the format, the less depth of focus.
Shorter focal length lenses have less depth of focus.

I think it is the opposite. More the film format is large (and thus its focal length), more depth of focus is reduced. :?:

On 16 mm Bolex camera, filters are inserted between the rear of the lens and the frame of the camera without optical correction with or without filter.

[bolextech]

Hi joelpierre,

Depth of focus is directly proportional to the circle of confusion.
Look at: http://www.dofmaster.com/dofjs.html
Select different formats and you will see how depth of focus gets shallower the smaller the format is.

As far as the Bolex camera is concerned, I must correct you.
Of course, the focus shift is not noticeable to the naked eye but any gel filter inserted in the filter slot has an immediate and visible effect when checked on the collimator.

If you like, we can continue this discussion on Repaire on which I am also a member.

Cheers,
Jean-Louis

[slashmaster]

Hey Bolextech, I've got a Yashica which never worked right so I overpowered it adding more AA batteries for slow motion. I focused it carefully for every shot. It took a little bit of footage for it to accelerate up to speed and around maybe 50 or 60 fps it got out of focus. I guess it must be because the film gate lifts up a little at high speed? Anyway, it's happened with all the cartridges I've used above a certain speed. What can I do to get better focus when it lifts up like that? Should I set the focus a couple feet closer to the subject than the camera actually is?

[bolextech]

Hi slashmaster,

It might be better to start a new thread on this subject.

Do you have any samples of this footage that you could post?

Without seeing your footage, I kinda doubt that the film actually moves out of the plane of focus. I suspect it might look blurry because of the film being jerked down during the exposure cycle by the cartridge take-up. Obviously the camera's mechanism wasn't designed to go this fast.

Cheers,
Jean-Louis

[slashmaster]

Hi slashmaster,

It might be better to start a new thread on this subject.

Do you have any samples of this footage that you could post?

Without seeing your footage, I kinda doubt that the film actually moves out of the plane of focus. I suspect it might look blurry because of the film being jerked down during the exposure cycle by the cartridge take-up. Obviously the camera's mechanism wasn't designed to go this fast.

Cheers,
Jean-Louis

Thanks Bolextech. Someday when I can do a transfer that's halfway decent I'll post it. It definatley doesn't look like the up and down streak kind of blur where the film moved before the shutter closed. And yes, the camera wasn't designed to go that fast!

[Jean Poirier]

I had the filter(s) removed and the lens recalibrated on a 4008ZM2. I focused by eye since there was a small difference at infinity setting. Results were sharp. Jean

[joelpierre]

Depth of focus is directly proportional to the circle of confusion. Look at: http://www.dofmaster.com/dofjs.html
Select different formats and you will see how depth of focus gets shallower the smaller the format is.

With the application Kodak Cinema Tools:

Super 8: circle of confusion: 0,0127 mm

Focal length: 6 mm
Subject distance: 3 m
f-stop: f/2
Depth of field: Infinite
Near limit: 0,96 m
Far limit: ∞
Hyperfocal distance: 1,42 m

Focal length: 66 mm
Subject distance: 3 m
f-stop: f/2
Depth of field: 0,10 m
Near limit: 2,95 m
Far limit: 3,05 m
Hyperfocal distance: 171,50 m

Super 16: circle of confusion: 0,0127 mm

Focal length: 66 mm
Subject distance: 3 m
f-stop: f/2
Depth of field: 0,10 m
Near limit: 2,95 m
Far limit: 3,05 m
Hyperfocal distance: 1,42 m

[bolextech]

Hi joelpierre,

I don't see what your posting adds to the discussion.

I think you are confusing depth-of-focus with depth-of-field which are two entirely different things.

From http://en.wikipedia.org/wiki/Depth_of_focus

"In small-format cameras, the smaller circle of confusion limit yields a proportionately smaller depth of focus. In motion picture cameras, different lens mount and camera gate combinations have exact flange focal depth measurements to which lenses are calibrated.

The choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with smaller formats. Placement of items behind the lens will alter the optics pathway, shifting the focal plane. Therefore, often this insertion must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus. It is often advised in 35 mm motion picture filming not to use filters behind the lens if the lens is wider than 25 mm."

Cheers,
Jean-Louis

[bolextech]

By the way, the circle of confusion for 16mm and super8 cannot be the same.

The circle of confusion for super8 is 0.0165mm

"The size of the circle of confusion that we can accept is dependent on how we view the projected film. ...the choice of an acceptable maximum diameter for the circle of confusion is based upon the projected film as it is viewed by some hypothetical viewer, a certain distance from a screen of a certain size. The choice that has been made for the circle of confusion for 16mm is 0.001 inch, and for 8mm 0.0005 inch. The circle of confusion for super 8-single 8 would be very nearly that of 8mm, actually 0.00065 inch."

Independant Filmmaking by Lenny Lipton p.163

That is:
16mm 0.0254mm
super8 0.0165mm
8mm 0.0127mm

Cheers,
Jean-Louis

[Will2]

This is the internet, you're not supposed to site sources, you're supposed to make it up and pass it off as fact.

[bolextech]

To say I'm old-school is an understatement. I guess that's what happens when you spend the whole day surrounded by stuff made in the 60s.

Cheers,
Jean-Louis

[joelpierre]

I don’t see what your posting adds to the discussion.
I think you are confusing depth-of-focus with depth-of-field which are two entirely different things.

Depth of focus
http://en.wikipedia.org/wiki/Depth_of_focus
Calculation
When the depth of focus relates to a single plane in object space, it can be calculated from:

T = 2 N c (v / f)

Where T is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length.

According to the formula, which is located at the rear of the lens would have no impact.