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Author Topic: Diffraction vs focal length  (Read 1004 times)

madlantern

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Diffraction vs focal length
« on: May 04, 2017, 06:17:18 PM »

Does diffraction at f/22 differ between say a 16mm lens vs a 200mm lens? I ask because the actual physical aperture on f/22 at 200mm would be significantly larger than on 16mm, and as far as I understand, it's the smallness of that physical opening that causes diffraction.
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BartvanderWolf

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Re: Diffraction vs focal length
« Reply #1 on: May 04, 2017, 07:06:02 PM »

Does diffraction at f/22 differ between say a 16mm lens vs a 200mm lens? I ask because the actual physical aperture on f/22 at 200mm would be significantly larger than on 16mm, and as far as I understand, it's the smallness of that physical opening that causes diffraction.

Hi,

No, it makes no difference, the (relative) aperture number (for a given wavelength) determines the size of the diffraction pattern.

The effective or physical aperture of the 200mm may be larger, but it is also much further away. So the angle of the light cone (numerical aperture) is the same as for the 16mm lens.

Basically, f/22 from any focal length produces a diffraction pattern with the same diameter.

Cheers,
Bart
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EricV

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Re: Diffraction vs focal length
« Reply #2 on: May 04, 2017, 09:04:14 PM »

Diffraction causes a point source to be imaged as a blurred spot at the lens focal plane.  The angular extent of the blurred spot depends (inversely) on the diameter of the lens aperture, independent of focal length.  This is generally what astronomers care about.  Photographers usually care more about the physical size of the diffraction spot at the focal plane, which is the angular extent multiplied by the lens focal length.  This quantity is proportional to the lens f/number.

Consider taking a photograph of a resolution target with two lenses of different focal length, then printing the images so that the resolution target is the same size in each print.  How blurry will the resolution target appear?  If I (photographer) am allowed to move closer to the target with the wider lens, so that the target has the same size on the camera sensor, then final blur depends only on f/number.  If I (astronomer) am constrained to leave the target at a fixed distance from the camera, so that the longer lens produces a magnified image of the target on the camera sensor, then final blur depends only on lens diameter.
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