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internal and external Filters


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#1 John Adolfi

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Posted 30 October 2006 - 02:58 PM

I got this idea from this forum a few months back and I want to place this before you people. Since the super-8 cameras are getting older and because of the newer film sotck demands, when we have a camera overhauled should we ask the Tech to remove the internal filter and then go with an external one. I'm assuming correctly that this flimsy pieces of plastic can or could deteoriate, get damaged? Also having the fullest control would mean chosing between a 85 or 85B to better balance a film temp.
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#2 André Bacher

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Posted 30 October 2006 - 04:37 PM

Hello!

please keep in mind that if you remove the neutral and wratten filter, the original optical setup inside the camera has changed! After removing the filter you have to re-adjust the camera optical system.
If not (only popped out the filter), you loose image quality!

greetings,
André
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#3 John Adolfi

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Posted 30 October 2006 - 06:07 PM

Hello!

please keep in mind that if you remove the neutral and wratten filter, the original optical setup inside the camera has changed! After removing the filter you have to re-adjust the camera optical system.
If not (only popped out the filter), you loose image quality!

greetings,
André

I'm not sure I follow you. What is out of adjustment? Optical? What exactly does that mean? How do you know this? Seems to me all you are doing is taking the filter inside out and replacing it with an external filter.
+filter-filter = -filter+filter. Simple algebra.
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#4 André Bacher

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Posted 30 October 2006 - 07:19 PM

I'm not sure I follow you. What is out of adjustment? Optical? What exactly does that mean? How do you know this? Seems to me all you are doing is taking the filter inside out and replacing it with an external filter.
+filter-filter = -filter+filter. Simple algebra.


Hi,

first, i forgot to wrote that this is guilty for the Beaulieu 4008 and maybe other S-8mm cameras with changeable lenses, sorry for that!

You are right that it makes sense to remove the old wratten filter because this 20-30 year old filter losts his optical quality most of the time.

I try to explain it for the Beaulieu 4008. Inside the camera body are two filters, one is the wratten (85) filter for daylight, the other one is a clear UV filter for tungsten.
if you look through the viewfinder, you cannot see a switch to "red" when you use the built in wratten because the filter effect happens after the viewfinder take-off.
The "optical-system" inside the camera is calculated inclusiv this little, thin filter (UV or 85). If you only remove the built in 85 filter, these minimum millimeters are gone and the back focus is not exact anymore. This will be no problem for long telephoto shots with f11 or f22 but using short 4, 6 or 8mm at f1.8 you can get a problem here!
This is also the reason why it is a difference to filter in front of a zoom lense or after the last element of the zoom inside the "flange focal depth".
I hope that makes sense to you?!

As far as i know the Leicina also has the two built in filters, but i dont know 100% for sure.

cheers,
André
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#5 Tom Doolittle

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Posted 30 October 2006 - 08:21 PM

If you only remove the built in 85 filter, these minimum millimeters are gone and the back focus is not exact anymore. This will be no problem for long telephoto shots with f11 or f22 but using short 4, 6 or 8mm at f1.8 you can get a problem here!


I'm not sure that's correct. The filter is not an optical element. It does nothing more than alter the color of light passing through it.

Slightly off-topic, but with regard to the removal of internal filters, some 40/160 cameras (compact Nizos, for example) have a separate little ND filter that pivots into the path of the photocell when the cartridge notch reader thinks the camera is loaded with ASA 40. This filter does not interfer at all with the light reaching the film, but merely "fools" the photocell into thinking there is less light available. Obviously this filter is calibrated for the difference between the two film speeds these cameras were designed to read, 40 and 160, or 1-2/3 stop. So, if some handy camera tech could replace THIS filter with one that had a rating of only 2/3 stop, would this not make the camera meter properly with 64T?

I hope I did the math right. Feel free to point out any errors in my logic.

oops, yeah, I can see it already...

You would need a filter that had a rating that was 2/3 LESS than the existing 1-2/3, making it a one-stop filter. Right? Man, my head hurts...
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#6 André Bacher

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Posted 31 October 2006 - 10:40 AM

Hello Tom!

The filter is not an optical element. It does nothing more than alter the color of light passing through it.

But not if the filter is inside the Flange Focal Depth, if this is the case, the filter IS relevant and changes the definition of this because it changes the standart c-mount flange focal depth (in case of the Beaulieu 4008).

The built in UV-clear filter is there because something must compensate the optical distance like the 85 filter when you shoot tungsten (there is no other logical reason for this UV filter on this place).
The modification (removing the filters and re-adjust the backfocus of the camera body) is done by camera technicians since the mid of 1970. In germany this cameras have the added word "Pro".
As i know, Beaulieu stopped using built-in filters on the higher models like the 6009 to 9008 because of this, you have to filter in front of the lens.

