Film-Tech Forum ARCHIVE: FOCUS, focus and refocus?

This is topic FOCUS, focus and refocus? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=005331

Posted by Richard C. Wolfe (Member # 431) on 09-15-2003, 10:34 PM:

I've been a projectionist as well as theatre owner for the past 35 years and still don't understand why there seems to be no standard for focus from one film to another. I'm quite aware of focus drift due to compression of the emulsion from one end of a reel to the other, but I'm talking here about from one lab to another, especially for preshow daters etc. All the short items that are spliced together for the preshow. I get my theatre logo and policy announcement from one company and my daters from another one, and I have to refocus almost each and every one. Sometimes various daters that come from the same company even have a slightly different focus.

What gives?

This should be a good question for John P.

Posted by Brad Miller (Member # 2) on 09-15-2003, 10:38 PM:

I'm betting your lenses are your primary cause. What kind of lenses do you have? What is the approximate throw and screen size? What make of xenon lamphouse are you running (with bulb wattage)? Finally, do you run on 2000' reels or 6000' reels, and how do you wind the film when running as well as when storing your snipes?

Posted by Paul G. Thompson (Member # 655) on 09-16-2003, 12:24 AM:

In addition to what Brad mentioned, I think it has alot to do with how hot the film trap is getting because of a damaged coating on a cold mirror, dichloric filters with the hot mirrors, lamphouse ventilation, lamp alignment, film trap mechanical problems and how big and how hard the bulb is being driven.

There are lots of variables.

I have ran some booths that were so rock solid that touching the focus knob was almost be unheard of.

I have also run some theatres where it was almost necessary to ride the focus knob. Those theatres usually had old obsolete lenses, bad mirrors in the lamphouses, mis-aligned lamphouses or dichloric filters that were just plain shot.

Posted by Chris Hipp (Member # 1788) on 09-16-2003, 12:35 AM:

I'm sure this isn't the case for you, but in my booth the only time I need to focus is when someone blind is running booth and changes the focus settings or when someone is just messing with things that they shouldn't.

This morning I had a lens that I had marked witha sharpie a few days ago when I took it out and someone had moved the lens itself about 3 inches from where the mark was. Some people are just stupid.

Posted by Brad Miller (Member # 2) on 09-16-2003, 12:50 AM:

To add to Paul's notes, I am assuming running a single screen that you have a fairly high wattage lamp. Do you have a water cooled trap and circulator? How about a heat filter? Also, what is your exhaust situation?

We should be able to track this problem down pretty quickly if you can provide enough specifics. For most booths, having to touch the focus knob at any point during the presentation is virtually unheard of. The only difference that I know of is that these are platter theaters running emulsion out.

I would look in the Picture Warehouse to familirize myself with your setup, but *ahem* someone *cough* hasn't sent in *cough* pictures of your *cough* theater yet. [Wink]

By the way, if your custom snipes have Photoguard on them (Filmack likes to use that), then you can forget them being in focus with the rest of the show.

Posted by Steve Guttag (Member # 268) on 09-16-2003, 06:37 AM:

The make/model of the projector and the trap/gate design also affect focus.

If you have a curved gate with tension bands on the trap, your focus stability from filmstock to film stock will be poor. Since the runners are the reference point for the film (on the gate), the thickness of the film will place the emulsion nearer/further to the lens and can cause a need for refocusing.

If you have a straight gate with hard trap runners then you will find that focus is more stable from print to print since the emulsion side is the reference however straight gates are less stable with high-wattage/high-heat systems since the film is allowed to pucker more.

The higher the wattage and the shorter the lens the touchy focus is apt to be. Also, the film stock and the amount of heat it will absorb will contribute. The film will actually swell towards the light source on projection so the lens is also having to contend with a moving target (fore and aft, hopefully not laterally or vertically).

One problems of 3-wing shutters in high-lamp situations is that it pulses the film 3-times and in a time frame that is too short for it to completely relax. They work great in smaller lamp situations (under 2KW).

