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Topic: Changable Focal Length??

Dave Bird
Jedi Master Film Handler

Posts: 777
From: Perth, Ontario, Canada
Registered: Jun 2000

posted 10-08-2000 10:40 PM

Back where I belong tonight (outdoors), we visited a drive-in I looked at buying about a year ago. Spent some time in the booth. He'd taken the Kalee/carbon c/o's out (one's in the snack-bar - nice touch) and installed my own setup - XL with SH-1020 soundhead converted for stereo. And you know how it is, you always forget to ask the question about something that caught your eye.

He's using B&L red ring scope lens, and he mentioned that one of the reasons he opened later in the year was that this lens was away in Montreal. Now, these look identical to mine, EXCEPT when he slides it in, I notice some open clamp-type things attached to it. As he slides the lens in, there's an actual gap between the front of projector and where the B&L lens is. Is this some kind of "focal-length adapter" or something?

BTW, throw of 400 feet to an 80 foot screen with a Xenex at 3K. We saw "Gone In 60 Seconds", a rather dark film, but the image was very good, no problems at all. There must be some kind of magic voodoo, I know he talked about his fast lenses, but even he was shocked at how bright it is. (The power had been disconnected for over 6 months, and therefore subject to new codes, so even though 3-phase runs right through the back lot, for him to connect it 50 feet or so, they wanted new poles for a mile!!! So they tried 3K in testing, and it worked.) Of course, this place is in the middle of nowhere, no light problems.

It actually snowed here tonight. Owner said snow's no problem, pretty rare this time of year, but no problem. Fog is much worse.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 10-09-2000 01:09 PM

Outside of prismatic anamorph they all have about the same light loss (focus contrast and colour are different issues)
The speed will basically be the back up lens

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Bill Purdy
Expert Film Handler

Posts: 139
From: Seattle, WA
Registered: Oct 1999

posted 10-09-2000 03:06 PM

I don't know if it is done anymore, but in order to take advantage of the higher speed of shorter focal length lenses, some folks used a prime lens wide enough to fill the screen horizontally and then mounted the anamorphic backward so as to shrink the image vertically to the correct aspect. They sure looked funny and you had to have a support for the anamorphic. I got involved because my lathe could cut the thread pitches needed for the special adapters necessary to marry the front of the prime lens to the front of the anamorphic.

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Bill Purdy
Component Engineering

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John Pytlak
Film God

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

posted 10-09-2000 03:21 PM

Bill: Otherwise known as "reverse anamorphics". Instead of "stretching" the image to fill the wide screen, it "scrunches" its' height. With a normal anamorphic, you choose the focal length of the prime lens to fill the height of the screen. With reverse anamorphics, you choose the prime's focal length to fill the width of the screen. AFAIK, was often used at drive-ins, where the throws could be quite long.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

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Bill Purdy
Expert Film Handler

Posts: 139
From: Seattle, WA
Registered: Oct 1999

posted 10-09-2000 05:42 PM

John: I think that's what I said.

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Bill Purdy
Component Engineering

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Dave Bird
Jedi Master Film Handler

Posts: 777
From: Perth, Ontario, Canada
Registered: Jun 2000

posted 10-09-2000 08:16 PM

The throw is 400ft. to 80ft. screen. The "modified" scope lens is VERY long. If they are using reverse anamorphics, would this account for the "brighter-than-seemed-possible" picture. I remember when I was looking at the property thinking it may be difficult to get a very bright picture. Would the combination of "fast" lens and the "scrunching" effect make for more brightness?

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 10-09-2000 08:19 PM

I thought that, generally, you want the prime lens to be as close to the anamorphic as possible. Is that true?

This "open clamp-type thing" that Dave describes; could there be some advantage in seperating the two?

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 10-10-2000 06:53 PM

I don't think that reverse anamorphic lenses allowed for a brighter picture. It was really only a way to use shorter focal length lenses (it was reduced by half.)

Scope generally is a brighter picture than flat.

But, a 400ft throw with a 80ft wide screen sounds like exactly the kind of application reverse anamorphics were made for.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 10-10-2000 07:03 PM

once the lens get much over 6" in focal length the efficiency drops unless you go to a 4" diametre one
The reverse anamorph allows for the use of the more efficient short lens

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