Film-Tech Forum ARCHIVE: 3-bladed shutters

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Posted by Matt Zeiner (Member # 1914) on 01-10-2004, 11:05 AM:

I have 2 old (black cast) XLs that have 3 bladed shutters in a screening room I service. There is plenty of light on screen. Wondering if there is any tangible benfit to using them as opposed to the standard 90 degree bowtie...these are some of the nicest running machines I have ever come across, btw. Incidentally, there are no heat sheilds installed on either machine.

Posted by Sam D. Chavez (Member # 1841) on 01-10-2004, 11:46 AM:

The benefit is the flicker rate goes from 48 to 72 so the image is smoother.

You lose 50% more light, relative to a two bladed shutter, not 33% as one would initially suppose.

Interestingly, the grain in the image becomes more pronounced with a three bladed shutter. There are varying opinions as to why. One possibility is that the film breathes less (shorter pulses of light) so you are getting a more accurate view at what is actually on the frame. Perhaps it is more complicated and has something to do with the persistence of vision phenomenon.

Posted by John Pytlak (Member # 331) on 01-10-2004, 12:13 PM:

I never heard that the shutter had any effect on perception of grain. Simplex's Willy Borberg and Kodak's Dr. Fred Kolb did quite a bit of reseach about the effects of radiant energy on the film frame back about 50 years ago. Borberg found that for very high levels of radiant energy, a two-blade shutter was preferred because the film frame had a longer time to achieve a stable position, especially during the second exposure:

"Effect of Gate and Shutter Characteristics on Screen Image Quality", Willy Borberg, SMPTE Journal, October 1957, Volume 66, pages 623-627

Dr. Fred Kolb published several SMPTE papers during the late 1940's and the 1950's, including the use of air pressure to cool and stabilize the film frame, and the use of wet gate projection. Dr. Kolb is still active in retirement, and was one of my mentors at Kodak.

Posted by Matt Zeiner (Member # 1914) on 01-10-2004, 12:16 PM:

OK - I have noticed an exceptional lack of motion artifacts, come to think of it. Also noticed the grainyness - even on full-frame anamorphic prints (I am also the projectionist there). Thanks.

Posted by John Pytlak (Member # 331) on 01-10-2004, 12:26 PM:

The relatively small screens used in most screening rooms don't require much radiant energy. So a three-blade shutter is almost always an advantage for a smaller screen, greatly reducing the visibility of any shutter flicker. It also is useful if you need to run the projector at less than 24 fps, showing silent films for instance, or allowing the lab color timer more time to judge the scene-to-scene color balance.

Posted by Warren Smyth (Member # 1838) on 01-10-2004, 04:37 PM:

I'm pleased the subject of these shutters have come up. I have considered doing a conversion to a 3 bladed shutters in a home theatrette on Kalee 19s. A loss of light output is no problem as arcs are used on a matt non perforated screen that is only 12 feet wide on scope. The carbons are currently under run, so a less efficient optical train would be an advantage. I could then raise the current to give higher arc stability. (The 6mm neg and 7mm pos don't burn perfectly at 38 amps). I can't raise the level at the moment because the light output would be too high. The highlights are on the point of flashing now. Besides the fact that the cut off periods must equal the exposed periods, are there any other reguirements with regards making the shutter?

I have never heard of the apparent grain problem however, which is a real concern and making me have second thoughts. Any suggestions for and against are very welcome.

I have also considered irising down the lens if there is still plenty of light, as I understand focus may be improved. The wide screen lens is two and a half inch and the image appears sharp over the full width, even with an angled light beam. What is the current thinking on irising down? Are there any disadvantages?

Posted by Stan Gunn (Member # 554) on 01-11-2004, 12:31 AM:

Hi Warren

My feeling is it would be the way to go as it would decrease light output and improve focus, [thumbsup]

the mirror can also be defocused a tad and should have no effect on focus or light colour.

I make an asumption you are useing a President lamp house [Big Grin]

,another way would be to fit a smaller mirror or mask off the mirror.

Kalees optical system was certainly very good when useing the full Kalee system, also the Kalee lamp houses were made to run a trim of 5mm and 6mm rods another way you may be able to take.

Posted by Warren Smyth (Member # 1838) on 01-11-2004, 04:58 AM:

Stan,
You are right. It's a complete Kalee plant - Presidents, 543 sound heads and even 6 channel G.K.Duosonic rack and panel valve power amps, ones I once ran for 70mm. They keep me warm in the winter (48 valves).

I agree the light output is pretty good from Kalee equipment, even though the 19s (and 20s)have the two bladed shutter, unlike their mate the 21 which had the single fast blade. I think the 21s were sometimes used in drive-ins in Aust because of their light efficiency.

