what is the difference between focal lenght and camera-to-object distance? DOF at DVinfo.net

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Old January 10th, 2005, 10:30 AM   #1
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what is the difference between focal lenght and camera-to-object distance? DOF

hi

I was reading about DOF and there are 3 things that you can do to change DOF.

1 focal lenght
2 camera-to-object distance
3 iris/ f-stop

what is the difference between focal lenght and camera-to-object distance?
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Old January 10th, 2005, 04:09 PM   #2
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I think (not sure) the difference is:

Focal length: when you are zoomed in on your subject

Camera to object: how close you are to your subject
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Old January 10th, 2005, 04:54 PM   #3
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Focal length is determined by your lens. 28mm, 50mm, and 85mm are focal lengths. A zoom lens has variable focal lengths, such as 24-70mm, whereas prime lenses have fixed focal lengths. The longer the focal length (bigger the "zoom") less depth of field you will have.

Camera to object is the physical distance from the lens to the subject, measured in feet, meters, etc. The closer the subject is the less depth of field you will have.
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Old January 12th, 2005, 05:43 PM   #4
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thanks for your answers. I apreciate them.

Now I am goin gto study the use of DOF in movies and series (E.R.) and try to understand which of the 3 solutions they use to make those movies. I know they use 35 mm film.I don't think they use ZOOM all the time, cause in E.R. they have moving cams so you can't have that of a steady movement when zoomed in. So I am guessint they are doing something with the IRIS (larger aperture of lens).

thanks
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Old January 12th, 2005, 05:59 PM   #5
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Defs.

Focal Length:

The distance from the optical center of the lens to the image sensor (ccd block or film plane) when the lens is focused on infinity. The focal length is usually expressed in millimeters (mm) and determines the angle of view (how much of the scene can be included in the picture) and the size of objects in the image. The longer the focal length, the narrower the angle of view and the more that objects are magnified.

Camera to object distance:

It's just that. The physical distance between the object and the film plane or ccd block of the camera.


Depth of Field:

The zone of acceptable sharpness in a picture, extending in front of and behind the plane of the subject, that is most precisely focused by the lens. You can control or exploit depth of field by varying three factors: the size of the aperture; the distance of the camera from the subject; and the focal length of the lens. If you decrease the size of the aperture, the depth of field increases; if you focus on a distant subject, depth of field will be greater than if you focus on a near subject; and if you fit a wide-angle lens to your camera, it will give you greater depth of field than a normal lens viewing the same scene.

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Old January 12th, 2005, 06:14 PM   #6
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Jose, since you mentioned E.R. and the use of 35mm film, there's another thing you missed.

The size of the film or sensor plane also affects depth of field, because of aperture. The actual aperture size determines the depth of field, and this will be constant regardless of the size of the sensor or the f-stop. Thus, on a smaller sensor you have a smaller lens with a much smaller aperture, in order to get correct exposure. Thus, depth of field is increased. This is why you can't get that nice background blur on your handycam, and why people spend $10,000 on mini35's for their DV cameras.

ER doesn't get their depth of field by excessively "zooming" or other sort of tricks. 35mm naturally has the most shallow depth of field because it has the largest "sensor", compared to 1/3", 16mm, etc...

In professional productions things tend to remain the same for consistency and quality issues. Apertures and lenses aren't changed that often, especially in a studio environment where lighting can be easily managed.
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Old January 13th, 2005, 08:05 AM   #7
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Yes, it is the iris that is generally used to control depth of field. Given that the sensor/film size is fixed other considerations determine the focal length and camera to subject distance such as composition and perspective (short focal lengths tend to separate things closer to the camera from those farther away while longer ones tend to flatten the image). If one wants great depth of field he stops down the lens and turns up the lights and/or increases exposure time. If shallow depth of field is sought he opens the lens, turns down the lights and/or decreases the exposure time and/or adds neutral density filters.
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Old January 20th, 2005, 11:49 AM   #8
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thanks for the answers. I understand it a little bit better. After having read this thread, I am looking at some selfmade home-adapters 35mm, cause I can't pay for the real thing. To be honoust...lots of threads being posted about DOF and the difference between HD and 35mm and the urge to accept HD as it comes. But I think film looks to much warmer and it really is about DOF and motion. The motion I can come by using adobe after fx. And I am saving to buy a 24 cam (canon xl2 maybe or other newer cams). I also have to save for a good microphone and some lights, so that is why I prefer to build one adapter myself. Seen 3 tutorials online and my borhter is goo din fixing things, so ...

who knows. I just need that shallow DOf which can be found in E.R. and Braveheart and stuff.
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