Please let me qote a sentence of Mr. Douglas C. Hart (Author of The Camera Assistant, page 256):

The Flange Focal Depth is the distance between the rear mounting flange of a lens and the film plane. This distance varies from camera system to camera system, but is a specific critical distance, measured in ten-thousendths of an inch.

I dont know exactly how thick a wratten-85 filter is, i think something between 0,01 and 0,1 mm - and then it counts in the world of "ten-thousendths of an inch"?!?

greetings,
André
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#7 Tom Doolittle

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Posted 31 October 2006 - 01:57 PM

...changes the standart c-mount flange focal depth (in case of the Beaulieu 4008).

The built in UV-clear filter is there because something must compensate the optical distance...


I am unfamiliar with the Beaulieu system, but I gather from your description that the filters are somehow integral to the lens mount. Am I correct? Is the lens displaced by the filter? Only then would the flange focal distance be affected.

In the case of most Super8 cameras, the 85 filter is simply a small blade of filter material that slips into a slot just before of after the shutter. Its presence or absence has no influence on back focus.

Consider the average Bolex, having a slot just behind the lens into which a user may choose to place a filter. The introduction or removal of said filter does not necessitate any compensatory adjustment to the lens. In most Super8 cameras the filter functions in exactly this manner, but is operated automatically rather than by hand.

Either way I'm not sure relying on behind-the-lens filters is good practice. Besides being impossible to access on most cameras (and therefore impossible to clean), they are likely to have faded considerably after twenty or more years. It is easy enough to pick up a new 85B at any camera shop and screw it onto the lens when needed.

As for the original topic: I'm not sure why one would bother removing the internal filter. If you just leave it on the "lightbulb" setting, the internal filter will remain displaced regardless of what film you put in the camera. If your fear is that you will inadvertently activate the filter, you could place a bit of tape over the switch, or remove the switch entirely. Either of these solutions avoids doing irreparable harm to the delicate shutter and aperture systems which reside in the same area as the offending internal filter. Your best bet is to simply flip the swtich (if you have one) to the "lightbulb" (tungsten) setting and leave it there. Buy a screw-on 85B and remember to use it whenever you shoot T-rated film outdoors.
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#8 Sean McHenry

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Posted 31 October 2006 - 07:02 PM

Before I begin, I am a television Engineer and not a film guy (but I am working on it) so to me it makes sense that the small possibly plastic filter may have discolored, especially if they are a gel type after all these years.

I would say that since most of these small film cameras either swing the filter out or in depending on the cart inserted that pulling the filter out completly should (I am not a film camera tech - should) have no effect on the focus of the lens elements. After all the camera itself will take the filter out of the optical path if the correct cart is used.

It is quite possible that this would be a good way to go on some older cameras but I haven't heard of anyone having color temp issues due to their built in filter aging. I am sure if you took a color temp reading off the light entering the cart area with some special light meters you may find it has changed but whether we can see that change?

Sean

PS,
I think Mr. Doolittle is correct. The clear filter, if it is a UV filter is not there to keep the focus from changing in the absence of the CC filter, if it is truely a UV filter it actually has a different purpose and isn't there just to place a piece of focus shifting glass in the optical path.

For the record, video cameras are exactly the same. A small filter wheel just after the lensing and before the optical block. You can populate the filter wheels with any number of filter - or none. Flipping filter in and out has no noticable effect on the focus or back focus.

I think the earlier gentleman may have been a bit confused by asking if the filters went between the lens and the lens mount. As far as I know, no filters work that way. All aftermarket filters, except the expensive filter wheel filters for video cameras mentioned above go in front of the lens.

Sean
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#9 Bernie O'Doherty

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Posted 31 October 2006 - 10:20 PM

Any filter, clear or otherwise, which is placed in the light path behind the lens, will shift the focus.
It will move the focal point slightlly Past the film plane, depending on the thickness of the gel.
Consequently, the lens needs to be calibrated to the new optical distance, i.e. Closer to the film plane.
Beautiful theory.
However, reality enters and we find that, over the years, those gels get dirty, concave and convex, ripple and, generally, degrade the image. It is better to get rid of them.
Now you've got a much clearer light path...But it's out of focus!
So we change adjust the lens calibration to bring the light to focus on the film plane, i.e. Closer in.
If that is not done your final image will not be sharp.

Cheers, Bernie
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#10 Tom Doolittle

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Posted 01 November 2006 - 01:28 AM

Surely Bernie is the expert here. As he mentioned, any element placed in the light path will, in theory, alter the back-focus.

