Film-Tech Forum ARCHIVE: IMAX lenses changing during trailers?

This is topic IMAX lenses changing during trailers? in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Thomas Pitt (Member # 4322) on 03-29-2018, 02:22 PM:

I went to see my first ever movie in multiplex IMAX (I call it IMAX Lite), and noticed something during the adverts and trailers. From what I could see, it looked like the projector lenses or something inside the projector was moving about during the black segments between trailers.

As I don't know much about this kind of projection system except that it uses two projectors, I'm wondering if this is normal. If so, why does it happen? Do different trailers use different lenses?

Posted by Ken Lackner (Member # 1002) on 03-29-2018, 03:08 PM:

You might be seeing the dousers or 3D polarizers move. An IMAX system does not use the projector's stock douser. There is a douser mounted outside the projector in front of the lens that moves side-to-side to block or allow the light from the lens to hit the screen. The polarizers are mounted to the same device. I am pretty sure they are only activated when 3D content is playing, but I couldn't swear to it.

Posted by Carsten Kurz (Member # 5396) on 03-29-2018, 03:38 PM:

The laser systems also use automated anamorphic lenses, so there is quite some potential to see things move in and out.

- Carsten

Posted by Paul Arch (Member # 10425) on 04-03-2018, 02:57 PM:

My local Digital IMAX does this during film trailers. I wasn’t sure whether it was the system switching between single and dual projection. As I have noticed that during some trailers (for IMAX features) that don’t have ‘in IMAX’ at the end there seems to be only one projector on and for others both are on. My other theory was it was switching between ratios and showing trailers in scope or flat, which ever is the true format of the film as IMAX is common width, they could possibly do this?

Posted by Jarod Reddig (Member # 6363) on 04-03-2018, 06:13 PM:

As said I think your seeing the 3D dousers moving. Ive noticed it before too. Ive never been able to look into the booth when it happened tho.