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Topic: Lens Types?

Dominic Case
Expert Film Handler

Posts: 131
From: Sydney NSW Australia
Registered: Aug 2003

posted 06-28-2004 06:43 PM

quote: Steve Guttag
.825/.446 = 1.84977578475....etc. Using proper math and limiting to 3 sig. figs...we get 1.85 but that is merely an approximation of the ratio using the data we have. By contrast, 16:9 is a precisely defined ratio using whole numbers and is always accurate.

This is a common argument that, in my opinion, confuses the point. The result of .825 divided by .446 is only as precise as the original numbers (in this case, 3 significant figures). If we started with .825 and .446 etc then it it perfectly accurate and equally precise to say the ratio is 1.85. It is only meaningful to have any more decimal places if we start with a dimension of .82500000.

Besides, anyone chasing a fourth decimal place is ignoring the realities of tolerances in engineering and optics.

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Andrew McCrea
Jedi Master Film Handler

Posts: 645
From: Winnipeg, Manitoba, Canada
Registered: Nov 2000

posted 07-31-2004 02:50 PM

I AM COMPLETELY UNEDUCATED WHEN IT COMES TO LENSES, SO:

Do they still make D150 lenses... Just in case someone ruined one, or if someone wanted to put one in for current 35mm presentation? What is the special name for these (integrated anamorphic, etc.) or do they just put an ellipse or something on a scope and flat lens?

A D150 here that closed in 2002 showed all the current blockbusters on that big screen.

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Manny Knowles
"What are these things and WHY are they BLUE???"

Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002

posted 07-31-2004 07:15 PM

A book I read in college explained that the purpose of the curved screen was to offset an optical defect in the original wave of CinemaScope lenses. If the screen remained flat, the image would appear slightly convex along the horizontal axis. In fact, I have seen a few 'scope movies on a flat screen and they do indeed appear to curve "away" near the left/right screen edges. I want to say that one of the movies I'm referring to was REBEL WITHOUT A CAUSE but I can't be sure since that wasn't a Fox release.

An alternative explanation -- which I have only ever heard and never actually seen published -- is that the curvature introduced a sense of depth, envelopment and panorama.

More explanations: (1) curved screens provided more uniform lighting; (2) focus issues; (3) to be distinctly different from "flat."

As for precision aspect ratios, I prefer to have everything expressed "to one" (:1). Call me old-fashioned but 16:9 doesn't readily tell me much. And besides, choosing one over the other based on decimal places is ludicrous IMHO.

quote: Dominic Case
Besides, anyone chasing a fourth decimal place is ignoring the realities of tolerances in engineering and optics

...and an old man's ability to file the plate. [Razz]

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 07-31-2004 09:42 PM

The D150 lens is just an adjustable magnifier that creates a spherical distortion that matches the deeply curved screen. It's adjusted to match the screen curvature and since making the adjustment also affects the magnification they work backwards and then choose the prime (backup) lens of appropriate focal length to fill the screen given whatever the magnification ended up being. Despite the way cool rectangular front it's not doing anything different in the horizontal plane than in the vertical. If you rotated the lens nothing would happen to the image except that it would vignette from the fact that the image is wider horizontally. The sperical distortion (correction) is actually working in the vertical plane too it's just that the much smaller vertical dimension of the image makes this negligable so it's not a problem on a screen that is flat in the vertical dimension.

They were made by Kollmorgen. No, they are no longer made. ISCO offers modern lenses for deeply curved screens. I don't know if they are a direct replacement however.

Read all about it here:
www.in70mm.com D150 Lens Article by Dick Vetter (its designer)

See also My D150 Page

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Bobby Henderson
"Ask me about Trajan."

Posts: 10973
From: Lawton, OK, USA
Registered: Apr 2001

posted 08-01-2004 12:17 AM

Aspect ratio is only one thing. It is a given any 35mm projector is going to have both a spherical lens and a 'scope lens. The other critical factor is having the correct focal length for lens chosen. This is really where the folks designing auditoriums need to spend a lot of time studying. The screen size, its curvature, the screen's distance from projector and lens size in the projector are basically the most important elements to get right. It is a shame you run into a lot of theaters that miss this part.

Schneider lenses seem to be the most popular brand in use for movie theaters. At least they always seem to be the kind I see used in a lot of theaters. The Carmike theater here in Lawton uses Super-Cinelux lenses on Simplex projectors. I forget the focal lengths, but I seem to remember them running between 50mm and 70mm depending on the size of auditorium (these auditoriums aren't really big, but do have pretty good size screens for the size of the rooms). The AMC Quail Springs 24 booth in Oklahoma City had Schneider Cinelux Ultra's on larger Simplex models for their large houses. These have focal lengths over 100mm.