Still, I believe that in the case of *most* Super8 cameras, removing the filter from the light path will not have any adverse affects on focus and will not necessitate an adjustment of the lens.

Follow my reasoning...

1. I currently own two Super8 cameras- a Nizo and a Nikon. Both cameras have internal 85 filters that swing out of the light path when not in use. In the case of the Nizo, I am 100% certain there exists no other filter, clear or otherwise, that fills the void left when the 85 is retracted. I have taken this camera apart several times, and am willing to swear on my mother's collection of antique Tupperware that when the color-correcting filter is out, there is only an empty space left behind. In the case of the Nikon, I have not seen far enough into its internals to verify this, but I suspect the same is true. My suspicions are based on the fact that when I look into the lens and put a flashlight in the gate, I can see twenty-seven years worth of accumulated crud adhered to the orange-colored surface of the daylight filter. When it is displaced, the view is crystal clear. If there were a "clear" or UV filter in there, it would have been subject to the same accumulation of junk as the other, and I would probably not see such a noticable improvement in clarity. (Consequently, I never use the internal filter of either camera.)

2. I once owned an old Sony Betamax camera. When it died I took it apart to see what made it tick. It contained three internal filters, each one a tiny leaf of gel that was rotated into position in response to the setting of a switch on the side of the camera. When no filter was selected, there were no filters in the light path, and nothing to offset the theoretical loss of critical back focus.

3. While I do not own one, I have seen many a Bolex with the filter slot behind the lens. When no filter is used, the filter holder is simply empty, and I have never heard mention of the requirement for what amounts to an "optical place-holder".

I agree that yes, in theory, the back-focus must be affected. But if the thickness of the filter were enough to throw the back-focus out, how then would these cameras compensate for that? As far as I can see, they don't. The thickness of the filter is apparently not enough to require any compensation. The only scenario I can imagine in which the insertion or removal of a behind-the-lens filter would require adjusting the back-focus of the lens is one where the filter was somehow integral to the positioning of the lens, perhaps screwed onto the back of it like an adapter, and by removing it you would have to put something back in its place to maintain the proper distance between rear-element and film-plane.

Surely there are many exceptions to this but I presume most Super8 cameras have internal filter systems of no more complexity than the above mentioned cameras. Assuming one has a camera of comparable design, I am pretty darned sure you could rip the filter right out without any worry about having to re-set the lens. Why you would bother doing that, I don't know, but that's what started this topic going.

It would be great to see the math done on this. If you really wanted to, you could get out your old Physics textbook and look up Snell's Law and figure out the angles of incidence and refraction all that crap but who has that sort of time? I'm guessing you'd need to see the original engineering drawings for a particular camera as well.

Please correct me if I'm wrong about any of this.
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#11 Bernie O'Doherty

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Posted 01 November 2006 - 09:38 AM

Hi, Tom,

You're probably right in your assessment that there is very little difference with or without the clear filter. On my collimator, however, there is an actual difference, and it runs between .001-.003". This is enough to change green to red on the collimator, meaning that there is in fact a visible focus shift. You'd probably only see this bad focus if you were blowing up from Super 8 to 35 and even then, it wouldn't be very apparent. I always end up recalibrating the lens to the new specs.

Video cameras (like the Sony Betamax) are different. Generally, the operator can change the focal point of the lens by simply twisting a knob and adjusting for best focus. They were designed this way because the designers knew that an internal filter ring would change the focal distance.

The point I was trying to make in my previous post was that you gain more in image quality when you pull out the old, crinkly gels than you lose in focus shift by not having a gel in place. So if you're not blowing up to 35 or 70, you'll be fine just removing them. Also, if you're using the regular cassette Kodak film with the plastic pressure pads, the whole argument is kind of beside the point anyway since the film cassette design is so unstable and sloppy. You don't need a crinkly filter or the lack thereof to degrade your image--the cassette takes care of it!

Cheers,
Bernie
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#12 Tom Doolittle

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Posted 01 November 2006 - 11:42 AM

Thanks for the clarification, Bernie.

The only thing I was trying to say was that many Super8 cameras do not have a clear or UV filter to offset the theoretical shift in focus.

The initial question was:

"...should we ask the Tech to remove the internal filter and then go with an external one. I'm assuming correctly that this flimsy pieces of plastic can or could deteoriate, get damaged?..."

My answer:

Although I see no point in it, removing it will do no harm. All you really have to do is flip the switch from the little sun to the little lightbulb. Doing so removes the filter from the lightpath. On many cameras, nothing replaces it, and nobody ever worried about what impact that had on back focus until now.
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#13 Erik J. Weber

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Posted 01 November 2006 - 11:45 AM

the whole argument is kind of beside the point anyway since the film cassette design is so unstable and sloppy. You don't need a crinkly filter or the lack thereof to degrade your image--the cassette takes care of it!

