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Trouble Finding the Correct Crop Factor Formula

lens directors viewfinder alan gordon mark v

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#1 Kendrick Satterfield

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Posted 21 August 2017 - 01:11 AM

Apologies if this question has already been addressed, but by what I could tell actual mathematical calculations haven't been brought up.

 

Right now I have an Alan Gordon Mark V director's viewfinder. I'm trying to figure out what the exact focal lengths (Give or take 2-3 millimeter) would be for commonly used sensor sizes such as full frame, Canon aps-c, and the Red Scarlet.

 

After calling Alan Gordon this is what I know for sure. The 35mm window is based off of super 35 3-perf film, 24.9 x 13.9. My baseline that would remain consistent on the viewfinder for these calculations would be the the 35mm window with a 16:9 aspect ratio. 

 

My question is as I find out the crop factor each sensor has in relation to the viewfinder do I need to apply that 16:9 aspect ratio to the sensor sizes? This part confuses me. In my mind I would want to leave the sensor sizes untouched. 

 

I suppose I'm just thoroughly confused. If someone can shed some light on what would be an appropriate course of action to take in figuring out what formula to use I would be so grateful.


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#2 Brian Drysdale

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Posted 21 August 2017 - 07:21 AM

You can work out a factor using this:

 

https://www.abelcine.com/fov/


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#3 David Mullen ASC

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Posted 22 August 2017 - 09:22 AM

If the aspect ratio doesn't change then you can just either the horizontal or vertical measurement to figure out the crop factor. For example, the crop factor between a format that is 36mm wide versus 24mm wide is 36 divided by 24.
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#4 Kendrick Satterfield

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Posted 24 August 2017 - 12:35 AM

You can work out a factor using this:

 

https://www.abelcine.com/fov/

Super helpful, thanks Brian!


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#5 Kendrick Satterfield

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Posted 24 August 2017 - 12:43 AM

If the aspect ratio doesn't change then you can just either the horizontal or vertical measurement to figure out the crop factor. For example, the crop factor between a format that is 36mm wide versus 24mm wide is 36 divided by 24.

David, that's a good point. Thanks for reminding me of that.


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