T3D Re: Focusing Formula ?

[Michael Watters <mwatters@xxxxxxxxxxxxx>]

John B states:
>P3D is such a zoo.  I give up.  I'll answer here where it won't get lost.

Hey, I tried starting a discussion about 2x2 viewers, but people were
 having too good a time talking about what they should talk about and
 if they needed more places in which to do their talking.  Maybe 

someone should suggest UC3D (Useless Chatter 3D).  :)

Marcus Warrington writes:
> Does anyone know of a formula that will allow me to compute what aperture and
> focusing distance I would need to use to allow the target item to be in focus 

> and anything before OR AFTER (the focus range) it to be out of focus.  ?
 

John B reponds:

>Given near point and far point, then the best focus is at:

>                       2*F*N
>best focus distance = -------
>                        F+N

>If you want to cover the range from N to F, then you want to stop down
>until your hyperfocal distance is (I had to look this one up):

I think what the guy was looking for was just the opposite.  He's looking to reduce
his DOF, throwing fore and backgrounds out of focus in order to isolate the subject.

I'll leave it to John to alter the formulas to give a precise technical explanation, but
basically it depends on HOW out of focus you need it to be.  If you're hoping
it'll be out enough that it won't impact your 3D at all you're out of luck.  The shift
between the lenses will insure that the viewer will always know there's some
sorta >thing< up close to the camera.  You CAN use limited DOF to eliminate things
in 2D photos, but that's only because they get thrown so out of focus that what's
left is an unrecognizable blob that blends in with the blobby background.  The blob
is still there though.  In 3D it'll be a stereo-blob.

Most extreme example I've taken myself was a 2D photo of a bird taken through a chain
link fence.  I was using a 135mm macro lens focused on the bird about 4 feet away.
The fence was TOUCHING the lens, the background was about 8 feet away and
I had the apeture set to f 3.5 (the lenses maximum).  The bird looked great, the background
was a total abstract blur, but in the corner there was a little grey part of the blur.
Nobody but me knew that the grey blur was coming from the fence.  If that had been
taken with a 3D camera, the blurs would have been in different places of course
resulting in a 3D blur, quite possible recognizable as a fence.

It's sorta connected to the running discussion about binocular vision giving a better
apparent resolution and all.  (I wasn't following the thread, forgive me if it veered
off of that somewhat).  Same basic idea.  I personally chalk it up to the wonders of
biology.  Same as the pulfrich effect.


mike
watters


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