What are the alternatives to the pentax hd da 560mm f 5 6 ed aw?

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From Pentax HD DA 560mm f/5.6 ED AW: Pentax HD DA 560mm f/5.6 ED AW: A prime lens by Pentax for APS-C cameras, 560mm f/5.6. A super-telephoto prime for birds and distant wildlife. Specs - Maker: Pentax - Focal length: 560mm (prime) - Maximum aperture: f/5.6 - Made for: APS-C sensors - Mounts: Pentax K - Stabilised: no - Autofocus: yes What it frames like - On APS-C (Pentax K): like 840mm on full frame; f/5.6 gives about the depth of field of f/8.4 there. ...

From Lenses: Aperture: The f-number says how wide the lens opens. Smaller numbers let in more light: f/1.8 lets in about ten times as much light as f/5.6, more than three stops. A wide aperture means faster shutter speeds in dim light and more background blur. Zooms with a constant aperture, like a 24-70mm f/2.8, keep the same exposure throughout the zoom range; cheaper zooms get darker as you zoom in.

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