Haha, lovely.

Erik
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#14 André Bacher

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Posted 01 November 2006 - 02:02 PM

Hello,

@Bernie: Thank you for your postings too!

...On many cameras, nothing replaces it, and nobody ever worried about what impact that had on back focus until now.

A lot of Super-8 users care about if the back focus is as good as possible. The discussion about this filter-remove-question is very old and not a result of this thread ;-)
Maybe it is also a question why or for what somebody is using the Super-8 format.

The only thing I was trying to say was that many Super8 cameras do not have a clear or UV filter to offset the theoretical shift in focus.

And the only thing I was trying to say was that some super 8 cameras have maybe a backfocus problem after removing the built in filters. Maybe, someone here did not know this and now, thinks about checking the camera befor using it for professional shootings. Thats all why i said here something about it.

The initial question was:
"...should we ask the Tech to remove the internal filter and then go with an external one. I'm assuming correctly that this flimsy pieces of plastic can or could deteoriate, get damaged?..."
My answer:
Although I see no point in it, removing it will do no harm. All you really have to do is flip the switch from the little sun to the little lightbulb. Doing so removes the filter from the lightpath. On many cameras, nothing replaces it, and nobody ever worried about what impact that had on back focus until now.

My answer:
I think it is better to remove dusty old filters and shoot the next holiday home movie with (maybe) little back focus problem and filters in front of the lens than to shoot with dirty old filters inside the camera.
But, if you want to use the Super-8 Medium for professional work, why not optimize all you can?

Which means:
- Use one of the better Super-8 cameras (Leicina, Beaulieu, Nikon, Nizo, etc.), think about the typ of lens and mounts
- Find an professional camera technician for a complete service check-up
- optimize the camera: remove internal filters (Beaulieu 4008, maybe other brands too?!), use sync options for stable 24 or 25 frames, use the FrameMaster to compensate a little the bad cartridge pressure plate
- think about using digital stabilization in post production
- shoot camera tests
- etc.


greetings,
André
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#15 Sean McHenry

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Posted 03 November 2006 - 10:45 AM

So in the end, it shakes out like this;
You are probably better off removing any old dirty crinkly wrinkled gel filters and live with a "possible" back focus issue that is likely negated altogether by the actual films flopping movement through the inexpensive gate mechanism.

In theory, the film flopping around could move the film in and out of the focal point while running causing a real but imperceptible shift. And you have to temper all this info by asking if we aren't trying to squeee High Def out of a Pixelvision.

All great usable info but for what most of us are using these for, I just know I will never see it. If it came down to less light and a dirty filter causing initial focus issues, I would probably pull that filter out (or have Bernie do it) and have it brought back into alignment if I was shooting something critical or just wanted to say my camera had been properly adjusted.

Sean
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#16 Fran Kuhn

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Posted 03 November 2006 - 03:06 PM

The introduction or removal of said filter does not necessitate any compensatory adjustment to the lens.


Hi Tom,

It's interesting to actually see the effect of placing a filter behind the lens. When I use my 8 x 10 view camera, it is often necessary to filter behind the lens because the lens' front element is sometimes too large for even a 4-by-4 inch filter to cover. Place even one micro-thin wratten gel behind the lens and the focus shift is immediately obvious on the camera's ground glass.
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#17 alfredoparra

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Posted 08 November 2006 - 11:27 PM

I have a Beaulei 4008, I removed the internal gels and I filed down the gate for wide screen format, I did some filming and the results were grate! but I will take Bernies advice and have the lens adjusted to the camera any ways! Its funny to read these threads lots of misleading info!
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#18 Anthony Schilling

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Posted 09 November 2006 - 01:45 AM

I got this idea from this forum a few months back and I want to place this before you people. Since the super-8 cameras are getting older and because of the newer film sotck demands, when we have a camera overhauled should we ask the Tech to remove the internal filter and then go with an external one. I'm assuming correctly that this flimsy pieces of plastic can or could deteoriate, get damaged? Also having the fullest control would mean chosing between a 85 or 85B to better balance a film temp.


There is no reason why you can't just disable the internal filter, and use an external 85B filter. Now days, all your color tungston films call for an 85B filter for daylight shooting. The standard 85 is a remnant from the old days and photo floods. Sure the 85 will still work fine with color negs and telecine/color correction... but if your going to get one filter, why not get the right one. The 85B also cuts more light than the 85, and that comes in handy since most of your T negs are pretty high ASA for daylight. For example, 200T is rated as 125ASA in daylight with an 85 filter, but 80ASA with an 85B.
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