I have tried 5mm and 6mm trims but the smaller crater size does compromise coverage for 35mm. I think these were used mainly for 16mm. They worked well with 16mm Bell and Howell projectors and Kalee Universal arcs but I'm not really happy with the results for 35mm with a slight hot spot. Mind you, that's a common occurance in plexes with xenons and high gain screens. No one seems to worry, but I'm a little traditional.

Posted by Leo Enticknap (Member # 534) on 01-11-2004, 05:29 AM:

The only reason I can think of for using three-bladed shutters now is for showing silent films which run at speeds significantly below 24fps. Most designs of 'bow tie' blade will block the light source for long enough at low speeds for the human eye to pick it up, resulting in a visible flicker on the screen. In extreme cases (e.g. a Vic 5 two-blade shutter running at 16fps) the flicker is so pronounced that I guess there could be a health and safety issue with epileptics and strobe effects.

The smaller blades of a three-bladed shutter will allow you to go right down to 16 without any visible flicker. As Sam points out the quid pro quo is, of course, greater light absorption, meaning you need a more powerful xenon. If I were setting up a projector to show 24fps most of the time but with the facility for occasional, regular silent screenings (which would be the case with most arthouse venues) I would go for a design of projector in which changing the shutter is a reasonably simple operation and a slightly overpowered xenon. The machine would run with a two-bladed shutter and low amps for 24, and with the three-blade fitted and the current wound up a bit for the silent shows.

If silent shows are being done very regularly or you are dealing with pre-installed projectors which make shutter changing very difficult, it might be worth going for permanently installed three-blades and living with the extra electricity and lamp costs.

Posted by John Pytlak (Member # 331) on 01-11-2004, 07:39 AM:

For a 12-foot wide screen in a small screening room, three-blade shutters would be fine. Even at 24fps, some people are sensitive to 48 Hz shutter flicker in brighter scenes, and going to 72 Hz greatly reduces the flicker perception.

Posted by Bruce McGee (Member # 131) on 01-11-2004, 09:08 AM:

I've got several Kodak Pageant 16mm machines that have the 'Super 40' shutter. At silent speed, the shutter is a 3 blade. At sound, it's a 2 blade. This makes the silents that I run look really nice. I also like the way sound film looks when I get a shutter that is stuck in the 3 blade mode. Sure, there is a bit of light-loss, but in a small room it is OK, at least for me.

Posted by Matt Zeiner (Member # 1914) on 01-11-2004, 01:10 PM:

I'm running mine on a 24' screen, pair of super lumex 2k lamps. Even with the 3 blade shutter I have tons of light. The picture is always beautiful, though especially on blowups the grain is more apparent than on 2-blade systems I have run. Anybody care to speculate as to why?

Posted by Leo Enticknap (Member # 534) on 01-11-2004, 02:32 PM:

OK, here's a complete and utter speculation. With a relatively overpowered lamp and a three blade shutter the intensity of illumination is more constant. Therefore you will see less of the 'attack' and decay' as the shutter blade covers and retracts from the light source than you would with a two-blade shutter. Because the light intensity is more static, you're looking more closely at detail in the picture - therefore seeing the grain - than you would do both with a lower overall light level (i.e. less footlamberts) and a more pronounced up and down cycle.

This is all I can think of...

Posted by Matt Zeiner (Member # 1914) on 01-11-2004, 03:37 PM:

Since we're speculating, here's mine: Maybe since we are not as distracted by the flickering at 48hz (especially in shots that utilize camera movement) and since there is less shutter "smear," the film grain is more clearly resolved on screen??

- I guess that's pretty much what Leo said...

Posted by Steve Kraus (Member # 476) on 01-11-2004, 07:53 PM:

Illumination more constant? How so? It still goes from zero to maximum, holds there, then drops to zero and holds, with rise and fall times dictated by the nature of the shutter (diameter, placement in the light beam, 3 blade or 1 blade running 3X etc.) If you mean more even across the frame that doesn't follow from 3X shutters either. If you have 2X and go to 3X you must raise illumination but not necessarily evenness. You could raise it even more and make the light more even but you could have done that without 3X shuttering with even less light.

Posted by Gordon McLeod (Member # 33) on 01-11-2004, 08:03 PM:

Actually lets do a little experiment
the smaller window of light gives less time for the eye to intigrate the volume of light (lux/time) as compared to the 2 blade that in effect has a greater amount of light over time
So as such there is less difference occuring for the brain to intigrate
Many years ago Imax did a lot of research into the whole issue of intigration of light over area and time and that was part of the determination re there screen brightness levels

Posted by Sam D. Chavez (Member # 1841) on 01-11-2004, 08:33 PM:

Glad to be involved in another debate.

I'm glad others have seen the apparent increase in grain. What is the visual version of phycho-acoustic?

The other thing I forgot to mention is there is also an apparent loss of contrast ratio.

