Best telephoto lens for the panasonic lumix gx9?

Here is what the site says about that.

From Panasonic Lumix GX9: Panasonic Lumix GX9: A mirrorless camera by Panasonic with a 20.3-megapixel Micro Four Thirds sensor, announced in 2018. A rangefinder-style body with stabilisation and a tilting viewfinder. Specs - Maker: Panasonic - Mount: Micro Four Thirds - Sensor: Micro Four Thirds, 17.3 x 13mm, crop factor 2.0 - Resolution: 20.3 megapixels - Stabilisation: in the body - Announced: 2018 - Type: mirrorless camera Focal lengths on it Lenses frame like 2.0 times their ...

From Sensor sizes: What it costs you: - Size, weight and price, especially of the lenses. A full-frame telephoto is much larger than its APS-C or Micro Four Thirds equivalent. - Less depth of field when you want more of it, for macro or landscape.

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