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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?
5

zonoskar wrote:

I know that the 2x TC you lose half the light: your equivalent lens aperture becomes 2x higher, so in case of the new RF tele lenses, you get F22 light gathering. But does that still hold for images quality due to diffraction? The physical aperture is still the same size.

An 800mm f/11 lens has an effective aperture of just over 72mm.  Stick a 2x Teleconverter behind it, it becomes effectively a 1600mm lens with an effective aperture of just over 72mm, i.e. f/22.

The physical aperture is constant so the minimum angle the lens can resolve will be constant. But a 2x Teleconverter will double the effective focal length of the lens and thus halve the maximum resolution of the lens on the sensor.

[Mathematically, the radius of the Airy Disc from a circular aperture is 1.22(lambda/D) where lambda is the wavelength of the colour of light you're considering and D is the physical diameter of the aperture. But D for a lens of focal length F is also (F/f no.), so the radius of the image on the sensor of a point source at infinity will be 1.22(lambda/f no.)].

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