Film-Tech Forum ARCHIVE: Would a Contemporary Lens Have a Greater Depth of Field?

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Posted by Bill Enos (Member # 440) on 04-07-2007, 07:38 PM:

Every year this is an issue on a couple of festivals that have subtitles. On scope if subtitles are sharp the center of the screen and up are soft. Flat is not an issue, there seems to be enough depth of field for both to be OK, never got a complaint.

Here is the info. Throw 110 ft., angle 14 degrees, screen height 16.5+- feet same for flat and scope, width scope 36'4",
width flat 26'. Lenses---some history here, no laughing please.
Scope prime Kollmorgen BX 163 5.5" with anamorphic B&L Cinemascope I. Flat Kollmorgen BX 241 3.75". The screen is not angled.

Flat gives sharp subs and image, scope is noticeably soft if subs are sharp. Would modern lenses improve on this depth of field problem on scope? Seems to me that the issue lies in the anamorphic since flat even with much greater magnification doesnt have this problem.

Posted by Louis Bornwasser (Member # 3063) on 04-07-2007, 08:16 PM:

Probably the projector is OK since flat is OK. (It is possible for this to be wrong since you are using the full height of the film only in scope.)

The only thing I see wrong on scope is that the BX-163 (I think) has a primitive magnecom built in; not the greatest idea.

I would get a modern scope backup and test to see if a full or partial cure happened. Louis

Posted by Steve Guttag (Member # 268) on 04-07-2007, 08:53 PM:

There are many factors in play here. The lenses you are using are notorious for poor focus uniformity due to the presumption that the film was flat in the gate. However, when heat hits the film, it swells towards the light source making the focus want to be at the screen plane and further away at the edges.

Of the old lenses, the BX163s are pretty bad as lens go (they are the black ones and have maybe 4 elements in them...I'd have check on that to get exact specifications.

Now, combine that with your steep down angle and you have yourself a risky situation. Note, for flat, you have less heat hitting the film (more hitting the aperture plate) so it will suffer less in this regard.

The Scope attachment should perform zero magnification in the vertical plane (Cylindrical lens elements, not spherical). However, it is possible for the elements to rotate with respect to themselves that will prevent one from EVER getting uniform focus.

I would think that modern lenses could have a rather marked improvement to what you have now (both prime and anamorphic attachment). The ISCO Blue Star is real hard to beat for anamorphic attachments. As for prime lenses, you are looking at a 140mm...they normally do pretty darn well with off axis projection. Either ISCO or Schneider will do you real good. If you have light to spare, Schneider can make stop down rings for their lenses greater than 100mm. These stop down rings will also improve corner-corner sharpness.

One thing you can check though it won't be conclusive is to pull the anamorphic and look at real target film and note the resolution...then see what the resolution with the anamorphic attachment on.

Louis is mistaken about the model you listed as having a crude MagnaCom in it...those were the short EF lenses. The BX294 is the lens series that had the Magnacom 69 in them...they also had an "X" in their name.

Another thing going for new lenses is that you ran carbons for many years and no doubt had smokers in the booth...I've seen many a lens collect soot from all that.

If you have straight gates in your XLs...you might also find that curved gates improve your focus situation.

So...change the prime lenses and you will see a marked improvement...change them both and you won't be able to believe how much better it can be.

Posted by Brad Miller (Member # 2) on 04-07-2007, 10:34 PM:

Lenses are a definite, but in situations where I have a top/bottom or left/right focus issue, it can almost always be corrected with some careful turret or gate adjustments, or sometimes shimming of the lenses in their holders.

I've got a nice condition used ISCO 135mm lens and anamorphic I can loan you for testing purposes if you pay the freight.

Posted by Louis Bornwasser (Member # 3063) on 04-08-2007, 06:40 AM:

I think Steve covered it all very well! Louis

Posted by John Walsh (Member # 168) on 04-08-2007, 09:51 AM:

A minor point ... If the films have subtitles burned in with a laser, they will be a little out of the focal plane of the emulsion, causing a bit more of a focus problem. But, yeah, a 14 deg tilt is gonna cause some issues.

Posted by Richard May (Member # 2627) on 04-08-2007, 02:38 PM:

We have had the same issues with scope in our theater. We upgraded 3 out of 4 lenses to schneiders with schneider anamorphics. What a difference.

Posted by Demetris Thoupis (Member # 833) on 04-08-2007, 04:03 PM:

I have found out that using one complete lens for cinemascope gives sharper and better focus in subtitled features throughout the image rather than using a 2 lens solution (lens and anamorphic) When using 2 lens solution the projector has to be 0 degrees throw to the screen in order to have a uniform sharp image. Laser subtitles are the main cause of problem. When using "wax" subtitles you should not have any problem.
Demetris

Posted by Richard May (Member # 2627) on 04-08-2007, 04:11 PM:

I tried getting an intergrated anamorphic but Schneider didn't make one for the focal length I needed. As for our theater, luckily we are at a zero angle.

Posted by Mark Gulbrandsen (Member # 72) on 04-08-2007, 05:26 PM:

A one piece really is actually two seperate lenses and many integrated anamorphics backup lens can be unscrewed from the anamorph. If you think two part lenses are bad you obviously have not tried a Blue Star anamorphic with one of the higher quality Isco backups. The difference is beyond astonishing!

