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Topic: Projection Lenses & Lamphouse Optics

Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 04-13-2000 12:21 PM

Does anyone make a projection lens faster than f/2.0 these days? Everything Schneider shows on their website is f/2 or slower.

What happened to all the f/1.8's and f/1.6's I used to see all over creation?

Also, does anyone know what the speeds are of the current crop of xenon lamphouses, such as those by Big Sky, Strong and Christie?

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John Walsh
Film God

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

posted 04-13-2000 12:52 PM

They make them slower to improve the focus (depth of field.) Usually only older lenses were faster.

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John Pytlak
Film God

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

posted 04-14-2000 05:58 AM

AFAIK, the new IscoOptic Ultra Star HD PLUS series of lenses (the ones with the red barrel) have a larger stop, and claim to offer up to 50% more light. But John is correct that as the f/stop is opened, the depth of focus becomes less. So if you already have a heat-related focus flutter problem, using a lens faster than f/2.0 may aggravate the problem. The really "fast" lenses of yesteryear are old designs that have less resolution, poorer field flatness and poorer contrast than modern lenses.

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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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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 04-14-2000 08:43 AM

In theory,both the reflector and lens are supposed to be the same f number for optimum performance. If you have an f1.8 reflector and an f2 lens then light, in theory is being wasted. While manufacturers can state that a lens is f2 there are complicatd mathematical formulas (these days requiring computers) that are used to exactly calculate it. That f2 lens could in reality be f2.16 or something like that. And yes, the f2 lenses do provide a bit more depth of field than an f1.8 or 9, but not that much. The biggest design change has been in the actual optics to be able to create flat focus from the entire film plane with a given focal length lens on the average. Also shouldn't projectors also be rated with an f number too really make things meaningful? The efficiency % thing as usually stated is really pretty meaningless unless you know the reflector and lens f numbers. Perhaps projectors should also be assigned f numbers. Alot can affect them too such as the angle of the cone of light going into the back end, the light baffles in the back end, the exact shutter opening and how close the aperture is to the film.
An interesting thing is that 70mm Cinerama and Dimension 150 lenses actually had fixed metal f stops installed to increase the depth of focus for the deeply curved screens they were used on. I don't know what f number they were but I'd guess that by the size of them around f3 to f 4 would be a fair guess.
Mark
Update: I just checked with Strong on the speed of their reflectors and they didn't know and couldn't find it in the data they have but they will check and get back to me.
I will check with others as well.

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 04-14-2000 11:12 AM

Thanks, guys for all the input. I agree, Mark. Projectors and lamps all used to be designated as to their speeds... what happened? I'm a little surprised to hear that Strong didn't know about their reflector. Evidently, none of this matters anymore.

f/2.0 used to be average. All the Magnarcs, Enarcs, Moguls and such; and the Supers and E-7's, the first Century C's, et.al. were all f/2.0. Drive-ins dictated the necessity for faster openings, lamphouses and lenses. Yes, I'm fully aware of focus differences, but it all amounts to trade-offs. Do you need light, or flat fields? I'm designing a drive-in, so I NEED LIGHT...

Thanks again, and I appreciate your diligence, Mark. I'll look for your reply soon.

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John Pytlak
Film God

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

posted 04-14-2000 12:43 PM

Tim:

I'd certainly take a look at the new "red barrel" IscoOptic Ultrastar HD PLUS series of lenses when getting more light on the screen is the primary goal. They provide more light, and are a modern design that addresses field flatness and minimizes flare very well.
Don't know what focal lengths are available.

Although most of today's lenses are very good, your final decision should be based on looking at objective test films like SMPTE 35-PA (RP40) and a variety of critical picture material, to choose the best lens for the application. Resolution, field uniformity, chromatic aberration, contrast, luminance, etc. should all be evaluated before you decide on a particular lens.

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Stefan Scholz
Expert Film Handler

Posts: 223
From: Schoenberg, Germany
Registered: Sep 1999

posted 04-14-2000 01:14 PM

At the time of writing, according to ISCO's lists, there are the following lenses available:
37.5 mm - 95 mm , within f = 37.5 and f = 60 at an interval of 2.5 mm, and out of the range with f = 5 mm interval.
Scheduled is 22.5 mm to 95 mm for "flat (1.85 !)" aspect ratio.
At this time a combined anamorphoser with base lens is avail. at 55, 60, and 65 to 85 mm focal length.
For additional information, you might contact Katja Korzen(Katja.Korzen@iscooptic.de). THey could supply test samples, John.

I have recently found a nice theory on projection and lamphouse optics in Richardson's "Handbook of Projection (for projectionit's AND managers)", named "the blue book of projection", written in 1922. I did not remember owning this book, and I really enjoyed the beautiful old projectors from Powers and others.

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 04-16-2000 09:23 AM

Thanks, John. I'll check into those. It sounds like what I'm looking for.

