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Native Aspect Ratio of 35/70/Imax film Negative


Fiza Chughtai

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Hi,

What are the Native Aspect Ratio of 35/70/Imax film negatives?

Also, I read that 35mm negative aspect ratio is 2.39:1, so how does some old films have the ratio of 1.37:1? (which gives almost 4:3 image on 16:9 screen with pillar boxing but not sure if that's exactly 4:3)

Regards

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65mm 15-Perf (Imax) 
1.34:1    Camera Full Aperture            2.772" x 2.072"        70.41mm x 52.63mm
1.43:1    Projector Aperture            2.740" x 1.909"        69.6mm x 48.5mm

35mm 4-perf Camera Aperture (Spherical)
1.33:1     Full/Silent/Super 35            .980" x .735"        24.89mm x 18.67mm

The original silent era 35mm movie format was 4-perf tall, which is 4:3 (1.33 : 1.)

In 1932, after the optical soundtrack started taking up space on the left side of the print, shaving the width of the image from 1.33 to 1.19 : 1, the Academy proposed the projector gate, besides covering over the optical soundtrack, also crop the top and bottom a little, creating a 1.37 : 1 shape.

4-perf 35mm CinemaScope in 1953 used a 2X anamorphic lens to squeeze a wider image onto the film. So if the original full aperture (silent) negative and print area was used, a 2X unsqueeze would get you 2.66 : 1 from a 1.33 : 1 negative. However, with the room needed for the soundtrack on the print, it eventually became today an approx. 2.39 : 1 shape (to round up to make it simpler to understand, think of a projector gate that is 1.20 : 1 in shape and then that image is optically stretched out by 2X to make it 2.40 : 1)

In 1960, Technicolor Italy came up with the 2-perf 35mm format (Techniscope). Half of a 4-perf 35mm full aperture frame (1.33 : 1) would be 2.66 : 1 naturally with normal spherical (not anamorphic) lenses. But back then only a 2.35 : 1 area was used to blow up & squeeze this in post to a 4-perf 35mm CinemaScope image.
 

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65mm 5-Perf (Super Panavision / Todd-AO)
2.29:1    Camera Full Aperture            2.072" x .906"        52.63mm x 23.01mm
2.20:1    70mm Projection Aperture        1.912" x .870"        48.56mm x 22.10mm

35mm 8-perf (VistaVision)
1.50:1    Camera Full Aperture            1.485" x .991"        37.71mm x 25.17mm
 

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With 5-perf 65mm, a 70mm print stock is used, there is an extra 2.5mm outside the vertical rows of sprocket holes on each side to make room for the soundtrack. So the picture area is the same within the perf rows and the image can be contract printed.  This is unlike 35mm, where the optical soundtrack is on the inside of the left row of sprocket holes, taking up some of the picture area.

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On 2/14/2021 at 7:15 PM, David Mullen ASC said:

35mm 4-perf Camera Aperture (Spherical)
1.33:1     Full/Silent/Super 35            .980" x .735"        24.89mm x 18.67mm

The original silent era 35mm movie format was 4-perf tall, which is 4:3 (1.33 : 1.)

In 1932, after the optical soundtrack started taking up space on the left side of the print, shaving the width of the image from 1.33 to 1.19 : 1, the Academy proposed the projector gate, besides covering over the optical soundtrack, also crop the top and bottom a little, creating a 1.37 : 1 shape.
 

Thank you Mr. David, big help.

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  • 1 year later...
On 2/14/2021 at 2:15 PM, David Mullen ASC said:

65mm 15-Perf (Imax) 
1.34:1    Camera Full Aperture            2.772" x 2.072"        70.41mm x 52.63mm
1.43:1    Projector Aperture            2.740" x 1.909"        69.6mm x 48.5mm

Does the camera aperture equal the exposed area on the negative ??  Meaning that IMAX true aspect ratio is 1.34:1 and not 1.43:1 like everyone says ??

Also, what is the need for masking(or let's say cropping) such a small portion for projection ??

 

Thanks

Edited by John Shell
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The camera aperture determine what goes on the film.

Projector aperture is always smaller, to allow for slight variations in camera gate position between cameras when material is intercut -not so relevant for IMAX but the principle remains. You don't want to run the risk of projecting the frame edge, which might show dirt or some other marks.

But regardless of camera aperture, the AR you see is what's projected, of course. A little more may come off on the screen masking.

Edited by Mark Dunn
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6 hours ago, John Shell said:

Does the camera aperture equal the exposed area on the negative ??  Meaning that IMAX true aspect ratio is 1.34:1 and not 1.43:1 like everyone says ??

Also, what is the need for masking(or let's say cropping) such a small portion for projection ??

 

Thanks

The correct aspect ratio is the one the filmmaker’s composed for, so I’m not sure what “true” means. We had movies shot for decades on 4-perf 35mm, which is 1.33 : 1 full aperture, but that area hasn’t been projected since 1932 for the most part, ever since soundtracks were put on prints. So is the “true” aspect ratio of 1.85 35mm films actually 1.33?

What about 1950s VistaVision films? That format has a full aperture of 1.5 : 1. If you actually projected that area, you’d things not framed to be seen, like off the tops of sets.

As for IMAX, I don’t know why the inventors chose to crop the full aperture a little, more at the top & bottom. Since the film runs horizontally, maybe they wanted to leave the option for a soundtrack on the print, I don’t know. Maybe when they built their first theater, it just worked out that the projector gate had to crop a little to fit the screen that they could build.

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On 2/14/2021 at 11:15 AM, David Mullen ASC said:

4-perf 35mm CinemaScope in 1953 used a 2X anamorphic lens to squeeze a wider image onto the film. So if the original full aperture (silent) negative and print area was used, a 2X unsqueeze would get you 2.66 : 1 from a 1.33 : 1 negative. However, with the room needed for the soundtrack on the print, it eventually became today an approx. 2.39 : 1 shape (to round up to make it simpler to understand, think of a projector gate that is 1.20 : 1 in shape and then that image is optically stretched out by 2X to make it 2.40 : 1)

David, and what is the common way for achieving a 1.85:1 and 2:1 aspect ratio with digital cameras? I see these aspect ratios are used a lot but Im not sure how to achieve them in camera (lens + sensor size necessary for it).

 

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You set up 1.85 or 2.00 framelines in the camera and you crop in post to those aspect ratios.  If you are delivering in HD or UHD, which have a fixed 1.78 : 1 ratio, you use letterboxing to achieve 1.85 or 2.00.

Sensor size is not relevant, you could shoot on any camera and frame and crop for 1.85 or 2.00.

Since 2.00 is not as common, you may have to generate custom framelines in the camera if possible.

Also, 16x9 (1.78 : 1) is only a few scanlines different than 1.85 so one could use 1.78 framelines and just leave room in framing for some very slight cropping top & bottom to achieve 1.85.

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