The F-number is defined as which ratio?

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Sharpen your skills for the Davies Publishing SPI Test with targeted flashcards and multiple-choice questions, complete with hints and clarifications. Prepare thoroughly for success!

Multiple Choice

The F-number is defined as which ratio?

Explanation:
The f-number measures how large the lens opening is relative to its focal length. It is defined as the focal length divided by the aperture diameter: N = f / D. That means a 50 mm lens with a 25 mm opening gives f/2. This ratio controls light gathering and depth of field: smaller numbers (larger openings) let in more light and yield shallower depth of field; larger numbers (smaller openings) let in less light and increase depth of field. The other ratios don’t match this definition—those involve the aperture over focal length, or include wavelength, which aren’t how the f-number is defined.

The f-number measures how large the lens opening is relative to its focal length. It is defined as the focal length divided by the aperture diameter: N = f / D. That means a 50 mm lens with a 25 mm opening gives f/2. This ratio controls light gathering and depth of field: smaller numbers (larger openings) let in more light and yield shallower depth of field; larger numbers (smaller openings) let in less light and increase depth of field. The other ratios don’t match this definition—those involve the aperture over focal length, or include wavelength, which aren’t how the f-number is defined.

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