Film-Tech Forum ARCHIVE: between lens sizes

This is topic between lens sizes in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Scott Norwood (Member # 30) on 10-15-2012, 10:05 PM:

This might be a stupid question, but what happens when a theatre is between lens sizes and cannot get a single lens that will accommodate both 1.85:1 and 2.39:1 aspect ratios? Are there any other options besides buying two lenses or using an anamorphic lens?

In particular, I am thinking of a house that has a 4.7 throw ratio for 1.85 and a 3.65 throw ratio for 2.39.

Barco sells a 2.8-5.5 lens that would work, but Christie only has a 3.0-4.3 and a 4.3-6.0. What are the options here, other than "buy the Barco"? And why does Christie only offer the high-contrast lenses in a small subset of focal lengths?

When using the anamorphic lens, where does the image get scaled to the height of the chip? Is it in the server, the projector, or an external box?

Posted by Brad Miller (Member # 2) on 10-16-2012, 12:28 AM:

Christie's high contrast lenses are series 1 lenses. They are absolutely better quality than any series 2 lens out there. I use one in my screening room as well as an anamorphic for scope. Nobody wants them though because they don't cover the entire zoom range for (properly designed) cinemas with side masking and require a wide angle converter or anamorphic to achieve scope.

Also many theaters are all about extreme high gain screens with a tiny bulb and scraping away at barely hitting 14fl on a new bulb. Those high contrast lenses WILL require you to burn a bigger bulb, but in my opinion they are VERY well worth it as they are sharper, have a substantially better contrast ratio and offer better colors. But again, there's that bigger bulb nobody wants to burn (and the wide angle or anamorphic lens they don't want to buy either).

You will also notice chromatic aberrations are essentially non-existent on the high contrast lenses, so you can get that convergence dead on target. With series 2 lenses, it is a compromise.

The high contrast lenses came about as a means to achieve an acceptable contrast ratio on the original Christie CP-2000 era projectors before the "dark chip" started shipping in the CP-2000-SB and later models. Even with the high contrast lenses in the early projectors, many theaters would intentionally run under light spec because it looked better to be darker and still have some degree of blacks. Many of these early installs targeted 10fl for this reason.

When using the anamorphic lens, you actually stretch the image vertically on the chip to use 100% of it (and the light). It also eliminates moire from the screen perforations and gives a more film-like image.