[MF3D.FORUM:428] Re: fl/30

["David Lee" <koganlee@xxxxxxxxxxxxx>]

I realized last night after I had sent the last message that I had neglected
to state that my intention is to keep the deviation constant (within
practical and artistic limits), no matter what the focal length of the
taking lens or the depth of the scene.

> David
>
>          I ran your formula side by side with John B formula and
> surprisingly enough, using a constant deviation in John's formula of 2.7,
> your answers were extremely close every time, regardless of the, near
> distance , far distance and focus distance.  So I guess you are both on
the
> same page...and when evaluating a scene, you both would use the same
stereo
> base.
>
>     But the question that arose earlier.....that is still not clear to me
is
> this.... if your formula is accurate than I guess the answer to the
> deviation question must be..... the deviation does not change even when
the
> fl of the camera lens is increased or decreased.  A few posts ago, it was
> suggested that when you double the camera  fl, you halve the deviation?

What you halve is not the deviation, but the stereo base you would have
gotten from using the 1/30th (dare I say) rule.

 But
> on the other side of the coin, when you go to shorter fl camera lenses you
> should never go below 2.7 deviation. When using longer fl lenses this
logic
> appealed to me, hence why I am pursuing this one till the bitter end :-)
>

The deviation would always stay at 2.7mm for medium format film (assuming a
constant viewing lens).


> Here is Johns formula if you do not have it handy...
> http://home.mira.net/~kiewavly/bases.html
>
>        David you seem to be very experienced at this, and I guess the
bottom
> line is this.... if you have very good success with this formula, than
maybe
> the deviation should not change at all, regardless of the camera fl lens
> being used?   Meaning yours or John B's formula (with constant deviation)
> would get us to the same stereo base.
>

This is exactly right.


>       Any one else have experience at this.... following the math is one
> thing, knowing what works is something quite different.  Thanks David...
> your input is very helpful to us beginners getting a handle on this... it
> seems we are now very close... I really look forward to seeing your paper
on
> this subject.
>

Interchanges like this will help get me inspired to finish it before the
convention. I think that the math is very straight forward, what I really
intend to delve into in the paper is how one actually does these things in
the field, even down to how one estimates how far away something is and how
accurate you need to be to get a good result.

David Lee