Steve

Posted by John Pytlak (Member # 331) on 09-16-2003, 09:31 AM:

Richard C. Wolfe wrote:

quote:
I'm talking here about from one lab to another, especially for preshow daters etc. All the short items that are spliced together for the preshow. I get my theatre logo and policy announcement from one company and my daters from another one, and I have to refocus almost each and every one. Sometimes various daters that come from the same company even have a slightly different focus.

Many daters, policy announcements, and rolling stock ads are NOT printed on Kodak print film, so there can be variability due to stock differences. I assume they are all on polyester print film at least?

Another problem can be the winding orientation and the way these short lengths of film were wound by the lab. Winding orientation has a significant effect on focus, especially if the film was wound on a small hub or core ("core-set" or clockspringing). I would suggest rewinding the film with the same orientation you normally use on your platters (emulsion out?) and letting the film "relax" and take on the new winding orientation. Do NOT use small hubs or small cores, regardless of the winding orientation.

Relative humidity in the projection room can also affect focus. Ideally, the film should be equilibrated to about 50 to 60 percent relative humidity. Really damp conditions can make the film take on "negative" curl, where the gelatin emulsion swells with the excess moisture. The heat of the projector drives out this moisture, shrinking the emulsion enough to cause a change in focus. Conversely, really dry conditions cause the emulsion to shrink, and may produce excessive "positive" curl.

Certainly the type of gate, gate tension, amount of radiant energy, etc. can affect focus. With your larger lamps, pay attention to proper lamp alignment and focus (good illumination uniformity), and use dichroic heat filters/reflectors:

http://www.kodak.com/US/en/motion/newsletters/pytlak/fall97.shtml

http://www.kodak.com/US/en/motion/newsletters/pytlak/winter97.shtml

Posted by Richard C. Wolfe (Member # 431) on 09-16-2003, 03:13 PM:

OK, here's the situation. First of all I think that some of you misunderstood what I was saying. The problem that I was speaking of is NOT with the feature, just the preshow.

I think that Brad may have hit the problem on the head. I get all my daters... things like: "Prevues of Coming Attractions", "Our Next Attraction", "Starts Friday", "Our Feature Presentation" and many more from Filmack, which of course is photoguarded. I get my theatre logo and policy announcements from Pike Productions. It may be that fact that is causing the problem. Some items with photoguard and some without. However, sometimes several items from the same company are in a different focus. Each of these items run for only a few seconds. The focus doesn't change throughout any singular item, but from item to item. Example: The daters are always spliced together as follows: Theatre Logo, Policy announcement (both from Pike), then Prevues of Coming Attractions, Our Next Attraction, Starts Friday(all from Filmack)then insert trailer, followed by "Remember this Attraction Starts Friday", Our Feature Presentation (both from Filmack), and of course then the Feature. Our focus knob is filed and painted with a line on it so that we can always see where the focus is set, as on a clock. We refer to the focus setting as 5 o'clock, 7 o'clock or whatever. It is not unusual for the "Prevues of coming..." to be at 5 o'clock, the "Our Next Attract.... at 7 o'clock and then back to 4 or 5 for Starts Friday. The trailer and Feature is almost always at 4:30 to 5 o'clock.

John P. may have a good point whereas they may not all be printed on Kodak film. As far as the winding orientation, I don't know if they are stored emulsion in or out, I'll have to check. We store all our daters not being used in a special film cabinet made just for daters. They are NOT stored on cores or hubs, they are hand wound and lie on their side where they can relax. The daters that are usually used always stay on a 6000' reel as they are used from show to show. There is one reel for Flat and another for Scope. The one that isn't being used is stored in a 70mm film cabinet where 2 reels fit in each bin. I just received a new shipment of daters from Filmack and have put all news ones on so dirt or oil buildup isn't a possibility as they are all brand new, yet still different in focus.

I will hereby list all the equipment in our booth, much of it having nothing to do with this problem, but I know many of you like to see what equipment is in a given booth.

We have a two projector changeover system with 6000' reels.

