Re: diffraction (was 4x5 focusing)

[kenneth.r.rockwell@xxxxxxxxxxxxxxxx]

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Original Text
>From Willem-Jan Markerink <w.j.markerink@xxxxx>, on 7/22/96 8:43 PM:
To: <kenneth.r.rockwell@xxxxxxxxxxxxxxxx>

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,

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     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
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The difference 
between men and boys
is the price of their toys

<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
Diffraction is very real.  I've been studying it for years.  

All the hobbiest need know is that diffraction is only partially visible to 
my eyes under an 8x loupe at  f/22 and really visible at f/32 and above.  
This is on Velvia, on IR film it's probably not much of a problem due to 
the limitations of IR film resolution in the first place.  It is visible as 
a simple loss of resolution or sharpness exactly similar to putting a black 
stocking over the lens.  Because f/stops above this are not found on puny 
35mm and most 120 cameras the casual hobbiest can forget about it.  It only 
becomes a real problem with 4x5 and above where f/32 and above is used all 
the time.

Its effect on point sources in a photo (streetlights, the sun, etc.) is 
more obvious, and usually due more to internal reflection than diffraction. 
 

Diffraction has been known about for hundreds of years and is predicted by 
the wave nature of light.  See any physics book or optics book for the 
formulae.    The funny part is, since I've been researching this for years, 
that not all these formulae agree exactly!  It has to do with the 
difference between those who solve using calculus and those solving using 
simple trigonometry.

Diffraction is diffraction.  Some people confuse diffraction with the 
reflections off the straight flat (not "knife") shiny black edges of 
diaphragm blades that lead to putting those lovely stars around light 
sources.  The number of points is equal to the number of diaphragm blades 
if an even number of blades, and twice that number if the number is odd.  
(14 points with 7-bladed diaphragms, 6 with 6 blades, etc.) Of course 
diffraction also plays a role here, too, but the pretty "star" effects are 
mostly simple reflections.  Just look through a lens stopped down just 
right and you can see these glinting off the edges of the blades sometimes.

A halo is a halo and is usually caused by the lack of an anti-halation 
layer.  Polaroid 55 negative film is pretty bad in this respect.These halos 
are HUGE compared to the "Airy" discs (Airy was a person) formed at typical 
photographic f/stops.  You won't see the Airy disc on film in real 
photography, so if you get an obvious ring you have a halo.  

Diffraction gets worse as color gets redder, but IR film has crummier 
resolution anyway. 


I could go on but won't .  The first and only PRACTICAL article I've ever 
seen on this just came out in the MAR/APR 1996 Photo Techniques mag.  Go 
read it, it sums up what I spent years developing independantly, and is 
brilliant.  It covers which is exactly how to determine the OPTIMUM f/stop 
to use when considering the simultaneously contradicting problems of 
diffraction AND depth-of-field.

Ken Rockwell
kenneth.r.rockwell@xxxxxxx
Hollywood, CA

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Topic No. 21