zoom - Is it exactly true that doubling the focal length makes everything look twice as big? - Photography Stack Exchange
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Thiery Legault - How to adjust the focal length ?
Why does the aperture number increase with focal length? What would happen if the aperture diaphragm was disabled and focal length was increased? - Quora
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Close-up lenses
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Solved 18.Using the same Converging Lens, set and keep the | Chegg.com
Thiery Legault - How to adjust the focal length ?
What changes will you see in the focal length of a lens if a part of the lens is covered? - Quora
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Photography Snapshot: The Power of Lenses | WIRED
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