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pCAM: Alexa LF Mini DoF vs Alexa Mini DoF


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Hi there fellas!

I'm currently working on a feature film as a 2nd AC and there's someting the focus puller and I have been discussing for the past few days that we've been unable to puzzle out much as we try. The camera we're shooting with is the Alexa Mini LF and a set of Arri Ultra Primes. Here's the thing:

We were facing a shot involving two actors and at some point while measuring distances we broke bout pCAM app to compare the DoF of the LF we're using and the Alexa Mini. To our surprise the app showed greater DoF in the LF than in the Mini, which definitely made our heads blow:

Camera 1
Alexa Mini LF Open Gate
Focus Distance: 2,5m
Aperture: f4
Focal Length: 50mm
Total depth: 0,45m

Camera 2
Alexa Mini Open Gate
Focus Distance: 2,5m
Aperture: f4
Focal Length: 50mm
Total depth: 0,38m

What do you make of this? Shouldn't there supposed to be shallower depth of field with a greater sensor as of the LF? To my understanding the greater the sensor the shallower depth of field, but there's clearly something I'm missing here...

Please help! Your insight will be very much appreciated! Find attached the screenshots.

Thanks

WhatsApp Image 2020-03-08 at 12.52.09 (1).jpeg

WhatsApp Image 2020-03-08 at 12.52.09.jpeg

Edited by Cristian Diaz
Typo
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I checked this against my pCam and got the same result.

I'd chalk it up to a programming error as a 50mm lens at T4 set to the same focus distance should net you the same result on both formats.

It's a misconception that a larger format means less depth of field as it doesn't have anything to do with the format size it self but rather the focal lengths you will be using to achieve the same FOV of the super35 images we are so used to.

If a lens had an infinite image circle that could cover any format, think of that image circle as our "pie".

The size of your format is effectively the "slice" of that "pie" that we are taking. The physics of the lens remain the same in regard to the DOF.

This is how on s16 a 50mm lens feels more like a 100mm lens in terms of field of view but it has the DOF of a 50mm lens.

If you put that same 50mm on s35 it would feel like a 50mm FOV (if we're using s35 as our baseline) you're now just taking a larger "slice" of the 50mm "pie".

On a "full format/LF/VV/" camera, that 50mm is going to feel more like a 32mm but its going to have the DOF of a 50mm, you're just now taking an even larger "slice" of the 50mm "pie" in terms of FOV.

Edited by Evan Walsh
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**Edit to the post above, It's not a bug in pCam so disregard that bit.

I didn't take into account that the DOF calculator is using a different circle of confusion between the two formats. While CoC does matter in depth of field calculations it doesn't make that big of a difference as you are arriving at similar numbers.

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