This is all anecdotal as I did the conversion on Dolby's DP75's in SF and used a reel of Mad Max which I knew by heart as a reference. The difference was immediate.

I went back of course and rechecked the light with a Minolta.

Posted by Steve Kraus (Member # 476) on 01-11-2004, 08:37 PM:

None of this is exactly selling me on the idea of 3X. [Cool]

Posted by Warren Smyth (Member # 1838) on 01-11-2004, 08:45 PM:

If there is an apparent increase in the perception of grain due to a reduction in the apparent flicker, it would indicate there is an apparent increase also of the overall resolution of the image. Has this been the experience of those of you who are in the position to compare 2 blades with 3?

If not, here is another theory as to the possible cause of the phenomenon which could easily be tested. Is it possible that those situations where 3 blades are being used, tend to have, due to smaller screens and less chance of flashing on highlights, the tendency to have higher illumination? A brighter picture maybe pushing up the greys and blacks enabling this density of emulsion to be observed better by the viewer. In short, perhaps the apparent grain is more a function of brightness than the apparent reduction of flicker.

It's interesting how, in order to improve the technical quality, there is so often a trade off.

Posted by Matt Zeiner (Member # 1914) on 01-11-2004, 09:44 PM:

this is getting interesting...I might install a 2-blade shutter in one of my machines at the next screening. Match the brightness and see. There are some things that I really like about the image out the 3X and some that I really don't. Come to think of it, Sam, I am pretty sure that I have always been displeased with the overall contrast onscreen. Everything is a trade-off.

Posted by Steve Kraus (Member # 476) on 01-11-2004, 10:00 PM:

An increase in perceptible resolution ought to be demonstrable with appropriate test films.

Posted by John Pytlak (Member # 331) on 01-11-2004, 10:23 PM:

I agree. Since the grain is what actually forms the image, if you see the grain more clearly, you also are seeing the image more clearly, i.e., it is sharper. I've always preferred the "look" of a three-blade shutter on a smaller screen, if only for the greatly reduced flicker perception.

I've never heard of a shutter having an effect on tone scale (contrast) if all else is equal (lens, port glass, luminance).

Posted by Mark Gulbrandsen (Member # 72) on 01-11-2004, 10:30 PM:

Contrast is not something that should be a side effect of a three bladed shutter. Overall contrast is more reliant on the optics and screen surface being used.

As for perceived sharpness I would suspect that the frame being projected is exposed to less heat and is popping out far less with the smaller shutter openings. This would allow the film to stay more in the shallow depth of focus that most lenses are capable of. Interestingly, myself and several others here have also noticed a sharper perceived image when using a high speed intermittent, but in this case with even wider drive inn openings.

In a pinch and to cut light level to permissable levels I have also used a two blade shutter with very narrow openings, in fact two two balded shutters on top of each other. A similar effect was also noted with more perceived sharpness in the image. This was one reason I speculated about there being less heat on the frame. In the 40's Kodak did some interesting 16mm high speed filming of actual 35mm frames popping when the light hit them. They also came up with a way to measure it using a Simplex -7.

Matt, A good visual test might be to use the Schnelder resolution film which is mainly black, absorbs more heat, and pops more when its in the framem. The differences in sharpness should be obvious between a 2 and a 3 blade shutter when running at the same light levels on screen.

Mark @ CLACO

Posted by John Pytlak (Member # 331) on 01-11-2004, 10:41 PM:

quote:
In the 40's Kodak did some interesting 16mm high speed filming of actual 35mm frames popping when the light hit them. They also came up with a way to measure it using a Simplex E-7.

Yup! That work was done by Kodak's Dr. Fred Kolb, and led to several important SMPTE papers. It also helped lead to curved gates and better heat filters to improve focus and stability. Paul Preo was another Kodak researcher who worked with Dr. Kolb.

Posted by Mark Gulbrandsen (Member # 72) on 01-11-2004, 10:43 PM:

John,
That would be a good group of papers to post in the manuals section if you could get ahold of them!!!
Mark

Posted by John Pytlak (Member # 331) on 01-11-2004, 11:08 PM:

I'll see if I can get permission for reprinting from the SMPTE (the copyright holder). jhawk got permission for Preo's winding orientation paper and my paper on the SMPTE Theatre Quality Evaluation Program, so it's likely they will let them be republished here.

Posted by Brad Miller (Member # 2) on 01-11-2004, 11:31 PM:

I have a water circulator running on my JJs with 2K lamps (Super Lume X). It is pretty obvious (at least to me) when the circulator is not on. The focus is substantially sharper when the circulator is running, and yes that's even with 2000 watt lamps.

I've been toying with the idea of 3 bladed shutters. If the contrast is lowered at all though, there is no way will I pursue it. I don't really see how that could be, assuming the light levels are matched.

