Re: ATM "Focaultgrams" images

["Jeff Baldwin" <bald@gotnet.net>]

Unsharp masking pulls details out in the texture of you polish.  You take
an overexposed black and white image of your Foucault shadowgram.  You then
place a thin sheet of plate glass between the negative and another
unexposed film.  Using a non-point light source you expose the negative
onto the film to make a slightly out-of-focus positive.  When the tow are
combined the result is that the out-of-focus positive will wipe out or
neutralize the image's general brightness level leaving only distinct
changes in intensities on both the bright areas and the dark areas of the
picture.  These distinct areas are of higher contrast now rather than all
too dark or all too bright on the negative.  The Lyot test may be a better
way of pulling out microripple texture.  I'm using a 135mm telephoto lens
generally at f/5.6 to shoot my Foucault shadowgrams of f/3.5 through f/6
mirrors with35mm format, or 250 mm lens with 120 format.  You may want to
use a longer lens to shoot slower mirrors.  The unsharp masking will
exagurate your microripple texture.  Bracket.
Jeff Baldwin
bald@gotnet.net

----------
> From: Ken Lowther <s0109366@cc.ysu.edu>
> To: Ricardo Dunna <rdunna@apit.furnas.gov.br>
> Cc: atm@shore.net
> Subject: Re: ATM "Focaultgrams" images
> Date: Thursday, September 18, 1997 10:36 AM
> 
> On Thu, 18 Sep 1997, Ricardo Dunna wrote:
> 
> >    It was about a 28 cm , 20mm thick tabletop glass, taken with 150 mm
> > telephoto lens. All surface details are easily visible and were
enhanced
> > with unsharp masking.
> 
> I have tried putting the knife edge in front of my camera lens, but have
> never been able to get any detail from the test.  What is unsharp
masking.
> 
> Ken Lowther  
> s0109366@cc.ysu.edu
> Youngstown, Ohio  USA
>