Does the Tamron 70-300mm for Sony a7 III provide a true 300mm focal length?

Asked 2/17/2021

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I use a Sony RX10 IV and learned that its advertised 24-600mm range refers to 35mm-equivalent focal length, not the lens’s physical focal length. I’m considering the Tamron 70-300mm f/4-5.6 for my Sony a7 III and want to know whether it reaches an actual 300mm focal length on that camera, or if there is a crop/equivalent factor involved.

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Yes. On a Sony a7 III, which is a full-frame camera, a Tamron 70-300mm lens gives its stated actual focal length range of 70-300mm.

The confusion comes from the RX10 IV: its built-in lens is often described using 35mm-equivalent focal lengths because the camera uses a smaller 1-inch sensor. Its lens’s physical focal length is much shorter, but it gives a field of view similar to 24-600mm on full frame.

By contrast, the a7 III is full frame, so a full-frame Tamron 70-300mm behaves as labeled, with the field of view you’d expect from 70-300mm on full frame. If you switch the a7 III into APS-C crop mode, the field of view becomes narrower, roughly like a 105-450mm equivalent, but the lens itself is still physically 70-300mm.

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