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Crop factor question


Dominik Bauch

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For scouting purposes, I'll be using a 5D mkIV. I'll be using an Alexa Mini on the shoot and I've used my high school math to arrive at a 1.3x crop factor vs the mk IV.

I've built a template in PS to mask out the Canon photos to equate them to the Alexa aspect ratio and resolution I'll be using. 

Question is, using the crop factor, how do I figure out what a full frame 50mm lens on the 5D would be equate to on an Alexa Mini. (To complicate things I'm using anamorphic lenses but that's just halving those focal lengths to arrive at the spherical equivalent for horizontal field of view...) Is it as simple as 1.3 x the focal length? I thought it might be more complicated as it's not like the traditional thing of using a full frame lens on a smaller sensor... kind of but not quite.

Yes I have Artemis but we wanted to storyboard with lens compression and nicer looking images than Artemis spits out.

 

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The full-frame focal length equivalent will always be longer than the Super-35 focal length so you’d multiply to figure out the full-frame value but divide the full-frame number to figure out the Super-35 value.

The thing is to be accurate you really have to know the actual width of the sensor area to be used for the final composition... and anamorphic complicates that if you have to crop the 2X image later.

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1.5X is for comparing a 36mm wide sensor to a 24mm wide one.

But he might be shooting the Alexa Mini at 3.2K (26.40mm) or Open Gate (28.25mm).  Between 36mm and 28.25mm, you have a 1.27X crop factor.  Between 36mm and 26.40mm, you have a 1.36X crop factor.

Also, if you record 4K video on the Canon 5D Mk4, you only use a 22mm wide sensor area out of 36mm.

 

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1 hour ago, David Mullen ASC said:

1.5X is for comparing a 36mm wide sensor to a 24mm wide one.

But he might be shooting the Alexa Mini at 3.2K (26.40mm) or Open Gate (28.25mm).  Between 36mm and 28.25mm, you have a 1.27X crop factor.  Between 36mm and 26.40mm, you have a 1.36X crop factor.

Also, if you record 4K video on the Canon 5D Mk4, you only use a 22mm wide sensor area out of 36mm.

 

ah ok got it .. thanks for the info.. also didnt know the 5D IV cropped in for 4K ..   

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Thanks all for the tips, so what's the best work flow for anamorphic?

Set 5D to 16x9, match horizontal FOV for full frame lenses to the anamorphic focal lengths I'll be using and then apply a 2.39 mask in PS on the output jpegs? Rough for sure but at least the horizontal FOV should be close.

Is there a more accurate way to approach this?

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Shooting 2X anamorphic on the Alexa Mini complicates the crop factor issue.  In truth your matching focal lengths are actually longer with anamorphic even compared to full-frame spherical.

The 2X anamorphic sensor area for an Alexa if you want to end up with a 2.39 : 1 image once unsqueezed is 21.12mm x 17.70mm.

But since there is a 2X horizontal squeeze, you can calculate horizontal field of view by pretending your sensor area is twice as wide, so 42.24mm instead of 21.12mm. So you are comparing a full-frame 36mm wide sensor using spherical lenses to a "virtual" 42.24mm sensor, which is a 1.17X crop factor in the reverse direction, i.e. a 24mm spherical lens on your Canon 5D full frame still image would match the horizontal view of a 28mm anamorphic lens on the Alexa Mini.  Or to put it another way, to match the view of the standard 40mm anamorphic lens on the Alexa Mini, you'd have to use a 34mm lens on the Canon 5D in still mode.

This virtual doubling of the format width by use of 2X anamorphics is one reason why the focal length numbers for anamorphic are similar to when shooting in 65mm spherical, i.e. double of standard 35mm cinematography.

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