This is topic Anamorphic lens question in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.


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Posted by Ed Inman (Member # 2583) on 06-05-2008, 01:58 PM:
 
Simple question: When properly adjusting the focus ring on a scope lens attachment, is the number of feet to the screen calculated from the tip of the lens, or from the actual film gate? In my case there is about a 1.66-foot difference between the two.
regards,
Ed
 

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Posted by Chris Slycord (Member # 4239) on 06-05-2008, 02:07 PM:
 
Neither

Focusing Anamorphics (from the tips section of this site)
 

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Posted by Ed Inman (Member # 2583) on 06-05-2008, 02:18 PM:
 
OK, fair enough. But if the distance numbers on the ring are not to be used why have them at all? The fact that they are there suggests there is some standard point from which they are to be calculated. That is my question.
 

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Posted by Justin Hamaker (Member # 2165) on 06-05-2008, 02:35 PM:
 
I can't answer you specifically, but I do know that when we actually measured the distance from the projector to the screen at our drive-in, the focus was best when we set the numbers on the lens barrel to match that distance (~350' if I remember correctly).
 

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Posted by Mark Gulbrandsen (Member # 72) on 06-05-2008, 03:36 PM:
 
Aargh! Its not a focus adjustment... Its an astigmatism adjustment! It is also best to adjust it using either RP-40 or Schneider test film... not by measuring the distance. The distance markings on lenses should never be trusted... they are usually pretty generalized.

Mark
 

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Posted by Kenneth Wuepper (Member # 1174) on 06-05-2008, 04:23 PM:
 
If you look at the scale on the anamorphic adapter, you will see that it is not very finely calibrated. The distance overall with or without the small distance from either the front of the lens or the aperture plate/film plane cannot be that accurately set.

We always found these marks to be valuable when there was a problem. If the operator changed the lens and twisted the front ring it was possible to move the anamorphic focus and then things looked bad. Easiest place to start trouble shooting was to check out the marks on the distance ring to see if they had been changed.

I supposed you could scribe your own lines on the ring and body of the adapter but that just goes against the whole professionalism thing. Like putting china marking pencil ticks on the meters of a broadcast transmitter.

KEN
 

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Posted by Ed Inman (Member # 2583) on 06-05-2008, 04:42 PM:
 
Thanks for the responses. FWIW my throw is only about 50 feet from the gate but from a visual observation (no SMPTE test film available) I seem to get the best picture with the ring set closer to 48 feet (closer to the tip of the lens rather than the gate).
Admittedly not the best way to calibrate--but for now it will have to do.
 

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Posted by Louis Bornwasser (Member # 3063) on 06-05-2008, 05:20 PM:
 
Just sent a reverse anamorphic lens to Schneider for "focus problems." Ring was miscalibrated. THEY were not able to adjust across the full range so they (quite neatly) remarked the scale. Louis
 

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Posted by Jim Bedford (Member # 4) on 06-06-2008, 12:25 PM:
 

quote: Ed Inman

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(no SMPTE test film available)

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Then get some. Trying to do this without RP-40 or some other sharp focus test film is like trying to do it without your glasses if you need them to see well.
 

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Posted by Ed Inman (Member # 2583) on 06-06-2008, 02:18 PM:
 
I'll put the test film on my to-do list, at least if it is for an actual paying gig, but right now I'm just projecting to a small screen in my back yard for fun--so it's not like I'm trying to focus a giant screen at a plex. Focusing to credits and other text seems to work pretty good. You should see the huge old 8.5-inch lens I have to mount the scope attachment on to keep the picture small enough. Quite a monster.
 

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Posted by John Hawkinson (Member # 1135) on 06-06-2008, 04:09 PM:
 
Ed, if your ring markings are only off by two feet, I think you're doing pretty well. I've seen some that are off by more like 40 feet.

