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F11 lens with 2x TC = F22?

Started Aug 5, 2020 | Questions thread
Sittatunga Veteran Member • Posts: 5,406
Re: F11 lens with 2x TC = F22?
1

makofoto wrote:

F11 is not a physically small opening in a 800mm lens. I find it hard to believe that Diffraction plays any part in this?

It does. The radius of the Airy disc on the sensor (the brightest image from a point light source through a lens with a circular aperture) is exactly proportional to the f/number.  See https://www.dpreview.com/forums/post/64229267 for the algebra.

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