Principal focal length: be careful
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The Myth of Equivalent Focal Length - IMF – World Bank International Photographic Society
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Focal Length: An Easy Guide to Using and Understanding Camera Lenses
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optics - What is the actual significance of the focal point of a lens? - Physics Stack Exchange
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What is Focal Length in Photography?
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Focal length - Wikipedia
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What is Focal Length in Photography?
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Focal Point | COSMOS
What Is Focal Length in Photography?
Focal length, focal distance, dioptric power, curved mirror, lens equation, microscope, photographic objective, focus, beam radius
Focal Length of a Lens
