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Frequent Contributor
Posts: 113
Registered: ‎01-04-2013

Re: Lens Diffraction vs Image Quality



@ebiggs1 wrote:

Ed,

This is not correct.

"the diffraction blurring does not depend on the lens focal length, but only on the f-number."

 

Diffraction is worse with WA and UWA lenses because of the short FL. Why?  Because the hole in the diaphram is smaller at any given f-stop compared to the same f-stop in a normal or tele typle lens. In a WA/UAW the light path is bent at a greater  angle.

 

__________________________________________________________________

 

EB,

 

After your interesting comments, I looked into this topic some more.  My conclusion about the importance of light diffraction in lenses was based on thinking about a singlet lens with an aperture right next to it. Of course, modern lenses are complicated affairs and designed my specialized optics engineers. Nevertheless, my conclusion still holds, namely, that the diffraction blurring on the camera sensor is proportional to:

 

                   2.44 * lambda * f-number

 

The entrance pupil diameter, which makes up part of the f-number, is defined as the image of the len's aperture stop as seen from the object (front) side of the lens. The link below corroborates the conclusion above. (Since I haven't tried to include a link on this site before, if it does not work just go to Wikipedia and search "Diffraction-limited System".

 

https://en.wikipedia.org/wiki/Diffraction-limited_system 

 

For me, it is always helpful to be able to understand things in the simplest, most reduced way, and this is an example. It makes learning easier.

 

Respectively,

Ed

 

Forum Elite
Posts: 14,424
Registered: ‎12-07-2012

Re: Lens Diffraction vs Image Quality

Ed,

I have no idea about math so that stuff is wasted on me. I only know from experience. WA and UWA suffer more diffraction from a given f-stop than teles do.

 

So I must repeat..........

This is not correct.

"the diffraction blurring does not depend on the lens focal length, but only on the f-number."

 

Sometimes stuff is made a lot more difficult by reading too much. Sometimes it is the simple explanation.

EB
EOS 1DX and 1D Mk IV and several lenses!
Frequent Contributor
Posts: 41
Registered: ‎06-22-2021

Re: Lens Diffraction vs Image Quality

Diffraction blur depends upon F number and the size of pixels on the sensor. It does not depend upon focal length or physical aperture diameter except that F number is calculated from both focal length and aperture diameter. For the same lens, a different sensor will have a different diffraction blur. There might be sources of blur other than diffraction blur, for example depth of field or motion. The combination of focal length and F number will change depth of field blur.

There is a simplified explanation with diagrams illustrating the math and example calulators at https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

When using a Richardson/Lucy deconvolution to remove diffraction blur, the radius must be chosen based upon the F number and the spacing of pixels on the sensor.

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https://www.rsok.com/~jrm/
Frequent Contributor
Posts: 113
Registered: ‎01-04-2013

Re: Lens Diffraction vs Image Quality

Agreed, johnrmoyer.

Forum Elite
Posts: 14,424
Registered: ‎12-07-2012

Re: Lens Diffraction vs Image Quality

"Diffraction blur depends upon F number and the size of pixels on the sensor."

 

I already stated that.

 

" t does not depend upon focal length or physical aperture diameter except that F number is calculated from both focal length and aperture diameter."

 

Yes it does. Why? Because the light rays are bent more in a WA/UWA than it is in a tele lens. As FL increase the diameter of the aperture is also increase. This bends the light less. This is simple to understand if you don't understand or reject it I have done all I can.

EB
EOS 1DX and 1D Mk IV and several lenses!
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