Depth of Field in Anamorphics

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#1 Lena Krause

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Posted 01 February 2016 - 07:11 AM

Hey!

I was trying to find out about how to calculate the depth of field in anamorphic lenses and i read several times that its the same formular for spherical and anamorphic lenses ...

Do you really use the same Formular to find out about the actual depth of field in anamorphic lenses? I always thought depth of field depends on the Front Nodal Point in the Lens itself. So since anamorphic lenses are made of two different focal lengths, aren’t there two different Nodalpoints ?

Maybe it does not make sense at all and  my thoughts are heading in the wrong direction.

Can anyone help ?

thanks a lot,

I'm confused!

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#2 Phillip Mosness

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Posted 25 February 2016 - 02:34 PM

Hi Lena,

There's some info on this subject in the FAQ on anamorphics at the top of this lens forum.

"To be safe one should calculate the depth of field based on the actual focal length of the lens."

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#3 Gregory Irwin

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Posted 25 February 2016 - 04:28 PM

Let's not complicate things. It's basically the same as spherical. If it really makes that minute of a difference, don't gamble!!!

G
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#4 Lena Krause

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Posted 26 February 2016 - 07:44 AM

but being basically the same means to me that it is not the same actually.... so there is in fact a difference, even though it is probably not noticeable and not of interest for most people.... but thats my question: what exactly is the difference? and how does is occur ?

so far it was impossible for me to find any info on that.

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#5 David Mullen ASC

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Posted 26 February 2016 - 10:18 AM

Trouble is that even spherical lenses vary by where the front nodal point is, I believe, so generally that aspect isn't taken into account if comparing, let's say, a 50mm spherical to the 50mm position on a zoom lens.

But with an anamorphic lens, the problem is that the circle of confusion is not a circle, it is an oval, so in theory you'd use the focal length of the anamorphic lens to calculate depth of field along the vertical axis but half that focal length to calculate the depth of field along the horizontal access.  But since real objects don't exist in only one direction but not the other, you can either take the tact that the actual depth of field is somewhere in between, or just use the more critical figure of the listed focal length of the anamorphic lens.

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#6 Gregory Irwin

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Posted 26 February 2016 - 11:00 AM

With consideration of 2x anamorphic, say, a 50mm focal length is very similar in it's verticle angle of view whether it's spherical or anamorphic but horizontally, it's twice as wide.  That would basically make it similar to a 25mm focal length spherically.   The depth of field would be calculated the same as a 50mm lens but the caviate is you have twice as much angle of view horizontally. That is what makes focus tricky with the anamorphic format.  Twice the angle of view with half of the depth to cover it.

G

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#7 Lena Krause

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Posted 26 February 2016 - 11:27 AM

thanks a million for helping me!

i'm asking because i am writing a bachelor thesis about anamorphic lenses.

so for the quality of the thesis, i'm trying to find the most accurate answer.... not for using it on set to find out about the exact depth of field.

so i assume there is no specific formular written down for calculating the depth of field for anamorphic lenses, so in conclusion you just have to guess and assume?

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#8 David Mullen ASC

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Posted 26 February 2016 - 11:30 AM

Like I said, you use the most critical figure, which is the one for the focal length of the lens, so you use the same chart for a 50mm spherical as you would for a 50mm anamorphic, even though in theory there is a little more depth of field in the horizontal axis compared to the vertical axis due to the oval of confusion.

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#9 Gregory Irwin

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Posted 26 February 2016 - 12:32 PM

Like I said, you use the most critical figure, which is the one for the focal length of the lens, so you use the same chart for a 50mm spherical as you would for a 50mm anamorphic, even though in theory there is a little more depth of field in the horizontal axis compared to the vertical axis due to the oval of confusion.

Exactly!!

G

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#10 David Mullen ASC

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Posted 26 February 2016 - 01:38 PM

The thing is that Depth of Field isn't really an exact science, anything based on an "acceptable" range of sharpness using a formula centered on a Circle of Confusion figure based on specific degree of enlargement, when people watch images on all sorts of screen sizes...

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