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Depth of Field


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#1 Gary Lemson

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Posted 18 April 2006 - 09:44 AM

Rookie comment here.

It is my understanding that reducing aperture will increase DOF by reducing light that creates observed circles of confusion. However, I've read that reducing light will decrease DOF. George Lucas commented on the latter in "The Making of American Graffiti". I'm a little "confused" myself. Please help.

Thanks.

Edited by otari99, 18 April 2006 - 09:45 AM.

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#2 Dominik Muench

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Posted 18 April 2006 - 09:54 AM

assuming that with more Depth of Field you mean: More Background gets in focus it is as followed:

small aperture: f1.3, f 2...... less depth of field (less background is in focus)

big aperture: f8,f11,f16.....more depth of field (more background is in focus)

depth of field depends on a few other things as well, such as the lens focal length and the shooting format (video has lots of dof, the bigger the negative the less dof you can get or the more you can play with it ) for example.

with a lens as wide as 12mm you get even on a small aperture a lot of depth of field.

also simply spoken, the more light you got and the higher your aperture (depending on exposure requirements) you also get more depth of field, maybe george lucas meant that. so:

less light - smaller aperture - less depth of field
more light - higher aperture - more depth of field

Edited by Dmuench, 18 April 2006 - 09:56 AM.

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#3 Gary Lemson

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Posted 18 April 2006 - 10:39 AM

assuming that with more Depth of Field you mean: More Background gets in focus it is as followed:

small aperture: f1.3, f 2...... less depth of field (less background is in focus)

big aperture: f8,f11,f16.....more depth of field (more background is in focus)

depth of field depends on a few other things as well, such as the lens focal length and the shooting format (video has lots of dof, the bigger the negative the less dof you can get or the more you can play with it ) for example.

with a lens as wide as 12mm you get even on a small aperture a lot of depth of field.

also simply spoken, the more light you got and the higher your aperture (depending on exposure requirements) you also get more depth of field, maybe george lucas meant that. so:

less light - smaller aperture - less depth of field
more light - higher aperture - more depth of field


Isn't the aperture smaller for a bigger "f" number?

Thanks again.

Edited by otari99, 18 April 2006 - 10:40 AM.

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#4 Dominik Muench

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Posted 18 April 2006 - 11:11 AM

Isn't the aperture smaller for a bigger "f" number?

Thanks again.



yes, the bigger the f or t number, the smaller the physical "hole" in the lens, and therefore the less light gets through, resulting in more depth of field.
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#5 Dominic Case

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Posted 18 April 2006 - 05:45 PM

However, I've read that reducing light will decrease DOF.

It's a bad way of describing the effect.

Depth of field is increased by having a narrow cone of light coming through the lens rather than a wide one. (The wider the cone of light, the more rapidly the spot of focus enlarges to an out-of-focus disc as the object distance and the distance of focus change).

So it could be said that reducing light by reducing the aperture (to a higher F number), would increase the DOF. It's not so much reducing light per se, but reducing light (in the camera) from the edges of the cone. That's what you've understood.

I guess what George Lucas is saying is that if you reduce light on the subject, then you need to open your aperture, thereby decreasing depth of field. Not sure if his explanation makes any great contribution to anyone's understanding of the issue.
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#6 Gary Lemson

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Posted 18 April 2006 - 08:42 PM

It's a bad way of describing the effect.

Depth of field is increased by having a narrow cone of light coming through the lens rather than a wide one. (The wider the cone of light, the more rapidly the spot of focus enlarges to an out-of-focus disc as the object distance and the distance of focus change).

So it could be said that reducing light by reducing the aperture (to a higher F number), would increase the DOF. It's not so much reducing light per se, but reducing light (in the camera) from the edges of the cone. That's what you've understood.

I guess what George Lucas is saying is that if you reduce light on the subject, then you need to open your aperture, thereby decreasing depth of field. Not sure if his explanation makes any great contribution to anyone's understanding of the issue.

Okay. Great explanation. I sort of figured there is a distinction between ambient subject light, and the light within the lens.
I've heard the suggestion that one might use an ND filter to reduce ambient light, and open the aperture to reduce DOF. This seems in concurrence with your statements (and Georges, too). Now I understand this aspect of DOF. Thank you.
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#7 Laurent Andrieux

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Posted 20 April 2006 - 06:13 AM

Opening the iris will lower the DoF. So If you have less light, you will need to open up.

If you close down the iris, you will increase DoF, but thet means you need more light, otherwise you would be underexposed !

If you put a neutral filter, thet will cause you to have to open up, reducing DoF, that means also you have enough light to do so, otherwise you'd be underexposed...

(don't forget the iris does not only acts on DoF, as a second point, the first point is it acts on the quantity of light. The purpose, whatever you do with DoF, is as a first point, to be correctly exposed as well...)
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