Re: diffraction (was 4x5 focusing)

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 22 Jul 96 at 2:37, Robert Long wrote:

> In a post elesewhere a month or two ago I read the statement that
> stopping down too far causes diffraction that increases the tendency
> of IR to create "halos."  I remain skeptical.  I'd like to hear
> opinions from our motly but fascinating crew.

Well, after a confusing thread on PhotoForum two weeks ago I hesitate 
to get into trouble again, but alas:
I didn't know better there was only one type of diffraction, ie 
bending of light around the aperture blades, and the magnitude 
related to the wavelength of light, ie IR suffers twice as much as 
visible. The latter is also stated in one of my technical German 
books. "Do not stop down to minimum aperture, 2-3 stops at most. 
Trying to bypass the IR focus correction this way is counterproductive."

But there seems to be a difference between diffraction and edge
diffraction; the first only occurring at aperatures equal to or
smaller than the wavelength of light, the latter with apertures as
used in normal photography. I can imagine it somehow, this
wavelength related stuff was even demonstrated in highschool with
running water waves through small openings, but it certainly does
not comply with my IR books....:-(( 

Any optical engineers among us, to set me straight? (again....:-))

BTW, I am almost certain none of the above is related to the halo 
effect. This is only caused by the lack of a (dyed?) antihalation layer in 
HIE; perhaps also a reason why HIE can be developed as a slidefilm 
(1600ASA as claimed in a German mag, see my homepage). Agfa Scala 
seems to have a similar 'deficiency', albeit it doesn't show the same 
halo's of course (maybe I am confusing things now....but I am sure I 
will be corrected if neccessary....:-)).

--
Bye,

       _/      _/       _/_/_/_/_/       _/_/_/_/_/
     _/  _/  _/               _/       _/  _/  _/
     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
                     _/_/_/  


The difference 
between men and boys
is the price of their toys

<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

------------------------------

Topic No. 18