Вот кстати частичный ответ на мой вопрос....
While the Canon EF 70-300mm f/4-5.6 IS USM Lens is reasonably well built, the 70-200 f/4 L features a significantly higher build quality. The 70-200 f/4 L also features fast and quiet-focusing Ring USM. Image quality is also better on the L lens - color, contrast and especially sharpness.
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