Film-Tech Forum ARCHIVE: Todd A O Cine Apergon lens

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Posted by Robert Throop (Member # 61) on 10-26-2007, 11:54 AM:

I just bought one of these off ebay. There's no focal length marked on it. There's apparently a magnifier on the front. Does anyone know what the magnification is? The lens supposedly came from a theatre in SLC. I'll be at a theatre tomorrow where I can check the EF with some RP40.
Thanks,
Bob

Posted by Louis Bornwasser (Member # 3063) on 10-26-2007, 12:45 PM:

The "magnifier" looking glass is a deep curved focus corrector (I think.) I have replaced many of these lenses. Typically they have yellowed and have focus problems at the edges. The Uptown in D.C. had these until they were replaced with Isco lenses back in the 80's, Sub titles were much improved as well as color and overall brightness.

In other places, I forced a Magnecom 65 fully closed to get the same effect. Louis

Posted by Mark Gulbrandsen (Member # 72) on 10-26-2007, 03:02 PM:

Bob,

It would most likely be from the Villa Theater and... yep... those lenses did mysteriously dissappear one day...... so conceivably it could be a hot item???? BTW: The magnifier is normally adjustable by screwing it back and forth in its part of the barrel.

Mark

Posted by Thomas Pitt (Member # 4322) on 10-26-2007, 03:25 PM:

Deep curved focus corrector? Is that a lens attachment that can compensate for curved screens, giving even focus throughout?

When I asked about why a movie I saw was out of focus, one of the projectionists at Leeds Light told me that they couldn't focus evenly on their two largest screens - due to the screen being slightly curved.

Perhaps they should invest in a couple of these corrective lenses [Big Grin]

Posted by Robert Throop (Member # 61) on 10-28-2007, 01:08 PM:

By my calculations the lens is about 135mm without the magnifier and 126mm with. About a 7% difference. Screwing the magnifier in and out has no effect on magnification.
Bob

Posted by Chris Slycord (Member # 4239) on 10-29-2007, 02:19 AM:

quote: Thomas Pitt
Deep curved focus corrector? Is that a lens attachment that can compensate for curved screens, giving even focus throughout?

If it were, it would be called a "curved screen corrector."

quote: Thomas Pitt
When I asked about why a movie I saw was out of focus, one of the projectionists at Leeds Light told me that they couldn't focus evenly on their two largest screens - due to the screen being slightly curved.

A curved screen is always going to defocus the edges a little but they get rid of pincushioning (screen distortion in which the sides bow in).

Posted by Thomas Hauerslev (Member # 566) on 10-29-2007, 06:11 AM:

The Todd-AO projection lenses, known as Cine-Aperagons, were specially designed for each cinema with aspherical surfaces on some of their elements. Available focal lengths

2,6" = 66mm
3" = 76mm
3,5" = 89mm
4,1" = 104mm
5,6" = 142mm
6,6" = 168mm

Posted by Larry Myers (Member # 738) on 10-29-2007, 10:52 AM:

Actually, any normal lens wants to follow a curve. If you take a long string and tie one end at the projector lens and place the other end at the center and both ends of the screen, you will have a natural curve. The longer the string, the less curve the screen needs to be. The lens has to be corrected if the screen is flat or if the screen has a greater curve then what is traced out with the string. A flat field lens would be for a flat screen. Then a drive in movie screen 400 ft from the booth needs no correction since the curve would be to shallow to notice.

Posted by Dave Macaulay (Member # 813) on 10-29-2007, 01:09 PM:

No.
A normal lens in a film projector with a flat film plane will focus exactly on a flat screen surface when the two planes (film and screen) are parallel. The rear and front focus distances are proportional: the centre of the film image is closer to the lens than the edges in exactly the same proportion as the centre of the screen is closer than the screen edges.
With a typical projection setup where the film and screen planes are not parallel plus either or both are curved... and with a short throw and very wide image... it's amazing how good the picture looks.

Posted by Larry Myers (Member # 738) on 10-29-2007, 02:30 PM:

That is true IF the lens is designed to do so. The issue is, what is the lens designed to do? Everything left alone, a simple lens wants to focus on a curved field with the film curved. Now enter new design elements and that condition changes that to what ever the lens designer wants the lens to do. That is, focus on a flat screen with the film being flat. Focus on a curved screen with the film being flat. Focus on a curved screen with the film being curved.

Posted by Steve Guttag (Member # 268) on 10-30-2007, 06:50 AM:

Its even more complicated than that...the film in the projector is not flat...not even with a straight gate...it will swell to the light source based on the heat of that light source. Furthermore...it pulsates with the shutter blade. It has also been proven with high-speed photography that "We" focus on the film when it is in its most deflected state.

Now take into acount the screen...how far is it away from the projector? What format is that lens being used for? This will trip up a "flat field" lens designer. You have the film with unknown deflection (based on wattage) in a projection system of unknown distance. That makes for a very tricky lens design and one that has keep constant lens design improvements coming from various lens manufacturers.

A flat screen will amost always exhibit a slight pincushion to the image which suggests that something is amiss in the flat-field design. A curved screen will virtually always exhibit a barrel distortion...depending on degree of curvature and the proximity of the projection system to the screen (including projector angle and skew). Some lenses definately focus better on curved screens (current designs). There is one absolute out there...there are no absolutes.

Posted by Dave Macaulay (Member # 813) on 10-30-2007, 07:42 PM:

OK, the issue is very complex.
I wanted to point out that what Larry implied about a "normal" lens focusing from a plane image onto a cylinder/sphere centred on the lens is incorrect. It's possible to make such a lens but in film projection we generally deal with "normal" lenses that work like a super efficient pinhole.
Lens design is very complicated and any practical design will be a trade off between several objectives. This makes comparing lenses "by the numbers" impossible. A lens with amazingly sharp resolution measured with a high contrast USAF test pattern may have poor depth of field and poor performance with lower contrast images. A lens designed with a deep focus to allow for screen and film curvature plus geometric misalignment will have poorer peak resolution, but often looks much nicer in real world use.