Re: Diaphragm Stars and Printing

[Marco Pauck <pauck@xxxxxx>]

ADavidhazy wrote:
> 
> > Andrew Davidhazy pointed out that "the anti-reflection coating is on
> > the surfaces of the lenses and not on diaphragm or shutter blades".
> 
> > I don't think I was getting confused. I think this may differ from
> > camera to camera since I had a Minolta SRT-101 where the manual
> > referred to an anti-reflective coating on the iris blades in their
> > Rokor lenses. It didn't seem to work though since I could get great
> > stars at f22 :=) Of course this could just have been bog-standard
> > anodising (?) given a marketing spin in 1960s Japan.
> 
> Agreed that having diaphragm blades that do not reflect light is a good thing
> but normally anti-reflection coatings are thought of in terms of lens surfaces.
> On the other hand, anti-reflection coatings on metal I do not think will have
> much of an effect on what happens at diaphragm blade _edges_ which is where the
> diffraction ocurrs. ... as substantiated by your experience.
> 
> > Presumably any coating that affected the edges of the iris diaphragm
> > would also affect edge diffraction effects? Or am I too long away
> > from high-school optics?
> 
> I am not sure I am putting this the right way but if this were the case we
> would not have diffraction limited lenses. It is precisely diffraction at lens
> or diaphragm edges that limits the resolving power of lenses. Smaller apertures
> result in less resolution.

Right, the degree of diffraction and the resulting 'stars' around
light sources is, as already described correctly, determined by
the light's wavelength, the diaphragm's diameter, and the lens'
focal length, *not* by other properties of the diaphram's blades.
So it's an issue of physical laws, not engineering.
(However, the number of blades determines the star's shape.)

An insufficient blackening of the blades simply causes overall
contrast degradation of the image. Some eastern lenses are prone
for this, e.g. the Ukrainian Arsat lenses or the lenses from
East-German Carl Zeiss Jena.

	Marco
-- 
Marco Pauck - http://www.pauck.de/marco/
For every complex problem, there is a solution that is simple, neat,
and wrong.  -- H. L. Mencken
*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************