F-number, explained by RP Photonics Encyclopedia; light throughput, focal length, depth of field, working f-number
The focal length of a normal eye-lens is about:$(a){\\text{ 1mm}} \\\\(b){\\text{ 2cm}} \\\\(c){\\text{ 25cm}} \\\\(d){\\text{ 1m}} \\\\ $
What is Focal Length in Photography?
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Lens f/number in Photography
What Is Focal Length in Photography?
f-number - Wikipedia
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f-number - Wikipedia
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Focal Length of a Lens
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F/# & N.A - Wavelength Opto-Electronic
