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I'm practicing with Sigma 28 - 300mm f/3.5 - 6.3 Macro lens. And the thing I've noticed is that at 300mm focal length my aperture can be set to as wide as f/5.6. And EXIF data confirms it:

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I'm wondering how is that technically possible and if the behavior is considered normal?

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This is probably because some cameras disable autofocus at slow apertures. The camera manufacturers think that performance won't be good enough, and so they disable it. The third-party lens is reporting inaccurate information to the camera so that it will work anyway — slow and sometimes incorrect, perhaps, but at least the camera will try.

Exposure might be off by a tiny bit as a result, but the third-stop difference between f/5.6 and f/6.3 basically comes out in the wash anyway.

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