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Andrew McCrea
Jedi Master Film Handler

Posts: 645
From: Winnipeg, Manitoba, Canada
Registered: Nov 2000

posted 08-01-2004 11:43 AM

Thanks for all the help!

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William Hooper
Phenomenal Film Handler

Posts: 1879
From: Mobile, AL USA
Registered: Jun 99

posted 08-02-2004 02:44 AM

quote: Ferdinando Innocenti
All right, only one thing: anamorphic is a generic word indicating a distortion in horizontal magnification

And one more thing too, it is not applied exclusively to motion picture images. It didn't even start with them! There were tons of anamorphic paintings made, most requiring not a horizontal "unsqueeze", but a cylinder!

http://www.btinternet.com/~peter.clements/morphic/morph.htm
http://library.thinkquest.org/3257/illusion.html?tqskip1=1
http://www.asu.edu/cfa/art/people/faculty/collins/Anamorph.html
http://www.jates.co.uk/anam1.htm

The distortions of trompe l'oiel require anamorphophic distortions, as do most picture compositions that are projections which must combat foreshortening, etc. I suppose that in that 'view', the entesis which is part of the very definition of a classical column is a sort of anamorphosis, since it was engineered to counter the optical problems with rows of columns.

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Bobby Henderson
"Ask me about Trajan."

Posts: 10973
From: Lawton, OK, USA
Registered: Apr 2001

posted 08-02-2004 05:48 PM

Correct if I'm wrong, but didn't Henri Chretien first develop anamorphic lenses for submarine periscopes?

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 08-02-2004 09:27 PM

AFAIK, it was telescope optics.

quote:
(Chretien) Hall, John S. Biographical Note: Henri Chretien and his magnificent contribution to the design of telescopes. (offprint, published 1983 in Bulletin AAS.)

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Ethan Harper
E-dawggg!!!

Posts: 325
From: Plano, TX, USA
Registered: May 2000

posted 08-02-2004 09:47 PM

How does a curved screen improve light reflectance? I can only come up with couple of theories and I do not know if they are correct.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 08-02-2004 09:59 PM

quote: Ethan Harper
How does a curved screen improve light reflectance?

Think of a gain screen as acting somewhat like a mirror: angle of incidence = angle of reflectance.

Then draw out a theatre plan with a fairly short throw relative to the width of the screen. You want to reflect the projector's light back to the center of the seating area for maximum light efficiency and illumination uniformity. If you curve the screen to a radius that is equal to ((throw + distance to center of audience) / 2), you will find by "ray tracing" the angle of incidence and angle of reflection that the projector's light is reflected back to the center of the audience from across the full width of the screen.

If you install a "gain" screen flat, you will find that the light from the projector that hits the sides of the screen is reflected toward the walls and not the audience, causing non-uniformity and loss of luminance.

Check out SMPTE Recommended Practice RP95:

http://www.smpte.org/smpte_store/standards/index.cfm?scope=1&stdtype=rp&CurrentPage=5

quote:
RP 95-1994: Installation of Gain Screens
This practice specifies the optimum installation parameters for gain screens used in motion-picture theaters.

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Andrew McCrea
Jedi Master Film Handler

Posts: 645
From: Winnipeg, Manitoba, Canada
Registered: Nov 2000

posted 08-04-2004 09:29 PM

Thanks for all the useful information...

From what I've gathered, it seems like there's no standard lens that you buy and put in the turret.

With the lens coming with a focal length and back focal length, how can you tell which lens you need (and buy) and the size of image it creates?

Thanks, and good night!

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 08-05-2004 11:12 AM

quote: Bobby Henderson
Correct if I'm wrong, but didn't Henri Chretien first develop anamorphic lenses for submarine periscopes?

Anamorphic lens systems were used in German submarines in WWI to provide the commander with a more panoramic view of what he was about to sink.

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Andrew McCrea
Jedi Master Film Handler

Posts: 645
From: Winnipeg, Manitoba, Canada
Registered: Nov 2000

posted 08-05-2004 12:24 PM

DELETING DUPLICATE POST

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Andrew McCrea
Jedi Master Film Handler

Posts: 645
From: Winnipeg, Manitoba, Canada
Registered: Nov 2000

posted 08-05-2004 12:25 PM

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