Do I really "Gain a stop?" Locked

Started 1 month ago | Questions thread
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A Marcus Junior Member • Posts: 49
Re: Do I really "Gain a stop?"

The optical industry uses the focal ratio (f-stop) as a measure of how much light energy one lens produces as compared to another. A giant lens 1 meter in diameter with a focal lenght of 10 meters, operates at f/10. Such a lash-up projects an image that will be equal in brightness to a miniature camera lens 100 mm in diameter with focal length of 1000mm. The f-stop is thus universal -- any lens operating at the same focal ratio as another delivers the same light energy.

A ratio (including a focal ratio) is dimensionless. The two key optical factors that deal with image brightness are the working diameter of the lens and its focal length. The focal ratio intertwines these two values. The result is a ratio that yields a valid image brightness comparison regardless of the dimensions that derived the value.

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