2 Simplex E-7 projectors
2 Simplex IPC 4 Star soundheads (now with Component Engineering ASR40S reverse scan red light readers)
2 ORC 2000 watt lamphouses model # XH-305
2 Eprad Co-operator automation units
LENSES: Scope: 2 Kollmorgen 2X model KA298 with Cinephor E.F. 5 in. f:2.0 backups
Flat: 2 Schneider Cinelux-Ultra 2/70mm-2.75 in.
Makeup table: Kelmar Systems RTV-8200
Splicer: NewTaper from Newmade Products Corps.
SOUND: Dolby CP45 Cinema Processor
Component Engineering MS-100 Booth monitor
Amplifiers: 4 QSC mx 1500 amp
SPEAKERS: Backstage - 3 Altec Voice of the Theatre A4s
Surrounds - JBLs (can't see the model #s)
SCREEN: 15' x 30' Flat picture is 15' x 27'. Scope is 13' x 30'.
The throw from lense to center screen is 90'.
The proscenium arch is 30' wide so the screen fills the entire width. We have movable masking to go from flat to scope. There are also two stage curtains. The 2nd is used for films while the front one is only used for stage shows.
The screen is curved with a 2' depth at the center and is tilted back somewhat toward the angle of projection.

I will have to make an effort to get some pictures to Brad to post. However, meanwhile there is a fairly recent booth photo in our website at www.roxytheaternorthampton.com. Click on the "photo" page and scrawl down to the bottom and click on the brass plate that says Roxy Theatre Past. Then click on the album cover... when it is open, scrawl to the bottom and click on next page and two shots of the booth will appear... one from the days of carbon arc, and one as it appears now.

If the information posted above gives anyone any more ideas as to the solution for my problem, I'll look forward to read them.

[ 09-16-2003, 09:46 PM: Message edited by: Richard C. Wolfe ]

Posted by Robert E. Allen (Member # 1351) on 09-16-2003, 03:24 PM:

I wish focus problems in today's multiplexes were all equipment related. They could then be easily corrected. It has been my experience that I rarely see a film in a multiplex that is focused or remains in focus not because of equipment or film problems but because there is no one in, or even checking on, the booth. So until the corporate big boys give the importance to the "Heart of the Theatre" it deserves we are going to continue to be subjected to sub-standard performances.

Posted by Brad Miller (Member # 2) on 09-16-2003, 04:00 PM:

With a 30 foot image and a 90 foot throw, you should be sitting pretty. However I would personally be looking at two things first:

*I'm betting that your scope image is harder to keep focused than your flat one. (Note it should be the other way around.) Can you verify or deny this?

*I would definitely make sure that all dater strips are stored on a core to make sure they do not curl unnecessarily, and under no circumstances anything smaller than a 2 inch core. 3 or 4 inch would be preferred. I think everyone will agree with me there. The part where people will start disagreeing is on the winding orientation. Especially with polyester film I stand firm with emulsion out winding, although there will be others who will claim emulsion in is better. You may just have to try it for yourself for a few months one way or the other, but I am wondering if *some* of your snipes are wound one way and the rest the other way. That would definitely be a major issue in terms of focus.

Also, if you demand for Filmack to not Photoguard your snipes they won't. That will help with your focusing issues big time and will also save you money on the cost of the snipes.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-16-2003, 04:21 PM:

I've recently had to replace a 70mm Cinelux-Ultra 2 due to focus problems. The cause was some mould / fungus growing inside the lens. You could literally time the focus drift (which I actually did for about a week) as the lens / mould heated up.

Likely not your problem, but I thought I'd mention it.

I'd agree with Brad about the PhotoGuard... from the little I've dealt with it, they don't seem to apply it evenly across different batches, so two different PhotoGuarded elements on the same stock could focus differently.

Posted by John Pytlak (Member # 331) on 09-17-2003, 09:15 AM:

On a platter, wind with whatever orientation gives you the best performance for focus stability and minimum static cling. Since the curl of the film varies with the relative humidity, the preferred winding orientation may vary with humidity.