Posted by Marc Jones (Member # 29) on 01-11-2004, 11:35 PM:

I don't know if this would apply, but at one time I was told that if you used a three blade shutter then you would get less flicker in the film if you recorded it with video recorder?.

Posted by Leo Enticknap (Member # 534) on 01-12-2004, 01:54 AM:

Not sure how that would affect videoing it in NTSC, but with PAL a two-blade shutter (in mechanical telecines) with the film running at 25fps is the norm - each frame scanned twice fits precisely into the 50hz scanning rate. Some machines I've seen also have a 16 2/3fps option, scanning each frame three times.

quote:
Illumination more constant? How so? It still goes from zero to maximum, holds there, then drops to zero and holds, with rise and fall times dictated by the nature of the shutter (diameter, placement in the light beam, 3 blade or 1 blade running 3X etc.) If you mean more even across the frame that doesn't follow from 3X shutters either. If you have 2X and go to 3X you must raise illumination but not necessarily evenness. You could raise it even more and make the light more even but you could have done that without 3X shuttering with even less light.

It was the time factor I'm thinking of. With a two-blade shutter the 'attack' and 'decay' phases take longer than with three blade (which is fitting an extra cycle into the same length of time), therefore the level of illumination at any given point on the screen across the entire cycle will appear to the naked eye to fluctuate less. In other (very crude) words, because you're seeing less of a flicker, you're able to see more detail in the projected image.

Agreed, though, that heat on the film is another factor.

Posted by Carl Martin (Member # 1146) on 01-12-2004, 02:39 AM:

i would think the "attack" and "decay" would take just as long regardless of 2 or 3 blades. assuming the blades are the same and spinning at the same rate.

or are the 2 "non-essential" blades smaller than the one that obscures the film travel? my interpretation of sam's statement that:

quote:
You lose 50% more light, relative to a two bladed shutter, not 33% as one would initially suppose.

is that a 3-bladed shutter has blades the same area as a 2-bladed shutter, blocking ~75% rather than ~50% of the lamphouse's output.

so you'd need twice as much light. is that right?

what about 5-bladed shutters? how do they let any light through at all? does one need a special fast intermittent?

carl

Posted by Leo Enticknap (Member # 534) on 01-12-2004, 04:16 AM:

quote:
i would think the "attack" and "decay" would take just as long regardless of 2 or 3 blades. assuming the blades are the same and spinning at the same rate.

Generally speaking the blades are not the same size - on three blade designs they tend to be smaller, in my experience.

Posted by Warren Smyth (Member # 1838) on 01-12-2004, 06:03 AM:

My gut feeling is that the apparent increased grain is a fuction of higher illumination pushing up the the lower tones to make the particles in the emulsion more visible.

This would also explain the observed decrease in contrast. If you push up the light, the blacks come up and although the whites maybe brighter, the iris in our eyes close to compensate for the extreme brightness. In short, the raising at the bottom end of the scale isn't matched by a perceived raising at the top end. This translates to an apparent reduction in contrast.

Now as I said, there maybe a tendency in 3 bladed situations to allow the image to be brighter for reasons I've given. However, let's assume that this is not so.

As we know, to produce standard illumination on the screen of film, the light output must be higher at the source with a three bladed shutter. It's been suggested double. If we freeze the picture on the screen for a moment in time, a single frame will be much brighter, (double in fact) than normal. It is this image that the brain perceives, with the raised lower tones (and accompanying perceived higher grain) and holds as persistence of vision. It maybe possible that the extra cut off from the 3rd blade is simply doing what our iris does and compresses the extreme brightness of the whites. More likely, that our brain and persistence of vision is doing the trick of limiting the brightness. The result, higher illumination of emulsion particles (grain) on lower tones resulting in more obvious grain to the eye, and lower apparent contrast.

Naturally, it's only a theory but might be worth considering. The fact is, we know very little about that from which we earn our living - persistence of vision.

Another interesting fact, is that we perceive flicker less on an image in the centre of our field of view. We are more sensitive to flicker on the outer edges of our field of view. This means that audiences sitting closer to the screen may notice the flicker more, as the image is also seen by the outer limits of their view.

Posted by Matt Zeiner (Member # 1914) on 01-12-2004, 08:36 AM:

I do have a bunch of that Schneider film - I'll check that out. I suspect that using this film on these machines may introduce focus issues into the test because of the fact that it absoarbs so much more heat than most films. Seems like I can get a loop of that stuff to last about 5 minutes before it starts curling in the gate without reducing the light source before using it.. As far as the contrast issue, maybe there is a "psycho-visual" component that hasn't been considered, although Warren may have touched on something valid is his above post...

