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Crop Factor 2K / HD / VFX

Arri Alexa Crop Factor

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#1 Olivier Martinez

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Posted 19 August 2013 - 05:17 AM

Hi all.

I have to shoot my first Real2VFX Spot. Varying 3D Models will be added to real shots. The 3D Technician asked me for the Crop Factor of the lenses we will be using while Shooting 2K and framing for HD. Does someone know how to calculate the Factor? I`m open for each additional advise!

Thank you!

Olivier


Edited by Olivier Martinez, 19 August 2013 - 05:18 AM.

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#2 Phil Rhodes

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Posted 19 August 2013 - 07:14 AM

Depends on the size of the sensor, surely?

 

Lenses don't have a "crop factor". I don't encourage the use of the term. A 50mm lens is a 50mm lens, no matter what sensor you put it on. The field of view changes given the size of that sensor. Attempts to discuss lenses in terms of a multiplication factor back to some often ill-defined norm are confusing.

 

I'm not sure what you're being asked for.

 

P


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#3 Olivier Martinez

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Posted 19 August 2013 - 08:33 AM

Hi Phil,

as far as I understood,, the 3D Technician needs all the lens and sensor Parameters to reproduce the same field of view in Computer. On top of that we have to shoot HDR shots with a mirrored silver ball. If it`s not a matter of the Crop Factor, what can I do to achieve everything a DP should look for in such a Situation (prodviding all data for CGI). As I already wrote, I`m pretty new to VFX Shooting. Any additional advise would be very helpful. Thank you. 


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#4 David Mullen ASC

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Posted 19 August 2013 - 11:09 AM

The 3D technician should just be able to work with the image area dimensions, which are 23.76 x 13.37mm for the 16x9 Alexa, 23.76 x 17.82mm for the 4x3 Alexa.

 

"Crop factor" is more for still photographers since it compares the FOV of the format to Full-Frame 35mm (8-perf 35mm 36 x 24mm).  The Alexa sensor is more or less the same size as 3-perf or 4-perf 35mm cine (Super-35).  The crop factor between Super-35 and FF35 is 1.5X (36mm ÷ 24mm).


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#5 Olivier Martinez

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Posted 19 August 2013 - 04:47 PM

Thank you, Mr Mullen! That was helpful to me.


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#6 Oliver Temmler

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Posted 20 August 2013 - 04:28 AM

The 3D Technician asked me for the Crop Factor of the lenses we will be using while Shooting 2K and framing for HD.

Hi Olivier,

 

for ProRes 2K recording, an ALEXA uses 2868 x 1612 raw pixels to create a 2048 x 1152 output (2868 x 2150 for 2K 4:3). At 8.25 microns per pixel, that equals an 22.16 x 13.3 mm aperture. When you take a 1920 x 1080 crop from this 2K image, the corresponding aperture is 20.77 x 12.47 mm.

 

You can find detailed technical information in the camera data sheets, available at www.arri.com/alexa/downloads.

If you require details about the angle of view of ARRI lenses, there are handy tables in the ALEXA Pocket Guides, which you can also find in the download section.

 

Best

Oliver


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