SMPTE Recommended Practice RP39, which specifies maintaining an emulsion-in winding orientation for optimum focus stability, was based on Kodak data for projection performance of TRIACETATE prints, which had different curl characteristics than polyester prints:

"Effect of Winding on the Projection Performance of 35mm Motion Picture Film" by Paul H. Preo and Eric A. Yavitz, SMPTE Journal, June 1965, Volume 74, Pages 521-525.

The recommendation to wind on the largest possible core or reel hub diameter IS still valid, regardless of whether the film is triacetate or polyester base. Fortunately, the inside diameter of the film roll on a platter is much larger than any reel or core, so core-set focus issues usually are due to small-diameter winding before the film ever gets on the platter.

Posted by Richard C. Wolfe (Member # 431) on 09-17-2003, 04:14 PM:

Brad you're right. We do have a more difficult time focusing our scope picture. We have to focus on the center as that is where most of the action is taking place and where patrons would normally be looking the most. The outer edges tend to be a soft focus which I have always attributed to the curved screen. I'm looking forward to your comment about this.

Posted by Chris Hipp (Member # 1788) on 09-17-2003, 05:15 PM:

Richard,
Look in the tips section. There is a page on focusing scope lenses. If your depth of field is not set correctly you won't be able to focus evenly across the screen.

Posted by Brad Miller (Member # 2) on 09-17-2003, 08:53 PM:

Chris, it is possible his anamorphic is a little out of focus, but I blame the lenses themselves. If those were junked and Schneider or ISCO lenses were put in their place I believe the difference in image quality would be a far greater increase in quality, than say comparing 35mm to 70mm.

Richard, if you would like to test that theory, I may have a prime lens that size (and anamorphic) I could loan you to try out.

Posted by Richard C. Wolfe (Member # 431) on 09-21-2003, 09:49 PM:

I thank you Brad for the offer to loan me a lense, but that would not be necessary as I can borrow one locally and save us the trouble and expense of shipping the lense back and forth.

Am I correct in assuming that when you say the prime lense you are referring to what I call the back up lense, or the lense that the anamorphic is attached to? That is a pretty old lense.

However, each time I think that there is something wrong with my lenses, I get a film that suggests otherwise. I am currently playing Seabiscuit, which of course is scope, and is sharp and clear throughout the image area. Why does this change so much from picture to picture?

Posted by Steve Guttag (Member # 268) on 09-21-2003, 10:33 PM:

Richard...I'd start with the anamorphic attachment...the KA-298 wasn't a great lens (the original Kasaka ones). As old lenses go, B&L were tops including the Super-Cinephores but modern lenses will help you there too.

If you were to update your scope lenses to an ISCO Blue-Star and Schneider or ISCO prime lenses (backup) then you would be shocked at the picture quality...not only sharper but much more vibrant.

Steve

(PS...tell Bob F. I said hi)

Posted by Larry Shaw (Member # 388) on 09-22-2003, 12:51 AM:

Steve,

Your comments on the 16th, while correct as usual, don't mention the (stand by for self-promotion) Kinoton gate design that provides a curve but doesn't have the focus issues associated with tension bands. IMHO it's the best of both worlds, especially combined with the slippery Delrin.

Posted by John Hawkinson (Member # 1135) on 09-22-2003, 01:31 AM:

John P. says, "SMPTE Recommended Practice RP39, which specifies maintaining an emulsion-in winding orientation for optimum focus stability, was based on Kodak data for projection performance of TRIACETATE prints, which had different curl characteristics than polyester prints: ¶ "Effect of Winding on the Projection Performance of 35mm Motion Picture Film" by Paul H. Preo and Eric A. Yavitz, SMPTE Journal, June 1965, Volume 74, Pages 521-525."

You neglected to include the link to it! SMPTE gave us permission to reproduce is, so here it is in the Manuals section.

--jhawk

Posted by John Pytlak (Member # 331) on 09-22-2003, 05:54 AM:

quote:
You neglected to include the link to it! SMPTE gave us permission to reproduce is, so here it is in the Manuals section.

--jhawk

Thank you! [Cool]