Film-Tech Forum ARCHIVE: Is image focus always a compromise?

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Posted by Steven J Hart (Member # 2340) on 09-20-2004, 08:54 AM:

When I switch film formats in my theater from flat to scope (or vice-versa) I usually run a loop of RP40 to check the focus. I notice that it is impossible to get a perfect focus across the screen. I always have to compromise the sharpness on one corner of the screen to get an acceptable image on another portion.

My theater is equipped with a 1600 watt kneisley xenex II, Century C with a 4 inch lens barrel and curved film gate, Isco 115 mm Ultra Star flat lens, Isco 115mm Ultra Star backup with Isco Reverse Anamorphic, non-curved 23 foot pearlescent screen. Projection distance is 116 feet. Bulb focus is good with an average of 21 fl illumination across the screen. The projection port is located about four feet to the left of center when looking at the screen.

Is there anything I can do to make my image sharper?

Posted by Adam Martin (Member # 641) on 09-20-2004, 03:48 PM:

To achieve proper focus, the lens needs to be aligned square to the film plane. Lamphouse alignment and theatre geometry (positions of the projector and screen) can also affect the focus of the projected image.

Posted by John Pytlak (Member # 331) on 09-20-2004, 03:56 PM:

Slight fall-off in resolution from center to edges is normally expected. But if you find that the focus "sweeps" across the screen as you rack focus, the film is not being held perpendicular to the optical axis of the lens, which is something you can fix. Your keystoning is minimal, but even more serious keystoning can be addressed.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-20-2004, 03:58 PM:

4' off axis with a 115mm lens isn't that bad. You shouldn't have any major focus issues caused by that factor alone. If focus comes in and out going from left to right (or right to left) and not either both sides in focus or the centre in focus, I'd look at your optical alignmnet, specifically lens alignment to the aperture.

Posted by Sam D. Chavez (Member # 1841) on 09-21-2004, 08:33 PM:

Uneven tension from the bands can also cause uneven focus from left to right. There has been many a Century where the gate shoe needed to be shimmed out on one side or the other to achieve even focus.

Posted by Gordon McLeod (Member # 33) on 09-21-2004, 09:29 PM:

You refer to it as a Century C with a curved gate
If it has a curved gate it is probably a H style and focus due to wear is probably an issue
Often the SA style doesn't align all that well in the C

Posted by John Pytlak (Member # 331) on 09-21-2004, 09:41 PM:

quote: Sam D. Chavez
There has been many a Century where the gate shoe needed to be shimmed out on one side or the other to achieve even focus.

Very easy to do, even as simple as using a gummed label as a shim on the A1-E-41 Film Gate Plate and Stud Assembly:

http://www.film-tech.com/manuals/CENMSCPART.pdf

Posted by Ken Lackner (Member # 1002) on 09-21-2004, 10:36 PM:

Correct me if I'm wrong, but isn't 16 fl the standard illumination? I have been told that having a bulb focussed too bright can also cause image focus issues due to the extreme heat on the film as it goes through the gate.

Posted by John Pytlak (Member # 331) on 09-21-2004, 10:43 PM:

Yes, excessive radiant energy (especially "hot spotting") can cause focus issues:

http://www.kodak.com/US/plugins/acrobat/en/motion/newsletters/archived/pytlak/heat1.pdf

http://www.kodak.com/US/plugins/acrobat/en/motion/newsletters/archived/pytlak/heat2.pdf

Posted by Jack Ondracek (Member # 1466) on 09-21-2004, 10:49 PM:

quote:
Lamphouse alignment... can also affect the focus of the projected image.

I've seen this before on this board, and it drives me nuts whenever it pops up. Will somebody please explain how this is possible? I don't get how lamphouse alignment can effect anything but uniformity of illumination... unless we're just talking about heat-related warpage caused by focusing the lamp down too tight. I don't see any other way that film focus, that's basically a function of physical positioning between the film, lens and screen can be affected by how the light source is lined up.

Posted by Randy Stankey (Member # 64) on 09-22-2004, 12:34 AM:

Well, in my opinion, getting good focus IS a compromise, though you should have only a very little compromise.

People sometimes only think about the "ideal" projector, whereby the film advances to a frame then it remains still in the gate for 1/24 of a second until the mechanism advances to the next frame.

In truth, the film is in constant motion. Even if you subtract out all the variables in the film mechanism itself such as pull-down time vs. dwell time; acceleration/deceleration forces on the film; movement of the parts of the gate itself and other things you still have to think about the fact that the film is actually "flexing" in the gate as the ratiant energy from the light source hits it.

Once the film has steadied itself in the gate, the shutter opens and the light hits it. On all but the most tame light sources, the film will instantly begin to absorb energy and heat up. This will cause the film to expand and "bubble up" in the gate. If I remember correctly, it bubbles toward the light. (Or toward the emulsion side.)

The film is actually out of focus for part of the time that any given fram is in the gate. It's just that the eye only seems to remember the part that was in focus.

So, a projectionist's job is to get the projector in a state where all the variables are minimized as much as he can and allow the eye of the viewer to perceive as much of the "in focus" time as possible.

Ideally, if you have your projector working the way it should be, the viewer should never perceive an out of focus condition. In reality you might have to "tune" the focus knob so that as much of the image is in focus as possible... preferably the area nearest the center of the screen.

I *DO* agree with the rest in that, if you find you have to make too many compromises, you will need to fix something.

My definition of "too many compromises" comes when an experienced viewer can notice an out of focus condition without trying.

If there IS a part of the screen that is out of focus, you should have to look to find it. (And, I mean you must have to search for it!)

Posted by Steven J Hart (Member # 2340) on 09-22-2004, 07:59 AM:

Thanks for all the info.
Gordon Wrote:

quote: Gordon McLeod
You refer to it as a Century C with a curved gate
If it has a curved gate it is probably a H style and focus due to wear

The model tag does indeed say that the projector head is a Century C, but when I replaced the film guides several years ago, I had my parts supplier stumped for quite awhile before he found the right parts. I wonder if this is a very late model projector where Century had made some of the changes, but not yet changed the model name? The head is mounted on a Century R5 sound head which was purchased new at the same time as the projector head (or so I'm told - way before my time) It is driven with a timing belt style belt.