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 04-16-2000 10:36 AM

Hi;
Just a quick post on Strong's behalf. You probably spoke to someone in Customer Service that simply didn't know the answer to the reflector question because they've never been asked before. This is about the first place it's come up that I can remember other than a few ITEA seminars I've attended. For the record, the two common Strong Reflectors (11- and 15-inch diameter)are about f/2.1.
You're right -- ideally, the entire optical system should be matched up, but that's just not the case in the real world. Lens designs vary a lot, as you've said. As larger images with 1.85/1 film have become prevalent, that led to shorter focal lengths. In order to get reasonable pictures, we had to go to slower lenses to get acceptable depth of focus characteristics. If there was one film format and one throw distance and one screen size, we could optimize one system, but that's just not the case.
You do remember the short lenses that we used to have all over -- the ones that focused about a half-inch from the film? They were fast and they didn't work very well. Remember film really is a moving image -- it does all kinds of things as it moves through the gate. As we got shorter focal lengths and larger images, the only way to maintain reasonable depth of field, focus and contrast was to make the lenses slower.
Optical design is improving; there are ways to improve light throughput while maintaining some of the optical qualities of a "slower" lens.
The PLUS lens will be distributed by Strong here in the US. This is a matter of inventory and getting all the required focal lengths. To be honest, the process is going to take a few months to fill in the entire line of focal lengths.
I suppose projectors do have an f/number but they are virtually irrelevant in current designs. I would think most are around f/1.7 now -- they would accept a pretty wide cone of light. However, the limiting factor is and always has been film image size and how light has to enter a projection lens. A very fast optical system would do a very good job of lighting up the aperture itself, but not necessarily a good job of getting light through the hole and into the lens. Therein lies the key.
Look forward to more conversation...

Pat

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Gordon McLeod
Film God

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

posted 04-16-2000 11:35 AM

The importance of matching optical elements can't be understated.
2 interesting observations
In the late 60's early 70"s Kolmorgen came out with a line of f1.7 lens that most people didn't notice much improvement over until they were used with a lamphouse that was the same speed many the SuperCorelite and then there was a lot of light

Another observation is a few years ago we had a theatre with carbons that wanted new lens so we sold them some Isco MC and they felt that image did not have the contrast there existing Kollmorgens had
The reason turned out after many conversations with Glen Bergren that the newer lens are cominated for a point light source as found in a Xenon system not the diffuse ball of light of the large 13.6mm positive carbon.
That was also explained to me that is why when we would convert a theatre from carbons to xenon the overall image sharpness wouold drop with there older lens at the same time

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 04-16-2000 01:18 PM

Thanks Pat!
I appreciate your input on this f factor thing going on here. I do wish all manufacurers would state this spec in the product literature that they print. I also agree that most projectors out there will, these days, accept a very wide cone of light, but knowing all this would help when installing new lamphouses on older projection equipment too.
Mark Gulbrandsen
Motion Picture Sales Manager
General Theatrical Supply, SLC

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 04-17-2000 04:28 AM

Mark;
The older projectors were really "slow", especially the Brenkerts and good old Motiograph's with the barrel shutters.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 04-17-2000 07:10 AM

Pat, You can say that again! They also won't accept a wide cone of light in the back without more substantial loss.
Mark

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 04-17-2000 08:13 AM

Lots of good replies, thanks to all! Yes, all the elements in the optical train need to be matched, else we're at the mercy of the slowest link. I had the opportunity a couple years ago to run a drive-in booth with an X-60-C, that still had as its mate a Super Core-Lite. Both V8 projectors (which had interfering baffles removed/cut back) and the f/1.x lenses were from the original installation. The Core Lite ran circles around the xenon, and I was running the trim in the 70mm position, too! If I bumped it up to the 35mm spot, where it should've been, there was too great a difference and I'd get car horns on changing back over to the Strong. (Yes, the X-60 was aligned correctly, and it was acceptable on the screen, it's just the Super Core-Lite was so much faster.)

The best analogy I've heard for describing matching the speeds of all the various elements in the optical train is: Imagine you're looking down the center of several doughnuts all in a line. If the hole in one of them is smaller, then that's the best the whole system can do. It's the weak link in the chain.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 04-17-2000 08:27 AM

Tim,
The big Ashcrafts are a hard ticket to beat! The quality of light form them has never been matched in my opinion. Chicago used to be full of Super Cinex's at one time, but there is only one pair left running that I know of(Copernicus Center). I've heard that there are many still running around the country though. There is one Arc lamp that will do better and that is the washing machine size Strong Jet Arc. This is a blown arc lamp that was designed in Switzerland. Sounds like a blow torch while running. After the show is done you can store the AA-2 inside it and lock it up at night. I know of one pair that still are run on occasion. NASA also used these in banks as solor simulators. A number of people also like the Peerless condenser arc as well, but mainly for rear screen use.
Mark

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