Film-Tech Forum ARCHIVE: Changing lenses on a regular basis

This is topic Changing lenses on a regular basis in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Antti Nayha (Member # 4980) on 04-06-2012, 04:40 AM:

I’m designing a D-Cinema system for a multi-purpose venue where two vastly different lens ratios are needed, depending on usage:

For regular cinema use, they will need a 1.6–2.4 lens to fill their big screen. And for some other uses (including backdrop projections for theatrical productions), they will need a much longer lens to project an image to a smaller screen that is also farther away from the booth.

Now, assuming that they will be changing between two motorized lenses let’s say once a week – will the focus/zoom/shift settings stay put for each lens? Or does a lens change always require running the ”lens home and return” procedure and creating new lens settings, to avoid problems? Any differences here between Barco/NEC/Christie? Or between 0.98/1.2" systems?

The non-cinema use doesn’t necessitate 100% perfect image quality, so at least focus plane and convergence don’t have to be adjusted each time the lens is changed. Those mechanical settings can stay optimized for the shorter lens.

Of course, I can tell them to use a separate (non-DCI) projector for the non-cinema use, permanently fitted with a longer lens. However, they would prefer a single-projector solution if that can be made to work reliably without too much pain.

Posted by Pete Naples (Member # 761) on 04-06-2012, 05:55 AM:

It'll need re-calibrated each and every time it's changed, and I think I'd be right in saying (certainly on Barcos & Kinotons) the lens files may not be accurate after the lens change, so those should be re-set as well.

An easier solution might be to spec 2 fixed lenses and re-size? Or use your 2 zooms but set up for fixed lens working. Not ideal in picture/light terms but it'd be a workaround.

Posted by Pietro Clarici (Member # 4937) on 04-06-2012, 07:29 AM:

You could spec a single lens, for the big screen. When needed, zoom out as far as possibile and then create a screen file scaling external sources to the matrix portion that fits the smaller screen. It will be less than 1080p (unless you have a 4K projector) and you will lose light, but it could be acceptable.

Posted by Antti Nayha (Member # 4980) on 04-06-2012, 07:49 AM:

Thanks for the replies, guys.

Pietro: that’s the first thing I had in mind, but the light loss might be just too much in this case. To make the small screen bright enough, I’d have to specify a huge lamp, which would then have to be dimmed down somehow during big-screen use…

I guess that using an anamorphic attachment for scope on the big screen would help as well, as that would allow me to use a longer lens (thus allowing me to zoom closer to the size of the small screen, losing less light).

Still, any real-life experiences on regular lens-changing – also on Christies and NEC’s – would be much appreciated!

Posted by David Zylstra (Member # 4230) on 04-06-2012, 09:05 AM:

Christie has a motorized auxiliary lens mount that can be used to drop an anamorphic lens into place, or in the case of sites with older lenses a 1.25X converter that is used for scope (i.e. a fixed height screen is fitted to flat, then this lens is used to blow up the scope picture to fit the height) . . . . you could do the reverse and find an auxiliary lens that will make the picture fit the further screen.

Posted by Chuck McGregor (Member # 6826) on 04-06-2012, 09:45 AM:

quote: Antti Nayha
including backdrop projections for theatrical productions

I'm just curious about front projection being used for this purpose. It is typically done with rear projection, there being the issue of actors, sets, or whatever between projector and the screen. But, this may not be the case with your physical layout but I have trouble envisioning this.

Typically, video projectors (e.g. Sony VLP series just as an example, not a recommendation) with really low throw ratios and throw distances are used because of limited upstage space. As well, the non-DCI imagery created for such use would likely be in some video projector friendly VGA format.

There can be other benefits to a separate (and portable) projector for theatrical use, but don't know enough details about your situation to comment.

Have these issues have been considered?

Posted by Antti Nayha (Member # 4980) on 04-10-2012, 09:43 AM:

Yeah, those issues were considered, but rear projection wouldn’t work in this venue due to the geometry…

However, in the end they’re leaning towards getting a secondary ceiling-mounted projector for backdrops only. This one will be installed closer to the secondary screen, and projecting from a higher angle (with heavy keystone correction) to avoid getting actors in the way.

The D-Cinema system will only project to the primary screen, so no need to worry about lens changes after all.

Posted by Dave Macaulay (Member # 813) on 04-12-2012, 10:47 AM:

"To make the small screen bright enough, I’d have to specify a huge lamp, which would then have to be dimmed down somehow during big-screen use…"
That doesn't sound right to me.
The smaller screen should need less light than the bigger screen.

Posted by Antti Nayha (Member # 4980) on 04-12-2012, 11:22 AM:

The smaller screen is also much farther away, remember? So without changing lenses, I wouldn’t be able to zoom close enough. I would have to scale the picture smaller digitally in that case, probably losing something like 75% of light and resolution.

I know, I should have drawn a picture in the first place. [Wink]