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Old 10-01-2016, 04:47 PM
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RickS (Rick)
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RickS is offline
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
The f-ratio is focal length divided by aperture, so it's pretty easy to calculate. For f/4 at 70mm focal length you'd want a 17.5mm aperture. At 200mm focal length the aperture would be 50mm to give you f/4.

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