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Alexa XT / SXT Sensor Size Confusion


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#1 James Malamatinas

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Posted 27 June 2017 - 12:22 PM

​I'm doing a little format research and comparing film to digital sensors I'm struggling to match things up with the Alexa XT/SXT sensor. ​According to Arri (alexa sensors)

 

 

The camera's custom developed CMOS Bayer sensor has the same height and width as a 35mm film frame. This has a number of advantages. The Super 35 mm width allows a cinematically shallow depth of field as well as plug and play use of the unparalleled range of Super 35 PL mount lenses.

 

What exactly are they referring to here? 

Super 35mm refers to the original silent film aperture of 24.89mm x 18.67mm ​(1.33) Looking at the SXT manual, and as shown here) the sensor dimension size at different formats are:

  • 28.17 x 18.13mm​(1.55) for Open Gate. 
  • 23.76 x 17.82mm (1.33) for 4:3 mode
  • 21.27 x 17.82mm ​(1.2) for 6:5 mode
  • 23.76 x 13.37mm (1.78) for 16x9 mode

​None of these made the original Super 35mm format size, what am I missing here? Is it simply that in 4:3 mode the dimensions are similar enough to Super 35mm to not make a significant difference? 


Edited by James Malamatinas, 27 June 2017 - 12:23 PM.

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#2 David Mullen ASC

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Posted 27 June 2017 - 01:17 PM

Yes, it's mainly a matter of being close enough -- after all we are talking about a digital image, there is no contact print off of the negative that will be projected.

 

Plus even with film, there is the dimension of the camera gate, the dimension of the area scanned, the dimension of the area used for the final image, etc.  For example:

 

scanning_format_table.jpg

 

Plus Super-35 4x3 is rarely a delivery format -- Super-35 was created to extract a widescreen format from it so rarely was the full height of the silent aperture area used other than for reframing for visual effects shots.


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#3 James Malamatinas

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Posted 27 June 2017 - 01:55 PM

Thanks David, that's very helpful and makes a lot of sense. As I understand it  when Super 35mm was being used for 1.85 flat it would have had a negative dimension of 24.89 x 12.97mm ​(taken from the American Cinematography Manual), with the image being cropped with a hard matte in the projector (most of the time anyway)?

​If so, I'm correct in thinking that with the Alexa XT/SXT sensor the ProRes 3.2k mode at 1.85 is larger than the equivalent S35 as it has dimensions of 26.10 x 14.67mm​? 

​At what point does the increase in the digital negative size start become an important consideration? Obviously if it gets larger than the image circles of 35mm lenses you'll end up getting vignetting, but at what point does it start significantly affecting depth of field (due to changing lens sizes) and other image characteristics, opposed to being "close enough" and presumably negligible as you put it earlier- in this example it is about a 20% increase? Perhaps a very subjective question! 





 


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#4 David Mullen ASC

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Posted 27 June 2017 - 03:46 PM

Super-35 was not meant to be projected from a contact print -- whether you shot 1.85 or 2.40, in the past you had to use an optical printer to either make a reduction down to standard 1.85 within Academy, or crop & squeeze to 2.40 anamorphic.

 

And then standard 1.85 was always projected in 35mm with a 1.85 projector mask.

 

There was always some trimming around the edges from the full aperture, which is why there are different dimensions listed for Super-35, the difference between the negative area exposed versus the frame lines of the picture area within that.

 

For example, look here -- Super-35 extraction for 1.85 and 2.40 use a 24mm wide area of the negative:

http://panalab.panav...atSpecs2010.pdf

 

Yes, 3.2K ProRes in the Alexa, or 3.4K Open Gate, use a wider area than standard 35mm or Super-35, so in theory there is slightly less depth of field if matching field of view and distance between the larger area versus Super-35.  You could try on online calculator that allows you to input custom sensor sizes to figure out how much you'd have to stop down once you matched field of view and distance by switching to a slightly longer lens for the 3.2K sensor area.  Keep in mind that technically the Circle of Confusion figure used would be slightly different between the two formats if both are being viewed on the same sized screen.


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#5 James Malamatinas

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Posted 28 June 2017 - 02:29 AM

As always, another comprehensive explanation, thank you. I'd not fully understood that was how S35 was used in regards to projection and reduction prints so that's helped make everything click into place a little more. The link is also indeed help in visualising a lot of this, much appreciated.  

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