Hi Andrew, Yes, when a lens is described as a 500 mm or 300 mm it is fixed focal length. A 150-500 is a zoom with varied Focal Length. Roy's advice is something else you need to take into consideration - the f stop controls the amount of light getting to the sensor or film. I use a 1000 mm Reflex Nikkor which is fixed at f11. This means I only use it on very bright and relatively static subjects if using it for birds. If birds are in motion I use a 500 mm f4 -P Nikkor which is a lot "faster". I also only use both from a firm tripod.
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