Re: Exact fl of lenses... 220 VR

[Bill Glickman <bglick@xxxxxxxx>]

Andrew


> I probably got into this discussion a bit late but I can not imagine how
the
> focal length of a lens would be a variable. Focal length by definition is
a
> fixed factor (zooms excluded of course!).

       On a non rotating camera it's a fixed factor.... a mute point indeed.
However, on a rotational camera the fl entered into the brain box determines
the amount of film that travels in front of the lens for a given angle.  The
greater the fl, the more film used.  Hence, if you do not know the exact fl,
you will get a film travel distance that does not match the fl... unless you
randomly guess it right.  How much problems this causes, as I mention in the
last post, is function of several variables.

>
> The things that _are_ variable in a rotating panning camera are the center
of
> rotation used, the amount of film that is transported through the camera
per
> unit of time,

        Yes, you said it right here... and the fl entered determines that
amount of film.


 the use of focus adjustments

    Yes, this has been discussed... gg and film alignment is critical for
proper focus on all cameras, including rotational cameras.

... and maybe a couple of other
> ones but I can't think of them right now.
>
> I could see how changing the rotation speed of the camera might lead to
> differences in the amount of film transported through it per revolution
but
> this is, imo, a highly unlikely scenario in a precision camera system.

       As I mentioned before, 4 mm off on fl, creates blurred edges in all
my tests,  if that is acceptable to some people, then this should not
concern them.  If you are after big enlargements, then one better be
careful...

Regards
Bill G

>
> Best regards,
>
> Andrew  o o  0 0 o . o  Davidhazy, Imaging and Photo Tech
>          \/\/\/\/\/\/          http://www.rit.edu/~andpph
> __________|        |_____________________________________
>
>