Re: Testing Focal Lengths of Lenses

[Dan Slater <dslater@xxxxxxxxxxxxx>]

Bill Glickman wrote:

> George
>
> RE: Optical bench testing lenses for roundshots.
>
>            In my opinion, this is a "hit or miss" proposition.  Here is why
> I feel this way.   There is two parts required for a rotational camera to
> provide its maximum resolution on film.   This of course assumes that all
> the procedures in my last email are being properly executed.  The two parts
> are:
>
> 1.  exact fl of lens.
> 2.  film length actually used for each fl and slit size entered.
>
>             If these two variables were independent of each other, optical
> bench lens testing of lenses would be a great idea.  However these variables
> are not independent of each other.  The optimal fl to enter into the brain
> box is a combination of the lenses true optical fl, and any correction
> factor of the film length used for each fl.  Remember, when changing the fl,
> you are changing the amount of film being consumed.  Unless the actual
> amount of film used matches EXACTLY the calculated length, then the exact
> lens fl is NOT the optimal one to use.

>
>
>         The one thing I would wish for to simplify this process....is some
> type of Polaroid 220 film whereas you can test, read results, adjust, re
> test, etc.  Of course no such product exist....so the only way I know to do
> this - is burn real film and run it up and back to the lab!  The tedious
> part is packing up your gear, going to lab, getting film processed and
> starting all over again and again.  It can take many many trips....

I would recommend adding a fiber optic or similar light source illuminating the
focal plane that projects backwards through the camera. Thread a piece of film
with a highly detailed image through the camera. The film could have been made
with a still camera or even just by scratching a piece of scrap film.

Project the film at the desired distance if the pan camera is focused close or
view with a telescope if at infinity. The telescope can be nothing more that a
still camera with a tele lens. There should be no lateral motion of the image if
the "focal length" is properly adjusted.

Dan Slater