Best telephoto lens for the pentax k x?

Here is what the site says about that.

From Pentax K-x: Pentax K-x: A DSLR by Pentax with a 12.4-megapixel APS-C sensor, announced in 2009. Runs on four AA batteries. Specs - Maker: Pentax - Mount: Pentax K - Sensor: APS-C, 23.5 x 15.6mm, crop factor 1.5 - Resolution: 12.4 megapixels - Stabilisation: in the body - Announced: 2009 - Type: DSLR Focal lengths on it Lenses frame like 1.5 times their focal length on full frame: 16mm like 24mm, 23mm like 35mm, ...

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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