Mark - I swear that this contrast issue exists and that it is not due to some deficiency in the installation. Brand new Schneider lenses. I have beautiful ports - dual paned and properly angled with high-grade optical glass and black velvet between panes to absoarb stray reflections. I promise you nothing is getting back into that lens!

Posted by Sam D. Chavez (Member # 1841) on 01-12-2004, 09:57 AM:

All blades have to be the same size. I played with making the cut off blade(s)smaller than the other and found as I was told the flicker became intolerable. Something in our brains is expecting all the pulses to be the same width.

Posted by Jeff Stricker (Member # 224) on 01-12-2004, 03:41 PM:

Sam, I appreciate hearing the results of your experiments with different size blade widths. I have thought many times to try that, but your efforts have save me from running the experiment and getting the same results.

Jeff

Posted by Warren Smyth (Member # 1838) on 01-12-2004, 06:01 PM:

Zeiss Ikon 'Picture and Sound' number 32/33e October 1954. A chapter on shutter blades contains the following.

quote:
An important point to remember in the elimination of flicker is that all obscuring periods are of exactly the same duration and have the same distances from one another. The sensation of flicker is not a value that can be measured, it depends on the particular properties of the human eye and is slightly different for every spectator

Cameron in his 14th edition (1959) of 'Motion Picture Projection' wrote a good chapter on persistence of vision emphasizing the role of the human brain in the perception of motion pictures. From the evidence he provided, it is clear that the brain fills in information (based on experience) that the eye does not see.

quote:
The mind can put together eye-reported fragments amounting to three percent and derive from that a sensation of seeing a 100 percent. The old saw about "putting two and two together" concerns a feat really trivial by comparison. We evidently believe a great deal more than we see. The eye reports facts but we see fancies.

In studying the possible cause of unexplained phenomenon, I believe we shouldn't under-estimate the possible role of the brain and that it sometimes tricks us.

Maybe the combination of a higher flicker frequency in combination with our brain, produces an apparent reduction in brightness that is NOT linear across the tonal range. Perhaps it does to light what an audio compressor does to dynamic range with sound.

Posted by Mark Gulbrandsen (Member # 72) on 01-12-2004, 08:18 PM:

"I have a water circulator running on my JJs with 2K lamps "
_________________________________________________________________

Brad,

If you think it looks good with hust a circulator you ought to try it with Cinefocus! Especially on 70mm stuff.

Mark @ CLACO

Posted by Fred Georges (Member # 505) on 01-12-2004, 10:30 PM:

I wouldn't think Cinefocus could work with a JJ turret. No lens barrel to blow through. But, I'm nibbling out some double dissolving 3 blade shutters for them this weekend. You have to keep shutter blade width constant. Otherwise the efficiency is upset between master & balance blades and the eye is very sensitive (at least mine are) to this pulsing. [Big Grin]

Posted by Matt Zeiner (Member # 1914) on 01-13-2004, 12:24 AM:

We used a cinefocus setup in a 70mm install with JJs at the Pequot museum in CT a few years back. CFS 70mm loop cabinets (ugh..)

Posted by Mark Gulbrandsen (Member # 72) on 01-13-2004, 08:33 AM:

About 10 years ago they had a 35mm CFS loop cabinet at the Chicago Museum Of Science And Industry for the Chevy Exhibit. You should have seen the lightning bolts that thing produced in a darkened projection room! The film layers also stuck together so badly that you couldn't even pull the film through by hand. At that time I considered it a very unsucussful loop cabinet installation. Luckily I wasn't the one that sold or installed it.
Mark

Posted by Matt Zeiner (Member # 1914) on 01-13-2004, 07:39 PM:

THe Pequots' projection booth is a sight to behold, let me tell you. It's basically a clean room scenario - no problems with static or dirt. The cabinet works flawlessly in this installation, though it can be a real pain to work on (or thread, for that matter)...

Posted by William Hooper (Member # 71) on 07-19-2004, 05:55 AM:

What's the ballpark price & availability of 3 bladed shutters for Simplex 35's?

Posted by Fred Georges (Member # 505) on 07-20-2004, 07:04 PM:

Strong machines them from a solid billet. Cost is about $500.00 [Big Grin]

Posted by Matthew Jaro (Member # 1786) on 07-21-2004, 09:13 AM:

I have three blade shutters on a pair of XLs. My screen is 16.5 feet wide (micro perf). I previously had 2 blade shutters with 800 watts xenon. I upgraded to 1600 watts (Super Lumex). The projection ports are optical glass and the lenses are the latest ISCOs.

1. I can assert that all three blades are the same size.

2. The price is around $500

3. There is absolutely no decrease in contrast. If anything, contrast is better because the picture is sharper.

4. There is less ghosting because the blades are much wider than you need to mask the pulldown.

5. The machine runs better, because the blades are balanced at the factory.

6. I can run 18 foot Lamberts on the screen with absoulutely no flicker. If I run the projectors with no film, there is still no flicker. I can even get a brighter image with no problem if I want to move to a 2000 watt lamp.

