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[OM] Re: Pixel diffraction effects on resolution [was Mini Macro questio

Subject: [OM] Re: Pixel diffraction effects on resolution [was Mini Macro questions]
From: Moose <olymoose@xxxxxxxxx>
Date: Mon, 19 Mar 2007 10:37:29 -0700
Winsor Crosby wrote:
> I thought it was pretty well accepted that you get aperture  
> diffraction in 35 mm film cameras by F16. You can see it in the  
> results in almost all lens tests. A quick review of Gary Reese's OM  
> lens tests on a film camera certainly reflects that. Why it would  
> disappear in the full frame lens you are using on your 5D is puzzling.
>   
According to the experts, pixel diffraction is independent of focal 
length, and only dependent on pixel size and physical configuration. See 
the last paragraph 
<http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm>.

Aperture diffraction, on the other hand, is a function of the absolute 
size of the aperture, not the F-stop it may happen to represent in any 
given focal length lens. Aperture diffraction is the reason pinhole 
cameras work. but it's a problem when it starts happening inside a lens. 
40 years ago, I could have done the calculations and described the 
physics, now I just know it happens. :-)

A 50 mm lens at f16 has an aperture diameter of about 3 mm (50/16). A 3 
mm physical diameter on a 100 mm lens is f32 (100/3). I'm not saying 
that is where diffraction effects will become noticeable in any given 
lens, just making a simple example.

In the world of complex lenses, I suspect the physical diaphragm 
location may not always be at the optical equivalent of the center of a 
simple lens, so the actual relationship between physical aperture size 
and F-stop may vary from lens to lens. It probably has to be fairly 
close to one of the nodal points, but how close, I have no idea.

The above example may be fairly accurate, though, as f32 is just where 
diffraction trouble becomes clearly apparent on the 90 mm lens I used. 
That's why I was careful not to attribute all the drop in sharpness in 
my tests at f32 to pixel diffraction.

So it doesn't disappear. I hope that makes it less puzzling.

Moose

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