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Old March 15th, 2005, 11:41 AM   #1
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Join Date: May 2004
Location: The Netherlands
Posts: 204
Questions about DOF

Hi all,

I have been following people's attempts to create a shallow DOF effect with much amazement and respect, and will be planning to try to make one of my own in the near future (probably using james' guide).

Now I also use a 35mm still camera, and I know a thing or two about photography and lenses, but some things about DOF in relation to lens-length still puzzle me. Maybe some of the gurus here know.
(I searched a lot about this, but sites about DOF tempt to be either too simplistic, or too complex (mathematical))
I've also read this article:

As far as I know, there are a few variabels that make up the differences in DOF: aperture, lens length, distance to subject and some "less important" or less controllable factors like circle of confusion and Entrance pupil factor.

Now about focal length and distance to subject. Imagine two shots of the same subject: one with the camera a large distance from the subject, using a long lens, and the other one with a small distance between camera and subject, this time using a short lens. Both shots are arranged so that the subject is exactly equal in size at both shots (of course, perspective will be different between both shots). Do they have the same amount of DOF? the first shot has a large distance (more DOF), but a long lens (less DOF), and the second one has a small distance (less DOF) and a short lens (more DOF).
So how do these two variables relate to each other? My logic would say that they are inversely proportional quantities, hence as long as you keep the subject the same size, you will always have the same DOF.
But there must be something wrong with that assumption, because why would people then used the conventional method of getting far away and use long lenses in dv-productions to get less dof? What's going on here?

My second question is also about focal length. Isn't it right that the only reason that a long lens has less DOF is because it has proportionally bigger apertures? I mean, if you compare a lens of 50mm and one of 100mm, the 100mm needs a twice as big hole to get the same aperture value. That is because an aperture value like f/2 is always a ratio of the "hole" radius and the focal length. That means that if a 50mm f/2 has a certain "hole"-surface x, the 100mm f/2 would need a "hole"-surface of 2 times x. So that would mean that in the end, the only important variable here would be the (absolute) aperture, measured in square mm from the diaphragm. Does that make sense?

Hope I'm being as clear as possible, and not asking to much strange questions, I'm just trying to get a hold of this whole DOF-thing. Hope one of you wise guys can help me out!

Much regards,

Steven Fokkinga is offline   Reply With Quote
Old March 17th, 2005, 08:46 AM   #2
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Join Date: Dec 2004
Posts: 749
Have you checked out?

This is the method by which I have been trying to accomplish my DOF machine - with the help of too many people to mention in this forum.
Leo Mandy is offline   Reply

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