7. I can perceive grain in some current release prints. This is because the image is so sharp, I can see every pore on peoples's skin. I would rather have sharpness than fuzzy focusing where you cannot see the grain. Earlier prints are generally sharper, especially IB prints. If you don't want grain, get a junky lens like a Kollmorgen.

8. Mot importantly, there are no motion artifacts. For example, the astute projectionist would notice that in any horizontal pan shot projected with a two blade shutter, there is a stepping effect. I think this is probably due to strobing of the shutter blade with the movement on the frame. The higher flicker rate greatly reduces this problem.

If anyone has adjusted multi-sync computer montitors, they will note that 60 Hz refreseh rate is intolerable and they will raise it. 48 is horrible.

All in all I can think of no reason why one would not use three blade shutters in any venue where there is sufficient light. After all, they are called "Studio Shutters" for a reason.

Posted by John Pytlak (Member # 331) on 07-21-2004, 10:36 AM:

If getting enough light is not an issue, 3-blade shutters will practically eliminate any shutter flicker, even at higher light levels. It also allows you to reduce frame rate for silent films. One of the best things to do if you have a larger lamp for a smaller screen.

Posted by Matthew Jaro (Member # 1786) on 07-21-2004, 02:14 PM:

Sam --- How could the contrast possibly be reduced? It would seem that this is physically impossible. Consider for a moment the higher-frame rate formats like Todd-A-O. There was certainly no loss of contrast. Everyone says that a higher frame rate is always better. Three blades would be similar to higher frame rates, except you are seeing the same frame more than two times so that motion would not be as smooth. But contrast?

Incidently, all motion (for example, water) looks much smoother. The effect is truely stunning.

Leo -- Why would you ever want to go back to a two blade shutter if you have a three blade shutter? Changing shutters on an XL is only a matter of minutes anyway.

Brad -- You have to try this. With your state-of-the-art system you don't know what you are missing. The difference is like night and day.

Posted by Michael Schaffer (Member # 1204) on 07-21-2004, 04:42 PM:

quote: Matthew Jaro
If anyone has adjusted multi-sync computer montitors, they will note that 60 Hz refreseh rate is intolerable and they will raise it. 48 is horrible.

Yes, but you cannot directly compare these. A monitor is building up the image line by line, and while the film projector also "builds up" the image by progressively revealing it, when the shutter is open, the image is statically present for a moment.
It's hard to compare the two, but subjectively I would say the amount of flicker you see in 48 Hz projection is comparable to about 70-75 Hz on a computer monitor. My eye needs about 85 Hz for a stable monitor image, but at 72 Hz on the big screen I don't see any flicker anymore either.
I don't know about the contrast since I never had the opportunity to see an A-B comparison, but I did work a lot with 3-bladed FP30Es, and these had a very stable image indeed.

Posted by Carl Martin (Member # 1146) on 07-22-2004, 02:33 AM:

quote:
1. I can assert that all three blades are the same size.

...

4. There is less ghosting because the blades are much wider than you need to mask the pulldown.

so the blades are what, 100 degrees or so? that makes 300 degrees worth of blades, only 60 degrees worth of void. can that be?

carl

Posted by Matthew Jaro (Member # 1786) on 07-22-2004, 07:57 AM:

Yes. The void part is barely wider than the aperture!

Posted by Steve Guttag (Member # 268) on 07-22-2004, 09:31 AM:

My this is an interesting debate!

Here are some facts...3-wing shutters can have ZERO affect on contrast ratio. However, I read Sam's post as "perceived" contrast...that is what one thinks they are seeing and that can not be measured with a meter. The meter will still measure the bright areas and the dark areas and come up with a ratio. As Sam says...what is the equivalent to psychoacoustics for vision?

However...while I'm on that subject...contrast on 3-wings. I have not observed the same thing. I have not seen contrast ratio change. I wonder when Sam did his experiments if his meter was calibrated to integrate properly for 72Hz as well as 48Hz to provide equivalent light levels.

I bring this up because of my work with Kinoton's "E" projectors. Most of the ones I work with have a shutter that operates as a 2-wing above 20fps and as a 3-wing below 20fps. It is one of the advantages of having the shutter as a separate motorized item and not tied to the gear train. There are merely 3 passes of the blades before advancement below 20 fps (note, one can have the machine configured for 3-wing at even higher speeds...it is all in how the software is programmed...they also have a "3-wing" verson of the projector that has a different shutter blade all together...it is actually a two-wing blade but with very narrow openings on the order of a Simplex or Century 3-wing...but I digress).

