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Mirror Focusing


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#1 David Regan

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Posted 18 June 2007 - 02:31 PM

I have sort of an odd question thats just made me curious, which is, how come you can rack focus on a mirror? I had a situation involving writing a word on a mirror, and then I racked to the reflection of a man in the background. At first I didn't think it would work, because I assumed the mirror would act as a flat plane, like a picture, you can't rack to the elements in the picture, as they are in the same 2D space. However it did work, just as a normal rack focus would. Can anyone explain why this works, I know its kind of an odd topic, but I'm curious if anyone can enlighten me.
Thanks
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#2 chuck colburn

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Posted 18 June 2007 - 03:08 PM

What you see is what you get.
In other words the distance from the subject to the mirror plus the distance from the mirror to your eye/film plane is the real focusing distance.
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#3 Nick Mulder

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Posted 18 June 2007 - 04:02 PM

A mirror reflects light - it doesn't emit it ...

hard to describe without ray diagrams !
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#4 John Sprung

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Posted 18 June 2007 - 06:08 PM

The image in a mirror isn't a flat 2D surface image like a painted picture. For purposes of focus, depth of field, and binocular distance finding, a mirror is just like a window, except that it changes the direction of the rays. You could do the same thing with someone writing a word on a window, and then racking to a person outside.




-- J.S.
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#5 Daniel Sheehy

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Posted 18 June 2007 - 10:00 PM

This image should help make it clear.. apologies for the quality.. I just whipped it up now.

Posted Image

As you can see, if you focus the camera at distance A, the writing will be in-focus, but not the object. The reason bing that to reach the camera (or the eye) the light reflected from the object must travel distance B+A.

Thus, in order for the object to appear in-focus, the distance focused at must be A+B.
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#6 David Regan

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Posted 19 June 2007 - 12:11 AM

Thanks a bunch guys, makes perfect sense, and satisfied my curiosity.
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