Can someone help me understand how AF can be erroneously focusing?

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Moses Regular Member • Posts: 319
Can someone help me understand how AF can be erroneously focusing?
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I searched this forum for posts on the Samyang 45mm f1.8 and found one that talked about a number of lenses that focused erroneously, and that manual focus is much sharper.

Can someone kindly explain to me how this happens?  I thought that the sensor is live during focusing, just like my eyes, and then focus based on a sharp image on the sensor.  Is that no so?  If it is so, how can mirrorless cameras focus erroneously?  Also, apparently the lenses need to be calibrated - how does calibration impact the sharpness of the image on the sensor?

Thanks in advance for any insight. I was going to purchase a Samyang 45 1.8 and a Tamron 28-75 2.8 ahead of the A7Siii....and wonder now if that is a good move. Or whether I should wait until the body comes?

Thanks.

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