Now consider this...as one slows down the projector and the machine switches from 2-wing to 3-wing mode...I don't see any shift in light or contrast on the screen (perceived). If the perception of contrast was based soley on the number of flashes per cycle, then one should see the screen image dramatically change but you don't. The flicker just diminishes from not bad to not there (you can hear the blade speed up when this happens).

BTW...if you do need to run a mix of silent and sound films...there is no substitute for me now...there is only the Kinoton "E" series of projectors. There is no switching of shutter blades...it is just done is software and it works flawlessly. It is really cool watching stuff at 16fps without flicker.

I will say, I have yet to meet the person that installed 3-wing shutters in a screening room or small screen venue that didn't think they made all the difference in picture quality.

Sam is also correct on his statement on the light loss being more than just 33%. A typical Simplex shutter has a 84-degree shutter (closed portion). This leaves us with a 53% shutter. A 3-wing shutter will also have 84-degree shutters but three of them. Leaving us with just 108-degrees for the three open portions or just 36-degrees each. The math is such that the true light is an integration over the entire cycle of the shutter blade. The three wing shutter is only 30% efficient leaving us with about 56% of the light of our 2-wing counterpart. Again presumming the closed portion of the 3-wing blade is still 84-degrees for both 2-wing and 3-wing.

On those machines with a 90-degree shutter with 50% efficientcy, the numbers fall such that a 3-wing shutter has a 30-degree opening so the efficientcy is only 25% which is only 50% of that of the 2-wing...as Sam stated above.

As to the rise and fall times of a 2-wing over a 3-wing...there is ZERO difference. The rise and fall times are a function of the shutter speed and the radius of the blade...period. A small shutter (i.e. Christie) will have a more pronounced flicker due to its small shutter than a large shutter (e.g. Norelco AA2 running a double speed). The frequency of the blades in both machines is the same...48Hz...the velocity of the blades as they cross the aperture are not. This is easily observed using John's solar cell connected to an O'Scope. Personally, I find it easy to observe on the screen. Century used a double shutter to increase the velocity of their shutter (double it) without increasing its size. One can also decrease the shutter closed portion as well since there are now two blades to further increase the light. However...there is a penalty...each shutter will have some bounce to it due to gear lash and the reversing shutters goes through several extra gears...since each blade is bouncing differently at different times, there will be a bouncing hot-spot on the screen. This is further accentuated since the center is the last part of the frame to close and the first to open. A light weight aluminum shutter will have a violent bounce...a heavy steel shutter will have a more damped bounce....I've seen several mis-diagnosed xenon lamps (flicker) that really was due to shutter bounce on a double shutter.

Now, consider this. I've had some of the same comments about 100-degree steel shutters (Century part C1-D-50S) as some have made here about 3-wings increasing the perceived sharpness. This is also in a screening room like environment with a relatively small screen and low wattage lamps.

Steve

[ 07-22-2004, 04:33 PM: Message edited by: Steve Guttag ]

Posted by Leo Enticknap (Member # 534) on 07-22-2004, 06:58 PM:

quote: Matthew Jaro
Leo -- Why would you ever want to go back to a two blade shutter if you have a three blade shutter? Changing shutters on an XL is only a matter of minutes anyway.

Light loss; especially if you're dealing with a venue which hardly ever shows silents and started with two-blade shutters. If you don't have much spare lamp capacity to play with then you're faced with deciding which is the lesser of two evils: the flicker from a two-blade shutter running at low speeds, or the light loss of a three-blade shutter? If you go for the latter, then obviously you want to replace the two-blade shutter when you go back to running at 24fps.

Posted by Don Furr (Member # 1469) on 07-22-2004, 08:07 PM:

I've got a friend here in Atlanta who, about a year ago, bought two new 3 blade shutters and installed them on his pair of XL's.

According to him, he could NEVER get the strobe out of the picture. After a lot of trying things, he returned the shutters.

Does anyone have an idea what went wrong? Is there a trick in the timing from a two to a three blade shutter?

Don

Posted by Michael Schaffer (Member # 1204) on 07-22-2004, 08:45 PM:

quote: Steve Guttag
A small shutter (i.e. Christie) will have a more pronounced flicker due to its small shutter than a large shutter

Why? Isn't the perceived flicker dependent on the relation of the open and closed portions?

Posted by Daryl C. W. O'Shea (Member # 1303) on 07-22-2004, 10:12 PM:

The edge of a larger shutter crosses the aperture faster than a smaller shutter. A larger shutter's edge also crosses the aperture more closely to a vertical wipe than a smaller shutter does.

Posted by Sam D. Chavez (Member # 1841) on 07-22-2004, 10:16 PM:

Responding to Don Furr, I would bet money that the blades were not all the same size. This will always create intolerable flicker as the brain is expecting to see even pulses.