Mark

Posted by Steve Guttag (Member # 268) on 04-08-2007, 08:34 PM:

Demetris...what you said makes ZERO sense. There is nothing inherently better with integrated anamorphics...in fact, they are the inferior ones. In the name of size and space, they are almost all 4-element designs that are inferior to the Isco 5-element, 6-element designs. Schneider's "integrated" anamorphic may be ordered as a separate piece as the "ES Anamorphic". As short anamorphics go, it is the best I've found. Once you get to 55-60mm though, the Blue-Star anamorphic attachment is notably superior to others. I've used them with all sorts of down angles as well as lens shifting.

One thing many people don't seem to realize is that focus starts with the lamp itself and how it is aligned as well as reflector design. They all work as a team.

Posted by Demetris Thoupis (Member # 833) on 04-09-2007, 07:24 AM:

I have this theater with extreme angle throw and we can not get even focus on scope films (talking about huge angle) no matter how hard we try. Combination of Galileo 105mm lens with Isco Optic Gold Anamorphic. This is mostly to the throw angle rather than the lenses. I do though have the same setup in a village cinema with the same result and very little throw angle. A small country can keep a stock of lenses just to check whether a problem can be fixed but in the second case, when I tried a single lens solution from Isco Optics which I borrowed from another cinema and then gave back, I managed to achieve perfectly sharp focus in all films including films with laser subtitles.
Demetris

Posted by Mark Gulbrandsen (Member # 72) on 04-09-2007, 08:13 AM:

With extreme down angles sometimes the screen has to be tipped back at the top to solve both keystone and focus issues. There were several theaters in Chicago like that.

I forgot to mention that the down tilt was so extreme in these theaters that the xenon lamps had to be installed in the reverse fashion as in a Super Trooper spotlight. This way they'd stay lit and just make warranty hours.

Mark

Mark

[ 04-09-2007, 09:44 AM: Message edited by: Mark Gulbrandsen ]

Posted by Jack Ondracek (Member # 1466) on 04-09-2007, 11:25 AM:

'scuse me if this sounds impractical, but could the trap/gate be angled somehow? It might not address keystoning, but wouldn't it fix the focus error?

Posted by Sam D. Chavez (Member # 1841) on 04-09-2007, 11:57 AM:

People were shimming Century gate shoes back in the "70's with business cards or beer can shims to get even focus. At that time it was due to abysmal build quality.

[ 04-09-2007, 06:42 PM: Message edited by: Sam D. Chavez ]

Posted by John Hawkinson (Member # 1135) on 04-09-2007, 04:15 PM:

"People were shimming Century gate shoe back in the "70's with business cards or beer can shims to get even focus. At that time it was due to abysmal build quality."

Sam, are you sure it was build quality? In Lou Berggren's article in SMPTE Journal (err, excuse me, "SMPTE Motion Imaging Journal") about lenses, he talks about how Kollmorgen got complaints about soft focus on the right side of lenses in 1962, now understood to be from the film bulging due to heat and not lying flat in the film plane, and how UA solved the problem with a a 0.002" (+/- 0.001") brass shim to balance the soundtrack. (There was also a problem with soft focus in the corners, for the same reason, but obviously not correctable so easily.)

[Berggren, Glenn. "The Evolution of the Cinema Lens--Part 2: The Cinema Revival: 1962-1974." SMPTE Motion Imaging Journal, Feb/March 2007. Vol. 116 No. 2/3, pp. 87-103]

The article also has this amusing anecdote (p.95):

quote:

Projectionist's Outlook
The projectionists, coping daily with the focus problem and having no applicable lens data, invented their own "junk science." First, they attributed the right-side focus problem to "crooked lens grinding." Yet, turning the lens in the lens mount, usually showed that crooked was not the problem.

It's a fun read (Part I was published in Dec. 2004, and to me, was not as interesting).

Steve G. mentions, "Of the old lenses, the BX163s are pretty bad as lens go (they are the black ones and have maybe 4 elements in them...I'd have check on that to get exact specifications. ."

You're right. [Cool]

According to Berggren, "the low-resolution 4-element BX 163 was a pre-1946 design," with the subsequent series being "the 6-element BX 241 f/1.9 series (in gold) designed between 1947 and 1952," and the "hybrid 7-element BX 290 f/1.7 series in 1954." I think the BX163 is an f/2.0 lens.

I think maybe the big shocker for me was to learn that Kollmorgen sold its projection lens business to Schneider in 1973. I had not realized!

--jhawk

Posted by Bill Enos (Member # 440) on 04-09-2007, 08:08 PM:

Seems like we could hardly do worse than these BX 163s, yep, they are F2.0. The good part is that we didn't buy 'em, they have been here since the '50s and this operation inherited them from the previous owners...I guess we'll replace them when it's possible. Anybody got some good, inexpensive replacements?

Did they recognize them for being this bad when they were new? If they were, why did anybody buy them, were they cheap or was there nothing better?

Posted by Sam D. Chavez (Member # 1841) on 04-09-2007, 11:01 PM:

Interesting stuff.

I saw a lot of instances where the bottom and top of the image were not in focus at the same time. The lens mechanism and gate were one assembly and not pinned so it was very possible for it to be cocked with reference to the trap.

Posted by Robert Throop (Member # 61) on 04-10-2007, 01:29 PM:

quote: John Hawkinson
the low-resolution 4-element BX 163 was a pre-1946 design

While this is true, Kollmorgan continued to make the BX 163 well into the 1960's. They were frequently used in long throw situations with older projectors that wouldn't accept 4 inch diameter lenses. I have a pair with an E.F. of 8.5 inches and I'm sure they were made in even longer focal lengths.
Bob