The utility of test film for this is so you have something where you can clearly distinguish the horizontal and vertical focusing. If you have something that works for you, great. You could probably even make something yourself with some black leader and a scriber (or anything sharp) and just cut continuous vertical lines and horizontal lines of the same width... [Of course, I've never tried this ]

--jhawk
 

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Posted by Brad Miller (Member # 2) on 06-06-2008, 05:51 PM:
 
The movement of the scratched lines would make it impossible to focus well on. To focus horizontal, simply rack a scope loop of something out of frame. To focus vertical, you can often get close with a loop of a green band. (Make sure you choose a trailer printed at Technicolor for either purpose, as those labs produce much steadier prints than the Deluxe labs.)
 

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Posted by Frank Angel (Member # 248) on 06-06-2008, 06:00 PM:
 
You would think by now that the better lens manufactures -- both of them -- would just abandon that whole business of marking footage the front ring of their anamorphics altogether.

Print the Forum Tip on how to set the astigmatism adjustment on the anamorphic lens in their product instruction sheet with the lens and also include enough RP40 Schneider's test film to make a couple of loops and be done with it, instead of having projectionist after projectionist diligently trying to measure the lens-to-screen distance and most often than not just doing an across the floor measurement, which means he's not calculating the hypotenuse, so he's not going to get an accurate distance anyway.

If there are no "distance" markings, he won't be deluded into thinking he can get the best focus by "measuring and dialing" He'll be forced to do it the most accurate way -- the Film-Tech way, which, when you come down to it, is MUCH easier than trying to lay a measuring tape thru the seats.

Hey, there are no distance markings on the projector focus knob and we all seem to be perfectly fine with that.
 

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Posted by Steve Guttag (Member # 268) on 06-07-2008, 07:21 AM:
 
This is not entirely accurate.

Many macines have a single line for focus to denote when it is correct. Unlike an anamorphic attachment, which must be calibrated to the theatre in relationship to the prime lens, a prime lens will have one point that it is in focus for a given throw. Calibration of a prime lens is also pretty straight forward...when the image is as sharp as can be, it is in focus, for that lens. All other points along the projector's focus range are of no consequence.

The anamorphic, being a cylindrical device (as opposed to spherical) isn't so cut and dried. The astigmatism adjustment is only working on one plane, not all. Thus, you could rack it through its range and never achieve the best results.

Looking back through the history of the anamorphic lens and lenses in general you can see the origins of the distance markings. First off, I don't know of any lens that has a focusable ring that doesn't also have distance markings. The same goes for apertures (iris). Zooms, however do not always get the same sort of thing though some do that correspond to the EF of the setting.

Looking back a bit more into history...the B&L anamorphic and even the older Kollmorgan anamorphics (by Kasaka) were multi-turn distance rings. As such, without some sort of marking, one could be wildly off base and it would take several turns to figure out where you were.

Most, if not all "modern" anamorphics are single turn devices (except the Schneider compact "ES" anamorphic...but it doesn't use a "ring" so much as a screw).

The ISCO Ultra (decendant of the Kiptar), seemed to have a relatively accurate ring and was calibratable via set screw. The index line also always seemed to be on the vertical center of the image.

The Schneider anamorphics of the '80s certainly had "misleading" index lines and generally also had shifted lens elements such that one could forget about decent focus, regardless of where you set the astigmatism adjustment...it was internally astigmatized!

The current line of lenses by ISCO seem to have the index line most anywhere relative to the lens being set correctly though I often find the index line between the horz and vert centerline of the image plane (10-11 O'Clock as one were to look down the mouth of the lens from the theatre). Often, the index footage marking does not exactly correspond to actual distance when optimal focus is achieved.

Schneider seems to have for for a different approach...they now sport a vernier that has relatively little useful information other than knowing at which extreme the astigmatism is set to.

In the end however, the distance markings should be not be use as anything other than a ball park or as an indication of gross errors. If you have a projection throw of 80-feet and you think the image looks best with the astigmatism set to say 40-feet...then the odds are, you have either done a poor job of getting BOTH the vertical and horizontal fields in simultaneous focus, OR you have some sort of damage to your lens(es).

The difference between 78 and 80 feet though...exceed the precision of the markings for cinema use.

Steve
 

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Posted by Frank Angel (Member # 248) on 06-07-2008, 01:38 PM:
 

quote: Steve Guttag

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Many macines have a single line for focus to denote when it is correct.

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