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Depth of Field and 65mm format.


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#1 Eric Soto

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Posted 05 January 2018 - 07:45 PM

 I was reading in American Cinematographer about the making of Thor : Ragnarok and and it says " to maximise the depth of field and exposure quality of the Alexa 65, Aguirresarobe (DoP) was determined to light to a relatively high T-Stop of 5.6, while rating the camera at ASA 1,250."

 

I wanted to know if anyone could explain the relationship between DoF and 65mm formats or any format really and why this was specifically done in the quote provided.

 

Thanks everyone!


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#2 Macks Fiiod

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Posted 05 January 2018 - 07:55 PM

The bigger the format, the shallower the depth of field.


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#3 Stuart Brereton

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Posted 05 January 2018 - 07:56 PM

The larger the format, the less DoF there will be at any given stop (assuming matching distance and Field of View). So, if you were shooting at f2.8 on s35, you would need to be at f4 on Full Frame, f5.6 on 65mm and f8 on 15/70 Imax, to have matching DoF.


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#4 Eric Soto

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Posted 05 January 2018 - 07:56 PM

The bigger the format, the shallower the depth of field.

Yea I figured, sorry I should've been more specific as to why this happens. 


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#5 Eric Soto

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Posted 05 January 2018 - 08:00 PM

The larger the format, the less DoF there will be at any given stop (assuming matching distance and Field of View). So, if you were shooting at f2.8 on s35, you would need to be at f4 on Full Frame, f5.6 on 65mm and f8 on 15/70 Imax, to have matching DoF.

Why does a larger format make the DoF smaller?


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#6 Stuart Brereton

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Posted 05 January 2018 - 08:06 PM

Why does a larger format make the DoF smaller?

Because larger formats require longer lenses to achieve the same FoV as smaller formats. Longer lenses have less DoF. There is also the issue of increased magnification and its effect on Circles of Confusion.

 

https://en.wikipedia.../Depth_of_field


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#7 Eric Soto

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Posted 05 January 2018 - 08:07 PM

Because larger formats require longer lenses to achieve the same FoV as smaller formats. Longer lenses have less DoF.

 

https://en.wikipedia.../Depth_of_field

I see. Awesome thank you for the quick replies.


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#8 Stuart Brereton

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Posted 05 January 2018 - 08:10 PM

I may have over simplified it there. Hopefully, the article explains it better than I can.


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#9 Eric Soto

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Posted 05 January 2018 - 08:11 PM

I may have over simplified it there. Hopefully, the article explains it better than I can.

It's fine. It's a good starting point to understanding the science behind it.


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#10 Stuart Brereton

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Posted 06 January 2018 - 12:00 PM

I need to correct myself here. The longer focal length is irrelevant. The decrease in DoF with larger formats is due to the increased magnification at the focal plane necessary to cover a larger area. As the image is magnified, CoC becomes more critical, which in return requires a deeper stop to achieve 'acceptable' focus.


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#11 David Mullen ASC

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Posted 06 January 2018 - 12:08 PM

It's even more complicated than that because a larger format will need to be enlarged less to fill the same sized screen, so larger formats actually have less critical circle of confusion figures...

 

Look at a depth of field chart for a 50mm versus a 25mm lens for example -- there is actually a 4-stop difference in depth of field when both are focused at the same distance, but put the 25mm lens on a format that is half as large to get the same field of view as the 50mm, and you have to use a CoC figure that is twice as critical because the smaller format will be enlarged by double to match the same screen size, so that 4-stop difference becomes only a 2-stop difference.


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#12 Stuart Brereton

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Posted 06 January 2018 - 01:19 PM

So, if I'm understanding you correctly, David, the increased magnification at the focal plane is offset by the decreased magnification in projection, resulting in a smaller net difference in DoF.


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#13 David Mullen ASC

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Posted 06 January 2018 - 02:04 PM

Yes
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