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Anamorphic vs Spherical FOV on Ursa Mini 4.6k Pro


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#1 Vladimir Cazacu

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Posted 20 December 2017 - 03:38 AM

So, I'm trying to figure out what FOV I'd be getting on the Ursa Mini 4.6k Pro when shooting 3k anamorphic, as opposed to 4.6k spherical but I'm having a bit of a hard time trying to wrap my head around it all.

 

25.34mm  x 14.25mm   4608 x 2592  4.6k -  spherical - 1.77

 

16.89mm  x 14.06mm   3072 x 2560  3k - anamorphic - 1.20

 

BMD doesn't list the sensor window size in mm when cropping in to the "3k" 6:5 anamorphic mode but based on the resolution chart that they provide and on the listed full sensor size, if my math holds up (I calculated that 1.5x is the difference between 3072 and 4608 and used it to scale down the sensor size), that would mean that there is a 1.5x magnification/crop factor difference between the two modes, right?

 

So for example: when using 2x anamorphics, where one would traditionally get something close to the FOV of a 20mm spherical when shooting with a 40mm anamorphic lens on S35, on the Ursa you'd be getting the FOV of a 30mm spherical due to the smaller sensor readout (20 x 1.5 = 30mm) when compared to the 16:9 - 4.6k open gate (shooting spherical), right?

 

Does this make any sense at all or is it a flaming mess?

 

Cheers.


Edited by Vladimir Cazacu, 20 December 2017 - 03:42 AM.

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#2 David Mullen ASC

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Posted 20 December 2017 - 11:38 AM

Yes, you'd expect to lose FOV when using an anamorphic lens on any camera that does not have a sensor that is 17.5mm tall since the 4-perf 35mm anamorphic projector gate is 21mm x 17.5mm.  It's often the height of the sensor that is the limiting factor since with 2X anamorphic photography, you only use a 1.20 : 1 area for a final image that is 2.40 : 1.

 

Most digital camera sensors other than in the 4:3 Alexa are widescreen, often 1.78 : 1 to 1.9 : 1, some even wider in shape.  So if you don't want to lose FOV, you'd either have to use a 4:3 Alexa or a VistaVision-size sensor camera because those have sensors that are at least 17.5mm tall.


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#3 Vladimir Cazacu

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Posted 20 December 2017 - 05:37 PM

Thank you very much for taking the time to answer me, Mr. Mullen. Your posts here and on other forums have been of great educational value to say the least, and your enthusiasm for the craft has surely rubbed off on me.

 

That being said, is my approach in figuring out the FOV difference between the 16:9 open gate and the 6:5 / 2.40 desqueeze correct?


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#4 David Mullen ASC

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Posted 20 December 2017 - 05:55 PM

Not sure what you are asking... are you comparing crop factor while also comparing anamorphic to spherical?
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#5 Vladimir Cazacu

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Posted 20 December 2017 - 06:30 PM

Yup.

 

If a 40mm spherical gives me roughly a FOV of 35 degrees when shooting 4.6k open gate, does shooting a 40mm anamorphic give me the FOV of a 30mm speherical (46 degrees) due to the sensor windowing?


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#6 David Hessel

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Posted 20 December 2017 - 09:11 PM

That seems to check out. I am getting a crop factor of 0.74 compared to 4.6k spherical so 40mm Anamorphic would have the same fov as 30mm spherical.

Edited by David Hessel, 20 December 2017 - 09:13 PM.

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#7 Dom Jaeger

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Posted 20 December 2017 - 09:20 PM

Yes that sounds about right. 

 

A 40mm anamorphic lens will give the (horizontal) FOV of a spherical 20mm lens across the cropped 16.89mm sensor width, which is about what a 30mm spherical would give across the full 25.34mm sensor width.


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#8 Vladimir Cazacu

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Posted 20 December 2017 - 10:13 PM

Cheers, guys.


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