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Projecting multiple spherical aspect ratios


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#1 Bon Sawyer

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Posted 13 March 2006 - 11:49 PM

Hi all,

My understanding is that most modern cinemas can only (correctly) project films in the 1.85:1 and 2.35:1 formats. I used to work at such a venue, where each screen had a 2-lens turret set up for these two formats.

To project another spherical format, such as 1.37:1 or 1.66:1, on the same constant-height screen without any vertical cropping, would one require a different focal-length lens? Or could one use the same spherical lens used for the 1.85:1 projection, and re-position/re-focus it to project the desired different format?

Also, are there 3-lens (or more) turrets out there that people use for this purpose? Or do venues that still bother with precise 1.37:1 and/or 1.66:1 generally change the lenses manually by hand?

Thanks,

Bon

P.S. I understand that there are other considerations too, such as masking (both in-projector and up at the screen), but I'm specifically wondering about the optical side of things.
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#2 Phil Rhodes

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Posted 14 March 2006 - 01:49 PM

Hi,

> My understanding is that most modern cinemas can only (correctly) project films in the 1.85:1 and 2.35:1
> formats. I used to work at such a venue, where each screen had a 2-lens turret set up for these two formats.

Yes, that's how it's generally done. That's how we do it.

> To project another spherical format, such as 1.37:1 or 1.66:1, on the same constant-height screen without
> any vertical cropping, would one require a different focal-length lens? Or could one use the same spherical
> lens used for the 1.85:1 projection, and re-position/re-focus it to project the desired different format?

We have a second spherical lens that we swap with the lens used for 1.85:1 material to do this, because it gives a better image size given the variability we have in the masking and screen drop. It is possible to project the whole 1.66 frame with the lens we use for 1.85 by tilting the projector down, but we don't quite have the screen drop to do it properly.

Obviously, either case involves moving the side masking in.

Because we don't project trailers, there isn't usually an issue with having to swap lenses or masking around on the fly.

Phil
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#3 John Pytlak RIP

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Posted 14 March 2006 - 02:06 PM

Although Schneider does have "Variable Prime" projection lenses whose focal length can be adjusted over a very small range, most projection lenses are fixed focal length. Therefore, to show a different aspect ratio at the same image height, you would need a lens for each format.

Some links:

http://www.schneider...ema_projection/

http://www.schneider...variable_prime/ ("Variable Prime" lens)

http://www.schneider...tre_design_pro/ Lens selection software download

http://www.iscopreci...c_cinema_e.html
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#4 Bon Sawyer

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Posted 15 March 2006 - 10:23 AM

Thanks for the info, guys!
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#5 Dominic Case

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Posted 15 March 2006 - 06:11 PM

To project another spherical format, such as 1.37:1 or 1.66:1, on the same constant-height screen without any vertical cropping, would one require a different focal-length lens? Or could one use the same spherical lens used for the 1.85:1 projection, and re-position/re-focus it to project the desired different format?

Just to answer the question precisely . . .

Yes you do need a different focal length lens for a different aspect ratio.
No you can't practically use the same lens unless it's a zoom lens (though many digital projectors go this way).
The only way you can get a larger or smaller image with the same prime lens is to change the throw (projector-to-screen distance). Not a practical option.
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#6 Bon Sawyer

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Posted 15 March 2006 - 10:41 PM

Thanks for the clarification. I presumed that that would be the case, but wasn't sure.

What techniques are used to correct the distortion of the projected image? No cinema I have seen has the projector perfectly aligned with the center of the screen, for obvious practical reasons. And of course, where multiple projectors are installed (due to the absense of a platter, or to project multiple formats), it's impossible to have both projectors aligned perfectly anyway.

Is it simply a matter of tilting/shifting the projector position in such a way that it offsets the distortion? Or is it something to do with the lens? (I see a "keystone corrector" lens attachment on one of the pages linked above by John, but I'm guessing that they aren't very common.)

Thanks,

Bon
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#7 Dan Goulder

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Posted 15 March 2006 - 11:41 PM

Hi all,

My understanding is that most modern cinemas can only (correctly) project films in the 1.85:1 and 2.35:1 formats. I used to work at such a venue, where each screen had a 2-lens turret set up for these two formats.

To project another spherical format, such as 1.37:1 or 1.66:1, on the same constant-height screen without any vertical cropping, would one require a different focal-length lens? Or could one use the same spherical lens used for the 1.85:1 projection, and re-position/re-focus it to project the desired different format?

Some movies, although rare, are still released in these aspect ratios. They are projected normally with the spherical lens, and pillar boxed within the 1.85/1 frame.
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#8 John Pytlak RIP

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Posted 16 March 2006 - 10:26 AM

Thanks for the clarification. I presumed that that would be the case, but wasn't sure.

What techniques are used to correct the distortion of the projected image? No cinema I have seen has the projector perfectly aligned with the center of the screen, for obvious practical reasons. And of course, where multiple projectors are installed (due to the absense of a platter, or to project multiple formats), it's impossible to have both projectors aligned perfectly anyway.

Is it simply a matter of tilting/shifting the projector position in such a way that it offsets the distortion? Or is it something to do with the lens? (I see a "keystone corrector" lens attachment on one of the pages linked above by John, but I'm guessing that they aren't very common.)

Thanks,

Bon


Yes, some lenses allow keystone correction, much as a tilting bellows on a view camera. But always best to design a theatre to minimize any keystone distortion.

Side-by-side projectors in a theatre are not usually an issue except for very short throws. A more common issue is a steep down angle, that might be found in theatres with balconies or steep "stadium" designs. Wide theatres that were poorly "twinned" without repositioning the projectors used to be a big issue too.
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