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image size, focal length, circle of confusion


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#1 Michael Althaus

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Posted 12 March 2006 - 05:54 PM

Why do I get bigger circles of confusion (narrower DOF) with a bigger image plane than I get with a smaller image plane. The DOF mainly depends on the f stop used, right? How much matters the focal length?

Does an image shot on 16 mm with a 25 mm lens look the same beside the DOF as an image with a 50 mm lens shot on 35 mm. Why is the DOF narrower on 35 mm.

It would be very nice if somebody could explain the relations between image size, focal length and circle of confusion.


Sorry for my bad English, I hope you understand what I trying to ask.
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#2 Stephen Williams

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Posted 13 March 2006 - 03:03 AM

Why is the DOF narrower on 35 mm.


Hi,

35mm has higher resoloution, is the simple answer. Take a lookat this.

http://www.vanwalree...optics/dof.html

Stephen
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#3 Adam Frisch FSF

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Posted 13 March 2006 - 06:49 AM

Very interesting link, Stephen.

It kind of supports my old hunch that DOF is actually constant with the same magnification. Taking a close-up of a person with a 500mm or a 20mm lens actually produces the same amount of bokeh per image area.
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#4 Dominic Case

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Posted 13 March 2006 - 06:30 PM

Why do I get bigger circles of confusion (narrower DOF) with a bigger image plane

The Circle of Confusion is not what you get - it's what you decide to accept.

The theory is that a point of light should ideally be reproduced as a point on the film - but away from ideal focus, it is reproduced as a small circle. If the circle is small enough that's OK - it will be too small to see the difference. The agreed CoC is based on conventional image size and viewing distance.

Conventionally 35mm motion picture uses CoC of .001", while 16mm uses .0006". The 16mm value is smaller because the image must be enlarged (in projection) more, to get the same viewing angle as for 35mm.

Why is the DOF narrower on 35 mm.

If you compare DoF for a 35mm lens using 35mm film, and for a the same lens on 16mm film, you will find that the 35mm has more DoF - because it is allowed a larger Circle of Confusion.

But if you compare the 35mm lens and film with a 16mm lens and 16mm film(they cover almost exactly the same field of view), you will find that the 35mm now has less DoF (at the same plane of focus). Even though it's allowed a larger CoC, the fact that it's a longer lens outweighs that. Focal length is a major factor in depth of field - for the same plane of focus.

Thats enough for one post. There are so many combinations of factors at play here, the term Circle of Confusion is well named. :blink:
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#5 Michael Althaus

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Posted 14 March 2006 - 06:26 PM

thank you for your replays. I'm still busy with reading the article. Will post again after I finished reading it and hopefully understand most of it....
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#6 Andy_Alderslade

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Posted 16 March 2006 - 08:17 AM

Thats enough for one post. There are so many combinations of factors at play here, the term Circle of Confusion is well named. :blink:


Thats probably one of the briefest, yet sharpest expanations i've read, cheers its cleared up some of my 'confusion,' :rolleyes: - sorry.

someone in the super 8 forum has asked why Super 8 has a larger circle of confusion ( according to ACM it is 0.002 inches) than 35mm. Is that because its uses mean it is enlarged less or because the grain is so large that focus isn't that critical.
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