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DOF and Imager sizes


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#1 Fred Stephens

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Posted 27 October 2010 - 01:47 PM

How does a 2/3", 1/2", and a 1/3" imager size affect depth of field? I have heard the larger the imager the shallower the depth of field when all other factors are equal. Anyone able to direct me to articles on this topic?
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#2 Fred Neilsen

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Posted 27 October 2010 - 03:48 PM

The imager size doesn't really effect the depth of field, it effects the field of view, meaning that a camera with a larger sensor will require a longer lens to produce an equivalent image as a camera with a smaller sensor would. This means that a 50mm lens would be required on a 35mm camera, while say, a 25mm lens would be required on a 16mm camera You probably already know that a longer lens will have a smaller depth of field than a shorter lens at the same t-stop, so the 35mm camera will have a shallower depth of field than the 16mm camera.

Hope this helps,

Fred
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#3 David Mullen ASC

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Posted 27 October 2010 - 05:17 PM

Yes, sensor size affects Field of View, which affects your choice of focal length.

Practically speaking, the general rule is that the crop factor is also the number of equivalent f-stops you gain or lose in terms of depth of field (not actual exposure) once you match field of view and distance and f-stop.

In other words, if the difference is 2X (like the difference between a 12mm wide sensor and a 24mm wide sensor, let's say) then the DOF difference is two-stops, meaning that the smaller sensor would look as if you stopped down the larger sensor by two-stops in terms of the increase in depth of field.

A 2/3" sensor (about 9 to 10mm wide) has a 2.5X crop factor compared to a 35mm cine sensor (24mm wide), so the difference is 2.5-stops in terms of the increase in effective depth of field.
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#4 Gabe Spangler

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Posted 27 October 2010 - 08:29 PM

Fred, I have to agree with you. All things being equal (aperture, focal length), the larger the format the shallower your depth of field will be. I don't know how anyone can disagree.

If you take a 35 mm sensor camera and a 1/3" sensor camera, put a 50 mm equivalent lens on each, set iris to f2 and focus on a human subject about 5 feet from the lens, you will see a huge difference in your depth of field, or appearance of depth of field. It's as simple as that. Scientific and mathematical mumbo jumbo aside, sensor size (or film size/format size) is a key piece of the depth of field puzzle, along with focal length and aperture.

Am I right in saying that smaller format has a smaller circle of confusion diameter, therefore more of the image is likely to be in "acceptable focus"? And with larger format, the circle of confusion diameter is larger, therefore out of focus areas will look really out of focus, giving us the pleasing out-of-focus bokeh that we all know and love? If I'm wrong ... oh well. I don't care about the technical stuff anyhow, just how to use it.
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#5 Tim O'Connor

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Posted 28 October 2010 - 02:01 AM

David, you keep educating. You are dedicated.
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