As to Guttag, I and other perceive what we perceive. Studio guys like Jim Nord and others have noticed the same thing.

I used a modern Minolta spot meter that I still own. It was alleged to be able to handle either flicker rate and the current increase came out correctly if you do the math.

I'm not saying It's astonishingly different but it is not the same. There were many good postulates offered on this thread and there is something to it.

Perhaps the PK 60 handles the film in such a way that it does not pop or breathe as much as the DP75's on which I did my tests.

Posted by Michael Schaffer (Member # 1204) on 07-22-2004, 10:36 PM:

quote: Daryl C. W. O'Shea
The edge of a larger shutter crosses the aperture faster than a smaller shutter. A larger shutter's edge also crosses the aperture more closely to a vertical wipe than a smaller shutter does.

You mean because a relatively large shutter usually has the edges at an angle that does not go through the geometrical center of the shutter? That makes sense.

Posted by Mark Gulbrandsen (Member # 72) on 07-22-2004, 10:47 PM:

"A small shutter (i.e. Christie) will have a more pronounced flicker due to its small shutter than a large shutter (e.g. Norelco AA2 running a double speed)."
_________________________________________________________________

Actually the Christie double bladed 1440 rpm shutter was so awful for flicker that Christie ended up going to a single blade double speed as in the AA-2. Although the shutter is still the same small diameter as before there is now no percevable flicker in a Christie thats been converted over. Sure it going to have more measaurable flicker than the larger AA-2 shutter has and you could measure it with the photocell method, but to me the perceived difference in this respect is far more important than a reading.

Mark @ CLACO
(Live from Winnemucca, NV.)

Posted by Daryl C. W. O'Shea (Member # 1303) on 07-22-2004, 10:58 PM:

quote: Michael Schaffer
You mean because a relatively large shutter usually has the edges at an angle that does not go through the geometrical center of the shutter? That makes sense.

That helps with the wipe being more vertical than diagonal, but it's the larger diameter that makes the edge move faster since the outside of a disc moves faster than the inside of (or smaller) disc.

Posted by Michael Schaffer (Member # 1204) on 07-22-2004, 11:06 PM:

Right. That`s what I had been missing all the time since I was fixated on the relative size of the blades and overlooked the total size of the whole shutter as opposed to smaller shutters. Thanks!

Posted by Steve Guttag (Member # 268) on 07-23-2004, 08:22 AM:

The vertical "wipe" effect has more to do with being a conical shutter rather than a flat one. If you look at a Simplex or a DP70, you will see that the shutter shaft is at an angle. Then the end of the blade is formed so that it cuts the aperture squarely.

When thinking of a small diameter shutter versus a large one...think of a clipped sine wave on an O'scope...a small blade will have less of its peaks clipped since so much time is spent actually opening or closing the aperture. A large shutter will present nearly a square wave since the transistion from open to close (or vice-versa) happens so fast. The frequency is the same, however...just like you can have square and sine waves of the same frequency but look and sound completely different.

Steve

Posted by Matthew Jaro (Member # 1786) on 07-23-2004, 08:45 AM:

This is just mere speculation: but do you think that the strobing problem that Don Furr aluded to could have been caused by lamp power supply ripple? 120 Hz ripple - 72 Hz shutter interruption rate = 48. Since the line frequency is not synchronized with the shutter, the effect would be strobing. Does this seem likely?

Posted by John Pytlak (Member # 331) on 07-23-2004, 09:28 AM:

Yes, if you have significant 60Hz current ripple in the DC being supplied to the xenon lamp, you can get a 12Hz "beat" with the 48 or 72 frequency of the shutter blades, which is very noticeable as flicker. This 12Hz beat pattern would be the same for both 2-blade and 3-blade shutters.

Posted by Matthew Jaro (Member # 1786) on 07-23-2004, 11:00 AM:

John -- Since full wave rectifier bridges are commonly used, wouldn't the ripple frequency be 120 Hz (one ripple for each half wave)? Rectification occurs on both the positive and negative swuings of voltage. Poor filtering due to defective filter capacitors or a shorted choke would cause this to occur.

In this case, the strobe would be more noticable with a three blade shutter, since the shutter interruption speed is closer to the full wave ripple frequency.

Posted by John Pytlak (Member # 331) on 07-23-2004, 01:36 PM:

When I've seen it, it was closer to a 12Hz frequency. Could it be that for some reason the ripple had a strong 60Hz component? (e.g., the full-wave rectifier was not balanced?)

Posted by Edward Jurich (Member # 1787) on 07-25-2004, 05:41 PM:

I would think high ripple would suggest bad filter caps. If the caps are that bad the voltage usually drops and my experience with bad caps and low voltage is the lamp is hard to strike at times. Now perhaps if one of the rectifier diodes was leaking or shorted on a 3 phase rectifier you could get some strobing.