I recently got a Nikon D3000 as my first DSLR camera. It came with a 18-55mm lens, but I'd like to also purchase a lens with a higher focal length for distance photography. I'm trying to figure out though if I should go for a lens with a 200mm focal length or a 300mm. Of course a 300mm will provide a better zoom, but I only need so much. Best way to figure out is to compare the how far away from the subject I can be with both focal points.

So say I wanted to take a full-body shot of a person from a distance (such as at a sport event), and I wanted them to take up at least 50% of the photograph. If I wanted to take a picture like that with a 200mm lens, about what is the furthest distance I can be away from the subject? What about a 300mm lens?

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